Using full frame lens on crop body cameras ?

[Ryan85]

GraFax I have and use the 16-35 f4 with the 5d3 and it is a great lens.

Nice. Glad your enjoying it. It's on my short list. So many lenses and so little time.

Yep I hear that.

 

[Joey]

I agree with you about the 24-70 2.8 on a crop sensor. You won't be able to get as shallow depth of field with a crop señor as with a ff. I think that's the point he's tying to make that you geting more like a f4 depth of field compared to a 2.8 on ff. But yes you still get the light of a 2.8 lens on a crop sensor

Depends on how you look at it. I'd argue that with the exception of the f-stop's effect on autofocus, you really don't get the light of an f/2.8 lens on a crop sensor. I mean ostensibly yes, if you have two sensors with the same pixel size and you look at a pixel-sized crop, you would see the same amount of light, but that's not the way people use cameras in practice.

People typically use cameras by framing a shot, and then viewing it at screen size or printing it at a desired print size. So the only truly interesting metric is the amount of light that makes up each square inch of output at a given size. Using that metric, because a crop sensor sees light from only about 39% of the lens, per square inch of output, a crop body gives you an image produced with only about 39% of the light that you'd get shooting the same shot with a full-frame body (ignoring any differences in light caused by moving closer to the subject, which if included, would make the crop body look even worse by comparison).

When it comes to the actual image projected on the sensor, there's no meaningful difference between using a crop body and using a teleconverter on a full-frame camera—just a little bit of IQ loss caused by the quality of the TC's glass, and maybe a tiny bit of light loss from the glass itself. And we say that using a 1.4X teleconverter makes a lens act like it is a stop slower. By that same standard, using a glorified 1.6X teleconverter (a crop body) makes a lens act like it is 1.35 stops slower. The only real exceptions to that rule are when either A. you'd be cropping the image on a full-frame to match the crop body (the reach-limited case) or B. you're talking about how the autofocus behaves. But in the more general case, you really don't get the benefits of an f/2.8 lens.

You may very well be right I don't know. I've always thought a 2.8 lens is a 2.8 lens whether it's on a ff or crop. Other than I know I'll get shallower depth of field with a ff and more depth of field with a crop at the same apertures.

An f/2.8 lens is an f/2.8 lens. f/2.8 is a formula, referring to the focal length of the lens divided by the effective maximum aperture of the lens (usually more-or-less the diameter of the outer element on a prime lens). Thus a telephoto lens needs bigger elements than a wide angle lens for the same f/2.8 (or whatever) aperture.
It's a red herring to introduce arguments about teleconverters into this discussion. A 1.4x converter doesn't 'make a lens act as though it's a stop slower', it really is a stop slower. The 1.4x converter changes the focal length of the lens combination but it doesn't change the size of the elements so the aperture formula changes. A 50mm lens with an effective diameter of 25mm has a maximum of aperture of 50/2 or f/2. Add a 2x converter and the lens combination now has a focal length of 100mm and the same 25mm diameter gives an aperture of 100/4 or f/4. It's maths. Nothing to do with the sensor.

The central paragraph of dgatwood's post is hard to follow but the physics works like this: the aperture of a lens defines how bright the image circle on the sensor is. Whether you use most of the image circle on a full frame sensor or less of the image circle (ff lens on crop body) the brightness of the image is the same - you're just using all or part of it. There will be no impact on image quality unless the individual pixels are of different sizes - and the pixels on a ff body are usually a lot bigger than the pixels on a crop body which has both benefits and disadvantages as discussed elsewhere on this thread and others. Some people including, I think, Tony Northrup, are confusing image brightness with 'total light capture' implying that unless you can use all the light that's coming through the lens you're losing quality which is plainly not so - we don't use, nor do we want to use, the edges of the image circle because the sharpness drops off and other distortions show up more.

The topic of depth of field - shallower depth of field on a ff and deeper dof on a crop body - is another subject and an interesting one. Make an image using a ff sensor of a particular scene and observe the depth of field - how much of the scene in front of and behind the point of focus is in acceptable focus. Now crop your image in software to the equivalent size of a crop sensor (in Canon terms, that's reducing the linear dimensions by 1.6) and look at the scene that is left. The depth of field hasn't changed, of course, it's the same image - you just have less of it. Now use the same lens at the same aperture from the same taking position on a crop body and take the same picture. You will get the same image you got after cropping the ff image. And the depth of field will be the same too because you used the same lens from the same taking position. Those are the elements that affect depth of field - taking position, focus distance, focal length of the lens and lens aperture. Sensor size does not of itself affect depth of field. HOWEVER - if you switch from ff to crop and change either the focal length of the lens or the taking position so that you get the same field of view as you had for the ff image UNCROPPED then you will get more depth of field. That's not because you're using a crop sensor, it's because you're using a shorter focal length lens or you're taking the picture from further away to get the same field of view as you got with the ff sensor. Since that's how we generally use our cameras, that's how we perceive it - we get more depth of field when we use a crop camera, because we're using shorter focal length lenses to get the same pictures.

 

[wyldeguy]

+1 to Joey. That is probably the best explanation I've ever heard and also one of the most intelligent arguments I've seen on a forum in years. I had actually believed that none sense about depth of field on crop bodies until now. Thanks

 

[Joey]

+1 to Joey. That is probably the best explanation I've ever heard and also one of the most intelligent arguments I've seen on a forum in years. I had actually believed that none sense about depth of field on crop bodies until now. Thanks

Thank you! I'm blushing...

 

[wyldeguy]

You're welcome

 

[weixing]

Hi,

+1 to Joey. That is probably the best explanation I've ever heard and also one of the most intelligent arguments I've seen on a forum in years. I had actually believed that none sense about depth of field on crop bodies until now. Thanks

Actually, it's not nonsense, it's just that most people assume that the image framing is the same (same field of view) when mentioning the depth of field on crop camera.... to get the same image framing (same field of view... eg. half body portrait on FF camera = half body portrait on crop camera), you need to be further away from the subject or use a shorter lens at the same distance... which will increase the depth of field.

Have a nice day.

 

[Helios68]

Hi,

I have never tested a FF camera intensively. But I do own a couple of EF lenses, including the 70-200 f/2.8 and 100L. What I can say is that they produce just amazing pictures even on my T5i/700D. I do not want to say that it performs as good as a FF body (which I don't expect), but it is definitely crap to say it is not usable at all !

I have compared the kit lens 18-55 (quite acceptable) with the 100L and 70-200L and it is worth the price ! But if it is just about a "Canon bashing", I can understand this point of view...

Regards.

 

[Jane]

Way to go, Joey. Well argued, well said.

 

[privatebydesign]

I agree with you about the 24-70 2.8 on a crop sensor. You won't be able to get as shallow depth of field with a crop señor as with a ff. I think that's the point he's tying to make that you geting more like a f4 depth of field compared to a 2.8 on ff. But yes you still get the light of a 2.8 lens on a crop sensor

Depends on how you look at it. I'd argue that with the exception of the f-stop's effect on autofocus, you really don't get the light of an f/2.8 lens on a crop sensor. I mean ostensibly yes, if you have two sensors with the same pixel size and you look at a pixel-sized crop, you would see the same amount of light, but that's not the way people use cameras in practice.

People typically use cameras by framing a shot, and then viewing it at screen size or printing it at a desired print size. So the only truly interesting metric is the amount of light that makes up each square inch of output at a given size. Using that metric, because a crop sensor sees light from only about 39% of the lens, per square inch of output, a crop body gives you an image produced with only about 39% of the light that you'd get shooting the same shot with a full-frame body (ignoring any differences in light caused by moving closer to the subject, which if included, would make the crop body look even worse by comparison).

When it comes to the actual image projected on the sensor, there's no meaningful difference between using a crop body and using a teleconverter on a full-frame camera—just a little bit of IQ loss caused by the quality of the TC's glass, and maybe a tiny bit of light loss from the glass itself. And we say that using a 1.4X teleconverter makes a lens act like it is a stop slower. By that same standard, using a glorified 1.6X teleconverter (a crop body) makes a lens act like it is 1.35 stops slower. The only real exceptions to that rule are when either A. you'd be cropping the image on a full-frame to match the crop body (the reach-limited case) or B. you're talking about how the autofocus behaves. But in the more general case, you really don't get the benefits of an f/2.8 lens.

You may very well be right I don't know. I've always thought a 2.8 lens is a 2.8 lens whether it's on a ff or crop. Other than I know I'll get shallower depth of field with a ff and more depth of field with a crop at the same apertures.

An f/2.8 lens is an f/2.8 lens. f/2.8 is a formula, referring to the focal length of the lens divided by the effective maximum aperture of the lens (usually more-or-less the diameter of the outer element on a prime lens). Thus a telephoto lens needs bigger elements than a wide angle lens for the same f/2.8 (or whatever) aperture.
It's a red herring to introduce arguments about teleconverters into this discussion. A 1.4x converter doesn't 'make a lens act as though it's a stop slower', it really is a stop slower. The 1.4x converter changes the focal length of the lens combination but it doesn't change the size of the elements so the aperture formula changes. A 50mm lens with an effective diameter of 25mm has a maximum of aperture of 50/2 or f/2. Add a 2x converter and the lens combination now has a focal length of 100mm and the same 25mm diameter gives an aperture of 100/4 or f/4. It's maths. Nothing to do with the sensor.

The central paragraph of dgatwood's post is hard to follow but the physics works like this: the aperture of a lens defines how bright the image circle on the sensor is. Whether you use most of the image circle on a full frame sensor or less of the image circle (ff lens on crop body) the brightness of the image is the same - you're just using all or part of it. There will be no impact on image quality unless the individual pixels are of different sizes - and the pixels on a ff body are usually a lot bigger than the pixels on a crop body which has both benefits and disadvantages as discussed elsewhere on this thread and others. Some people including, I think, Tony Northrup, are confusing image brightness with 'total light capture' implying that unless you can use all the light that's coming through the lens you're losing quality which is plainly not so - we don't use, nor do we want to use, the edges of the image circle because the sharpness drops off and other distortions show up more.

The topic of depth of field - shallower depth of field on a ff and deeper dof on a crop body - is another subject and an interesting one. Make an image using a ff sensor of a particular scene and observe the depth of field - how much of the scene in front of and behind the point of focus is in acceptable focus. Now crop your image in software to the equivalent size of a crop sensor (in Canon terms, that's reducing the linear dimensions by 1.6) and look at the scene that is left. The depth of field hasn't changed, of course, it's the same image - you just have less of it. Now use the same lens at the same aperture from the same taking position on a crop body and take the same picture. You will get the same image you got after cropping the ff image. And the depth of field will be the same too because you used the same lens from the same taking position. Those are the elements that affect depth of field - taking position, focus distance, focal length of the lens and lens aperture. Sensor size does not of itself affect depth of field. HOWEVER - if you switch from ff to crop and change either the focal length of the lens or the taking position so that you get the same field of view as you had for the ff image UNCROPPED then you will get more depth of field. That's not because you're using a crop sensor, it's because you're using a shorter focal length lens or you're taking the picture from further away to get the same field of view as you got with the ff sensor. Since that's how we generally use our cameras, that's how we perceive it - we get more depth of field when we use a crop camera, because we're using shorter focal length lenses to get the same pictures.

I agree that this is generally a nice explanation, particularly about light intensity. However regarding the highlighted section and on, it is wrong.

DOF relies on two factors only, physical aperture (not numerical value) and subject magnification. Aperture seems easy for most of us to understand, though we must also understand the difference between apparent physical aperture and the numerical aperture; where so many slip up is subject magnification. The print or screen size, and viewing distance, both impact DOF.

There are three situations for comparison and you have to decide which method you use because the outcome is different.

1/ Two people stand next to each other, one has a ff the other a crop camera, both have the 400 f5.6 and are shooting a nesting eagle a long way away. Both have to crop their images to get the framing they want. Obviously the ff image is cropped more but that is irrelevant, the eagle is projected on to both sensors the same size and both images are cropped to the same framing and reproduced the same size (on screen the same size) so the DOF is identical, indeed both images are identical. DOF is the same.

Same reproduction size, same physical aperture = same DOF.


2/ Both photographers decide they want to show a scene setting image and change their lenses to the widest they have, both have a 35mm f2 IS and use that to take a shot, obviously the framing is different. Both show the resulting image full screen on the same sized monitor, the crop image shows each element of the scene larger because it has less fov (framing). The DOF for the crop camera is less than the ff camera. Why is this? Because the reproduction ratio of the crop camera is higher and reproduction ratio is key to DOF. DOF is less with the crop camera.

Different reproduction size, same physical aperture = different DOF.


3/ The crop camera photographer remembers in the bottom of his bag he has an 11-22, they both stand next to each other and use the same settings, with different focal lengths to take 'the same' image. When viewed on a screen the same size the crop camera has more DOF even though it is enlarged more, why is this? Well the actual subject size on screen is the same for both images, but the aperture, although the same number, is physically smaller for the crop camera. The FF camera has a 35mm @ f10, or a 3.5mm aperture opening; the crop camera has a 35mm/1.6= 21mm @ f10, or a 2.1mm aperture opening. The smaller the apparent aperture the more dof. DOF is greater on the crop camera.

Same reproduction size, different physical aperture = different DOF.


So, as can be seen, sensor size in and of itself does not impact dof, how you choose to display that resulting capture does. You cannot disassociate end reproduction size from the dof calculation, you can make any comparison you like but you need to understand the impact of those different choices.


The DOF is not greater on crop cameras in the most common comparison because we use a shorter focal length, it is greater because we use a smaller physical aperture to achieve the same exposure settings.

 

[Joey]

I agree with you about the 24-70 2.8 on a crop sensor. You won't be able to get as shallow depth of field with a crop señor as with a ff. I think that's the point he's tying to make that you geting more like a f4 depth of field compared to a 2.8 on ff. But yes you still get the light of a 2.8 lens on a crop sensor

Depends on how you look at it. I'd argue that with the exception of the f-stop's effect on autofocus, you really don't get the light of an f/2.8 lens on a crop sensor. I mean ostensibly yes, if you have two sensors with the same pixel size and you look at a pixel-sized crop, you would see the same amount of light, but that's not the way people use cameras in practice.

People typically use cameras by framing a shot, and then viewing it at screen size or printing it at a desired print size. So the only truly interesting metric is the amount of light that makes up each square inch of output at a given size. Using that metric, because a crop sensor sees light from only about 39% of the lens, per square inch of output, a crop body gives you an image produced with only about 39% of the light that you'd get shooting the same shot with a full-frame body (ignoring any differences in light caused by moving closer to the subject, which if included, would make the crop body look even worse by comparison).

When it comes to the actual image projected on the sensor, there's no meaningful difference between using a crop body and using a teleconverter on a full-frame camera—just a little bit of IQ loss caused by the quality of the TC's glass, and maybe a tiny bit of light loss from the glass itself. And we say that using a 1.4X teleconverter makes a lens act like it is a stop slower. By that same standard, using a glorified 1.6X teleconverter (a crop body) makes a lens act like it is 1.35 stops slower. The only real exceptions to that rule are when either A. you'd be cropping the image on a full-frame to match the crop body (the reach-limited case) or B. you're talking about how the autofocus behaves. But in the more general case, you really don't get the benefits of an f/2.8 lens.

You may very well be right I don't know. I've always thought a 2.8 lens is a 2.8 lens whether it's on a ff or crop. Other than I know I'll get shallower depth of field with a ff and more depth of field with a crop at the same apertures.

An f/2.8 lens is an f/2.8 lens. f/2.8 is a formula, referring to the focal length of the lens divided by the effective maximum aperture of the lens (usually more-or-less the diameter of the outer element on a prime lens). Thus a telephoto lens needs bigger elements than a wide angle lens for the same f/2.8 (or whatever) aperture.
It's a red herring to introduce arguments about teleconverters into this discussion. A 1.4x converter doesn't 'make a lens act as though it's a stop slower', it really is a stop slower. The 1.4x converter changes the focal length of the lens combination but it doesn't change the size of the elements so the aperture formula changes. A 50mm lens with an effective diameter of 25mm has a maximum of aperture of 50/2 or f/2. Add a 2x converter and the lens combination now has a focal length of 100mm and the same 25mm diameter gives an aperture of 100/4 or f/4. It's maths. Nothing to do with the sensor.

The central paragraph of dgatwood's post is hard to follow but the physics works like this: the aperture of a lens defines how bright the image circle on the sensor is. Whether you use most of the image circle on a full frame sensor or less of the image circle (ff lens on crop body) the brightness of the image is the same - you're just using all or part of it. There will be no impact on image quality unless the individual pixels are of different sizes - and the pixels on a ff body are usually a lot bigger than the pixels on a crop body which has both benefits and disadvantages as discussed elsewhere on this thread and others. Some people including, I think, Tony Northrup, are confusing image brightness with 'total light capture' implying that unless you can use all the light that's coming through the lens you're losing quality which is plainly not so - we don't use, nor do we want to use, the edges of the image circle because the sharpness drops off and other distortions show up more.

The topic of depth of field - shallower depth of field on a ff and deeper dof on a crop body - is another subject and an interesting one. Make an image using a ff sensor of a particular scene and observe the depth of field - how much of the scene in front of and behind the point of focus is in acceptable focus. Now crop your image in software to the equivalent size of a crop sensor (in Canon terms, that's reducing the linear dimensions by 1.6) and look at the scene that is left. The depth of field hasn't changed, of course, it's the same image - you just have less of it. Now use the same lens at the same aperture from the same taking position on a crop body and take the same picture. You will get the same image you got after cropping the ff image. And the depth of field will be the same too because you used the same lens from the same taking position. Those are the elements that affect depth of field - taking position, focus distance, focal length of the lens and lens aperture. Sensor size does not of itself affect depth of field. HOWEVER - if you switch from ff to crop and change either the focal length of the lens or the taking position so that you get the same field of view as you had for the ff image UNCROPPED then you will get more depth of field. That's not because you're using a crop sensor, it's because you're using a shorter focal length lens or you're taking the picture from further away to get the same field of view as you got with the ff sensor. Since that's how we generally use our cameras, that's how we perceive it - we get more depth of field when we use a crop camera, because we're using shorter focal length lenses to get the same pictures.

I agree that this is generally a nice explanation, particularly about light intensity. However regarding the highlighted section and on, it is wrong.

DOF relies on two factors only, physical aperture (not numerical value) and subject magnification. Aperture seems easy for most of us to understand, though we must also understand the difference between apparent physical aperture and the numerical aperture; where so many slip up is subject magnification. The print or screen size, and viewing distance, both impact DOF.

There are three situations for comparison and you have to decide which method you use because the outcome is different.

1/ Two people stand next to each other, one has a ff the other a crop camera, both have the 400 f5.6 and are shooting a nesting eagle a long way away. Both have to crop their images to get the framing they want. Obviously the ff image is cropped more but that is irrelevant, the eagle is projected on to both sensors the same size and both images are cropped to the same framing and reproduced the same size (on screen the same size) so the DOF is identical, indeed both images are identical. DOF is the same.

Same reproduction size, same physical aperture = same DOF.


2/ Both photographers decide they want to show a scene setting image and change their lenses to the widest they have, both have a 35mm f2 IS and use that to take a shot, obviously the framing is different. Both show the resulting image full screen on the same sized monitor, the crop image shows each element of the scene larger because it has less fov (framing). The DOF for the crop camera is less than the ff camera. Why is this? Because the reproduction ratio of the crop camera is higher and reproduction ratio is key to DOF. DOF is less with the crop camera.

Different reproduction size, same physical aperture = different DOF.


3/ The crop camera photographer remembers in the bottom of his bag he has an 11-22, they both stand next to each other and use the same settings, with different focal lengths to take 'the same' image. When viewed on a screen the same size the crop camera has more DOF even though it is enlarged more, why is this? Well the actual subject size on screen is the same for both images, but the aperture, although the same number, is physically smaller for the crop camera. The FF camera has a 35mm @ f10, or a 3.5mm aperture opening; the crop camera has a 35mm/1.6= 21mm @ f10, or a 2.1mm aperture opening. The smaller the apparent aperture the more dof. DOF is greater on the crop camera.

Same reproduction size, different physical aperture = different DOF.


So, as can be seen, sensor size in and of itself does not impact dof, how you choose to display that resulting capture does. You cannot disassociate end reproduction size from the dof calculation, you can make any comparison you like but you need to understand the impact of those different choices.


The DOF is not greater on crop cameras in the most common comparison because we use a shorter focal length, it is greater because we use a smaller physical aperture to achieve the same exposure settings.

Are you using DOF (depth of field) to refer to something other than what I understand it to be - How much of the scene in front and behind the point focussed on is acceptably in focus? The four factors I list and you highlighted in red certainly do affect depth of field (just checked with Wikipedia to make sure I hadn't drifted into a parallel universe...)
However, I may possibly get your point to an extent: Technically, depth of field is calculated using the circle of confusion - the size of the circular out of focus image created by a point of light. The further out of focus, the larger the circle. Only one plane in any image is actually 'in focus' but a range of planes in front of and behind that plane are considered acceptably in focus because the circles of confusion created at these distances are small enough to be seen as points. It is true that what is 'acceptably in focus' will change with enlargement of the final image and the viewing distance but that is a subjective matter. There are figures for 'acceptable' circle of confusion for various formats and the figure used for 35mm cine for instance is larger than that for 16mm. However in still photography we are much more rigorous and 'acceptable' circle of confusion is vanishingly small. I generally consider that if I consider an image (or region of an image) out of focus on a 12 x 16 print, it will also be out of focus at 6 x 8, less so, but still unacceptably out of focus. Similarly the difference between full frame and crop sensors.
Interestingly, and touched on by the Wikipedia article, the difference in the amount of enlargement involved in producing a given image from a crop sensor rather than a full frame and the effect on the acceptable circle of confusion size should mean you get LESS depth of field from a crop camera. Most people's perception though, as I mentioned in my earlier post, is that you get MORE depth of field from a crop camera, which is because you use shorter focal length lenses or work from a greater distance to get the same field of view. I think that's part of the point you're making with your three scenarios. As Wikipedia puts it: "The comparative DOFs of two different format sizes depend on the conditions of the comparison. The DOF for the smaller format can be either more than or less than that for the larger format. "

Your last statement "The DOF is not greater on crop cameras in the most common comparison because we use a shorter focal length, it is greater because we use a smaller physical aperture to achieve the same exposure settings." and the reasoning earlier in your post, seems to make an argument where no argument is necessary. We refer to lens' aperture as, for instance, f/2.8 in order to factor the focal length of the lens into the actual aperture of the lens to give a figure describing how bright the image circle will be. That's why we can say that focal length and aperture number both impact the depth of field.
I have re-drafted this response several times in an attempt to get it just right - apologies if you read an earlier version of this post...

 

[Joey]

Way to go, Joey. Well argued, well said.

Thank you Jane.

 

[stefang]

I read a lot of bashing on the video by Tony Northrup over here, so I was curious and watched it. What I see is that he basically is right in his conclusions:
- The best APS-C lenses may be sharper than an equivalent FF lens on the same body
- When you are forced to crop you photo's to at least APS-C size, you may be better off with an APS-C body
That there may be very good reasons other than sharpness to buy FF lenses and bodies does not invalidate his conclusions....

 

[Ryan85]

Hi,

I have never tested a FF camera intensively. But I do own a couple of EF lenses, including the 70-200 f/2.8 and 100L. What I can say is that they produce just amazing pictures even on my T5i/700D. I do not want to say that it performs as good as a FF body (which I don't expect), but it is definitely crap to say it is not usable at all !

I have compared the kit lens 18-55 (quite acceptable) with the 100L and 70-200L and it is worth the price ! But if it is just about a "Canon bashing", I can understand this point of view...

Regards.
[/quote
+1

 

[Chipcago]

Hi --

New member and not super techie, so please be nice. Also, I am a Tony/Chelsea fan, -- they are nice, goofy and earnest, IMHO, so again, please be nice. That said, I do get very confused by Tony's explanations. Here's why -- I think -- prior to digital phtotgraphy, using a 35 mm camera meant that light got to film causing a photochemical reaction which produced a picture. So, all the math was based on a constant. Now, there are ff digital cameras, which are "true" 35 mm equivalents and crop sensor cameras, which are camera with sensors that are smaller than 35 mm by varying degrees. In addition, the medium is not film, so instead of a photochemical reaction, the cameras are taking in information communicated by light and reacting electronically.

Since both ff and crop sensors are pocessing the same light and then electronically converting that to an image, I don't understand his comments about the same amount of light getting to the sensor. In other words, if person A is using a ff to take a picture of statue 1 with a field of view of X and person B is using a crop to take a picture of statue 1 and adjusts his position to have the same field of view of X and both are using the same lens and the same focal length, isn't everything the same? Both cameras are working to take the same eaxact picture, with the same light coming to the sensor. The only difference I can think of is that if both camera's are 20 MP then the FF pixells are going to be larger. So, I get confused about his "sensor light collecting" statements, because it seems to me that this would only be relevant if the argument was about comparing a 35 mm film camera to a smaller sized film camera. It seems to me that the argument with ff versus crop has to do with how a 20 MP ff processes the electronic information versus a 20 MP crop, not about the light getting to the sensor.

Thanks in advance for the assistance.

Chip

 

[Ryan85]

Hi --

New member and not super techie, so please be nice. Also, I am a Tony/Chelsea fan, -- they are nice, goofy and earnest, IMHO, so again, please be nice. That said, I do get very confused by Tony's explanations. Here's why -- I think -- prior to digital phtotgraphy, using a 35 mm camera meant that light got to film causing a photochemical reaction which produced a picture. So, all the math was based on a constant. Now, there are ff digital cameras, which are "true" 35 mm equivalents and crop sensor cameras, which are camera with sensors that are smaller than 35 mm by varying degrees. In addition, the medium is not film, so instead of a photochemical reaction, the cameras are taking in information communicated by light and reacting electronically.

Since both ff and crop sensors are pocessing the same light and then electronically converting that to an image, I don't understand his comments about the same amount of light getting to the sensor. In other words, if person A is using a ff to take a picture of statue 1 with a field of view of X and person B is using a crop to take a picture of statue 1 and adjusts his position to have the same field of view of X and both are using the same lens and the same focal length, isn't everything the same? Both cameras are working to take the same eaxact picture, with the same light coming to the sensor. The only difference I can think of is that if both camera's are 20 MP then the FF pixells are going to be larger. So, I get confused about his "sensor light collecting" statements, because it seems to me that this would only be relevant if the argument was about comparing a 35 mm film camera to a smaller sized film camera. It seems to me that the argument with ff versus crop has to do with how a 20 MP ff processes the electronic information versus a 20 MP crop, not about the light getting to the sensor.

Thanks in advance for the assistance.

Chip

Don't worry to much if there a little mean to you they don't like Tony Northrup here much. I don't agree with everything he says but he's ok. I think he gives his honest opinion.

I'm not a techie either but I'll explain your question the best I can. I'm sure others can and will explain it better than me or correct me if I'm wrong.

So a FF sensor is the equivalent of a 35 MM film camera. A crop (sensor aps-c) has a smaller sensor. So a 100mm lens on a FF is 100mm lens. On a Canon crop sensor you have to multiply the the lens by 1.6. Since the sensor is smaller. So that same 100mm lens on a crop sensor camera would be a 160mm. A 50mm on FF is a 50mm. On a crop sensor a 50mm is a 80mm lens. And so on.

Now 100mm 2.8 on a FF is a 100mm 2.8. That 100mm 2.8 on a Canon crop sensor is 160mm 2.8. You multiply the focal length by 1.6 on your crop sensor but the aperture/ f number stays the same.

Now with a FF your going to have a shallower depth of field. With a crop sensor you're going to have more depth of field. I don't know why but that's how it is. It's the sensor size and physics. I think when Tony say there's less light on a crop sensor he's actually meaning a 2.8 lens acts like a f4.5 lens because you don't get as shallow depth of field with a crop sensor.

I hope that helps with your question. I wouldn't worry to much just shoot and enjoy it.

 

[tcmatthews]

Hi --

New member and not super techie, so please be nice. Also, I am a Tony/Chelsea fan, -- they are nice, goofy and earnest, IMHO, so again, please be nice. That said, I do get very confused by Tony's explanations. Here's why -- I think -- prior to digital phtotgraphy, using a 35 mm camera meant that light got to film causing a photochemical reaction which produced a picture. So, all the math was based on a constant. Now, there are ff digital cameras, which are "true" 35 mm equivalents and crop sensor cameras, which are camera with sensors that are smaller than 35 mm by varying degrees. In addition, the medium is not film, so instead of a photochemical reaction, the cameras are taking in information communicated by light and reacting electronically.

Since both ff and crop sensors are pocessing the same light and then electronically converting that to an image, I don't understand his comments about the same amount of light getting to the sensor. In other words, if person A is using a ff to take a picture of statue 1 with a field of view of X and person B is using a crop to take a picture of statue 1 and adjusts his position to have the same field of view of X and both are using the same lens and the same focal length, isn't everything the same? Both cameras are working to take the same eaxact picture, with the same light coming to the sensor. The only difference I can think of is that if both camera's are 20 MP then the FF pixells are going to be larger. So, I get confused about his "sensor light collecting" statements, because it seems to me that this would only be relevant if the argument was about comparing a 35 mm film camera to a smaller sized film camera. It seems to me that the argument with ff versus crop has to do with how a 20 MP ff processes the electronic information versus a 20 MP crop, not about the light getting to the sensor.

Thanks in advance for the assistance.

Chip

Tony went a little off the reservation when it came to ISO. A full frame sensor only gathers more light because it has a larger area. Crop cameras are not providing more amplification to get the same brightness because they are smaller. More than likely the 7d and 5d II provide close to the same amplification per pixel. ISO performance(noise) is largely a function of the efficiency and size of the image pixels.

His ISO noise math only works out because all the cameras used have close to the same amount of pixels. It is true when comparing film cameras. But it brakes down completely in digital where pixel pitch takes over.
Perhaps I just made his point that ISO is a bad measurement for digital. But personally I think that constant exposure behavior between camera formats is more important.

For an Ideal lens (constant intensity across the image circle) the intensity of light hitting a full frame, APC or m43 will be the same. This will lead to the same ISO, same shutter speed etc. Light intensity effects ISO. Full frame cameras have a larger Area and typically have larger pixels. Therefore it collects more total light because of its larger area. To give a crop or m43 the same total light you have to raise the light intensity.

It seems to me that the argument with ff versus crop has to do with how a 20 MP ff processes the electronic information versus a 20 MP crop, not about the light getting to the sensor.

Pretty much. Furthermore an argument about light hitting the sensor is a bit of an over simplification. Not really helpful without a discussion of pixel pitch.

 

[BozillaNZ]

When you mount the same FF lens to a APS-C 1.6x crop sensor, you are essentially only capturing 39% of the light (photons) the lens collects, and throws away the rest of the 61%. No matter how you spin, that is a loss.

So yes, using FF lens on APS-C is wasteful, because you could as well use a native APS-C lens with the same focal length and aperture, and it will be a lot smaller and lighter because the glass in it collects light from a narrower angle of view so it doesn't have to be as large.

 

[Don Haines]

Hi --

New member and not super techie, so please be nice. Also, I am a Tony/Chelsea fan, -- they are nice, goofy and earnest, IMHO, so again, please be nice. That said, I do get very confused by Tony's explanations. Here's why -- I think -- prior to digital phtotgraphy, using a 35 mm camera meant that light got to film causing a photochemical reaction which produced a picture. So, all the math was based on a constant. Now, there are ff digital cameras, which are "true" 35 mm equivalents and crop sensor cameras, which are camera with sensors that are smaller than 35 mm by varying degrees. In addition, the medium is not film, so instead of a photochemical reaction, the cameras are taking in information communicated by light and reacting electronically.

Since both ff and crop sensors are pocessing the same light and then electronically converting that to an image, I don't understand his comments about the same amount of light getting to the sensor. In other words, if person A is using a ff to take a picture of statue 1 with a field of view of X and person B is using a crop to take a picture of statue 1 and adjusts his position to have the same field of view of X and both are using the same lens and the same focal length, isn't everything the same? Both cameras are working to take the same eaxact picture, with the same light coming to the sensor. The only difference I can think of is that if both camera's are 20 MP then the FF pixells are going to be larger. So, I get confused about his "sensor light collecting" statements, because it seems to me that this would only be relevant if the argument was about comparing a 35 mm film camera to a smaller sized film camera. It seems to me that the argument with ff versus crop has to do with how a 20 MP ff processes the electronic information versus a 20 MP crop, not about the light getting to the sensor.

Thanks in advance for the assistance.

Chip

Hi,

An awful lot of people get mixed up with "equivalent"

let's start with the crop factor... I pick up a crop camera and put a 100mm lens on it and take a picture of an object 100 feet away. Standing in the same spot, if I want to take an identically framed picture on a FF camera, I need to use a 160mm lens on it. That is your 1.6X crop factor. very easy to understand....

Things get fun when you think about depth of field..... shooting at F2.8 and at 100 feet that 160mm lens has a DOF of about 20 feet, while the 100mm lens has a 55 foot DOF... you would have to stop down the 100mm lens on the crop camera to F1.2 to get the same depth of field as the 160mm lens on the FF camera.

that's what happens if you are stationary, sometime you can zoom with your feet. If you shoot with the FF camera at 100 feet away, you can get the same field of view with the crop camera from 160 feet.... there's that 1.6X crop factor again.

If you shoot with a 100mmF2.8 lens on the crop camera and move to 160 feet with the crop camera and the same 100F2.8 lens , you get the same field of view AT THE FOCUS PLANE, but everything else changes. Your depth of field with the FF camera is still 20 feet, but with the crop camera it is now 160 feet of DOF. To make matters even more confusing, you now have a different perspective on the two shots so they are not and can never be identical. The crop camera is now going to be seeing distant objects larger and near objects smaller in relation to how the FF camera shows them in relation to the object at 100 feet that you are focused on.

As far as light goes, that 100mm F2.8 lens remains as a 100mmF2.8 lens no matter which camera you mount it on. It collects the exact same amount of light. Your exposure times and ISO settings are identical between the two cameras. The difference is the amount of the light that is used. The FF sensor is 2.56 times larger than the crop sensor (1.6 squared). If both cameras are the same number of megapixels, this means that the pixels in the FF camera will get 2 1/2 times the light as the pixels do in the crop camera. If you are in poor light this is a big deal, but in good light it really does not matter.... it's kind of like the difference between shooting at ISO 200 and 500 (good light, not noticeable) or between shooting at ISO 10,000 and 25,600 (poor light, very noticeable difference)

and then we have the corners/sweet spot tradeoff.... Lenses are always sharpest at the center and that's the part crop used most, while FF cameras can have problems with the corners.... but since this is so incredibly lens dependant, there is no answer here....

another factor that virtually nobody considers is economy of scale and limited design and manufacturing resources. Looking at that 100mmF2.8 lens, you could design one specifically for FF and another one specifically for crop, but there would be no improvement in image quality and the price of both would be driven upwards.

In short, there is no such thing as equivalence between a crop lens and a FF lens and there is no reason to not use a FF lens on a crop sensor.

 

[Don Haines]

When you mount the same FF lens to a APS-C 1.6x crop sensor, you are essentially only capturing 39% of the light (photons) the lens collects, and throws away the rest of the 61%. No matter how you spin, that is a loss.

So yes, using FF lens on APS-C is wasteful, because you could as well use a native APS-C lens with the same focal length and aperture, and it will be a lot smaller and lighter because the glass in it collects light from a narrower angle of view so it doesn't have to be as large.

The only elements that would be smaller are the small elements nearest the camera. The largest grouping would be the same. on lenses longer than 50 or 60 mm it makes a hardly recognizable difference.

 

[privatebydesign]

Are you using DOF (depth of field) to refer to something other than what I understand it to be - How much of the scene in front and behind the point focussed on is acceptably in focus?

No, we are referring to the same thing.

The four factors I list and you highlighted in red certainly do affect depth of field (just checked with Wikipedia to make sure I hadn't drifted into a parallel universe...)

They are just factors to define the two core elements, aperture size and magnification, that is it, those are the only two numbers you need to work out DOF after you define a size for your CoC. All those other number you plug in to dof calculators are simply working out the subject magnification, the aperture size and coming out with a number for a pre defined CoC. You cannot work out DOF if you do not know those two numbers, if you know the aperture number but not the focal length you cannot work out DOF, if you know the size of the aperture opening you can.

It is true that what is 'acceptably in focus' will change with enlargement of the final image and the viewing distance but that is a subjective matter.

DOF is entirely subjective. That is why in the definition of DOF the word "acceptably" precedes sharp; acceptable to what?

There are figures for 'acceptable' circle of confusion for various formats

Yes but what does that refer to? As a file on a HDD an image has no DoF, am image does not have an intrinsic DoF value until it is given an output size and viewing distance, these have become normalised but they are the defining factors. The aperture size creates the DoF, the output magnification defines it. The CoC figure is generally an 8"x10" print viewed at 12", which corresponds to any other print viewed at the distance of its diagonal. People with better acuity will always find 'standard' DoF wanting.

Interestingly, and touched on by the Wikipedia article, the difference in the amount of enlargement involved in producing a given image from a crop sensor rather than a full frame and the effect on the acceptable circle of confusion size should mean you get LESS depth of field from a crop camera.

That is exactly what I wrote in my scenario number 2, "The DOF for the crop camera is less than the ff camera." if a comparison is made of that shooting scenario.

As Wikipedia puts it:"The comparative DOFs of two different format sizes depend on the conditions of the comparison. The DOF for the smaller format can be either more than or less than that for the larger format. "

I know, that is why I gave examples of how that can be illustrated, along with the third option, scenario 1, where the DoF can be the same from both formats. As I said, "There are three situations for comparison and you have to decide which method you use because the outcome is different."

So the question can be boiled down to a core element, why does a shorter focal length result in deeper dof? The answer is a shorter focal length results in smaller subject magnification, subject magnification is a core element of DoF, and, for the same exposure value a shorter lens has a smaller physical aperture for the same numerical aperture value, and aperture size is the other core element of DoF.