How (and why) does sensor size change DOF?

Pi

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privatebydesign said:
Pi said:
Matthew19 said:
The simplest answer is from Gale Tattersall, DP of the TV show HOUSE : a larger sensor requires a longer lens to achieve the same field of view. The longer the lens the less DOF. (aperture staying the same of course)

Which aperture: the f-stop or the physical one?

If it is the former, start with the 200/2 on crop. How do you get more DOF with 320mm on FF?

Mr Tattersall was over simplifying.

How do you get more DOF with a FF than a 200/f2 on a crop camera. Easy, use a lens with a smaller physical aperture than 100mm, a 100-400 f4-5.6 would do the job.

I meant: how do you get less DOF, which was consistent with the post I replied to. And, to be more precise, how do you get 1 1/3 stop less, which is what the difference in the sensor sizes suggests.
 

privatebydesign

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Pi said:
privatebydesign said:
Pi said:
Matthew19 said:
The simplest answer is from Gale Tattersall, DP of the TV show HOUSE : a larger sensor requires a longer lens to achieve the same field of view. The longer the lens the less DOF. (aperture staying the same of course)

Which aperture: the f-stop or the physical one?

If it is the former, start with the 200/2 on crop. How do you get more DOF with 320mm on FF?

Mr Tattersall was over simplifying.

How do you get more DOF with a FF than a 200/f2 on a crop camera. Easy, use a lens with a smaller physical aperture than 100mm, a 100-400 f4-5.6 would do the job.

I meant: how do you get less DOF, which was consistent with the post I replied to. And, to be more precise, how do you get 1 1/3 stop less, which is what the difference in the sensor sizes suggests.

Shoot with a 300 f2.8 on your ff @ f2.8. That is how.
 

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Pi

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privatebydesign said:
Shoot with a 300 f2.8 on your ff @ f2.8. That is how.

And I get 1 1/3 stop less DOF? Really? You calculator shows the same DOF (in fact it is a bit less with FF but far from 1 1/3 stop).

What if I shoot with the 200/1.8 on crop?
 

privatebydesign

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Pi said:
privatebydesign said:
Shoot with a 300 f2.8 on your ff @ f2.8. That is how.

And I get 1 1/3 stop less DOF? Really? You calculator shows the same DOF (in fact it is a bit less with FF but far from 1 1/3 stop).

What if I shoot with the 200/1.8 on crop?

I don't understand what you are getting at. You will always be able to do things with one format you can't do with another, that is why one is not "better" than the other for everybody. The simplest workaround for your contrived example is to buy a 1.4 TC for the ff camera, that gets you within 1/3 stop of equivalence values.

However, let me flip your scenario, tell me how to take a one shot crop camera equivalent of a ff that is using a 17 TS-E? Or a 15mm fisheye f2.8? Or a 200 f2/1.8? Or a 50mm f1.2? Or an 85mm f1.2? Tell me how you could do 1:1 images of anything over 22mm with a crop camera, the list goes on and on. For some people most of these will never be important, for others just one makes a particular sensor size over another worthwhile

I recently advised a pro photographer on an equipment purchase. His ideal camera for his intended output, low resolution web based event work, was a 4/3 sensor. That gave him deeper dof and more than adequate output quality.
 

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privatebydesign said:
Obviously a 300 f2.8 on a ff camera outperforms a 200 f2 on a crop camera with regards thin dof, it also makes smoother and much more blurred backgrounds. People who espouse the 200 f2 as having a "unique look", are missing the point, a 300 f2.8 demonstrates all the "unique" qualities of the 200 f2, isolation, big background blur, narrow dof, only it does them all "better".


I presume the same thing applies to the 135 f2 and 200 f2.8. Personally I've always found the latter to be the better all round lens
 

Wild

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privatebydesign said:
Also here, http://www.josephjamesphotography.com/equivalence/ for a very detailed insight into sensor sizes and their interaction with focal length, dof, aperture and iso. Yes, even iso has a crop factor!
Privatebydesign, thanks for the great link. Lots of fun explanations in there. However, I'm still not sure how print size and viewing distance affect DOF. Is there another explanation somewhere, or maybe some examples?

Btw, I've tried just resizing some of my photos on my monitor and seeing if they seem to have more/less DOF and can't really tell a difference...maybe I'm doing it wrong.
 

Pi

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privatebydesign said:
Pi said:
privatebydesign said:
Shoot with a 300 f2.8 on your ff @ f2.8. That is how.

And I get 1 1/3 stop less DOF? Really? You calculator shows the same DOF (in fact it is a bit less with FF but far from 1 1/3 stop).

What if I shoot with the 200/1.8 on crop?

I don't understand what you are getting at.

Going back to my post yesterday - in reply to Mathew, I think. You cannot just say - keep the aperture (the f-stop, to be more precise) the same with FF, and you get less DOF. Sometimes you just cannot keep the f-stop the same because there is no, say, 320/2 lens or similar. Most of the time, larger sensor does allow for less DOF with the same AOV but this depends on what lenses you have available, and what lenses you could possibly have available.

When you go to telephoto lenses, the limiting factor is often the physical aperture, not the f-stop. You just cannot increase the FL and keep the f-stop the same in many cases. I did not say anything about other scenarios.

I recently advised a pro photographer on an equipment purchase. His ideal camera for his intended output, low resolution web based event work, was a 4/3 sensor. That gave him deeper dof and more than adequate output quality.

That might have been a good advice with one exception - the m43 does not provide deeper DOF! You can get as much DOF as you want with any sensor size if you stop well enough until diffraction starts making everything so blurry that talking about DOF becomes meaningless.
 

privatebydesign

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Wild said:
privatebydesign said:
Also here, http://www.josephjamesphotography.com/equivalence/ for a very detailed insight into sensor sizes and their interaction with focal length, dof, aperture and iso. Yes, even iso has a crop factor!
Privatebydesign, thanks for the great link. Lots of fun explanations in there. However, I'm still not sure how print size and viewing distance affect DOF. Is there another explanation somewhere, or maybe some examples?

Btw, I've tried just resizing some of my photos on my monitor and seeing if they seem to have more/less DOF and can't really tell a difference...maybe I'm doing it wrong.

Here is an example. I took this image for an artists show, it was printed to 46"x31". As a 700px web image most would agree the zip picture left, by her right cheek, is within acceptable focus, at f7.1 with a 100mm lens it is well within a dof calculators range. The second image is what that zip looks like when I printed it at 46" and viewed from the same distance. Clearly it is not now in acceptable focus. The only thing that has changed is the subject magnification. We have increased the CoC to such an extent that it no longer holds true, we can clearly differentiate between a point and a circle. To bring it back into acceptable focus we all we need to do is increase our viewing distance, step back from your monitor, across a room, and the zip will become sharp again.

Cool isn't it? :)
 

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privatebydesign

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Pi,

I still don't understand what you are getting at, other than making a point that some lenses don't exist, but that is so obvious I didn't think it needed saying.

For equivalency purposes there are many more situations where a crop camera can't replicate a single shot from a ff camera than the other way around. A ff and 300 f2.8 might not give appreciably less dof than a 200 f2 on a crop camera, but what if you want less dof with the crop camera too? Of course you could always use a Nikon 300 f2 and an adapter to get less dof with the ff camera. However, my point was you can take essentially the same image with both formats with available lenses in your scenario, you can't in any of the multitude I laid out.

As for the 4/3 providing more dof, it depends how you look at it. Take a scenario where output is modest print and web sized, so iso and noise is not a concern, you have a set EV/exposure, of say 1/60 sec, f5.6, iso 400. Now if I shoot a ff capture at 10 feet with those settings and a 50mm lens I'd have a total of 4.25 ft dof, not bad but not very practical for event shooting and group shots. However, on my 4/3 camera I'd get the same fov with a 25mm lens, if I used the same settings for my exposure, 1/60 sec, f5.6, iso 400 I'd get a dof that included everything from 7 feet to 17 feet, a far better range for group shots.

Now you could argue that I could replicate my 4/3 shot with the ff camera at f11 and iso 1600, and you'd be correct, but the 4/3 is going to cost much less, be much smaller and lighter etc etc. That is why it was sound advice for him to get a 4/3 camera.
 

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ecka

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privatebydesign said:
Wild said:
privatebydesign said:
Also here, http://www.josephjamesphotography.com/equivalence/ for a very detailed insight into sensor sizes and their interaction with focal length, dof, aperture and iso. Yes, even iso has a crop factor!
Privatebydesign, thanks for the great link. Lots of fun explanations in there. However, I'm still not sure how print size and viewing distance affect DOF. Is there another explanation somewhere, or maybe some examples?

Btw, I've tried just resizing some of my photos on my monitor and seeing if they seem to have more/less DOF and can't really tell a difference...maybe I'm doing it wrong.

Here is an example. I took this image for an artists show, it was printed to 46"x31". As a 700px web image most would agree the zip picture left, by her right cheek, is within acceptable focus, at f7.1 with a 100mm lens it is well within a dof calculators range. The second image is what that zip looks like when I printed it at 46" and viewed from the same distance. Clearly it is not now in acceptable focus. The only thing that has changed is the subject magnification. We have increased the CoC to such an extent that it no longer holds true, we can clearly differentiate between a point and a circle. To bring it back into acceptable focus we all we need to do is increase our viewing distance, step back from your monitor, across a room, and the zip will become sharp again.

Cool isn't it? :)

This is only an illusion of sharpness. The truth is what really matters (the information). Looking at the print from far away only proves that human vision is very limited. At close-up you can see all the information captured by your camera, both sharp and blurry parts. So, sharpness = information. Then from the distance you see much much less information despite that it looks sharper. This kind of sharpness ≠ information. This trick is about the CoC of your eyes, DoF has nothing to do with it.
 

Pi

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privatebydesign said:
Now you could argue that I could replicate my 4/3 shot with the ff camera at f11 and iso 1600,

Consider that I said it. :)

and you'd be correct, but the 4/3 is going to cost much less, be much smaller and lighter etc etc. That is why it was sound advice for him to get a 4/3 camera.

I did say that it was probably a good advice. But the fact remains: smaller formats do not allow you to get deeper DOF.
 

Wild

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privatebydesign said:
Here is an example. I took this image for an artists show, it was printed to 46"x31". As a 700px web image most would agree the zip picture left, by her right cheek, is within acceptable focus, at f7.1 with a 100mm lens it is well within a dof calculators range. The second image is what that zip looks like when I printed it at 46" and viewed from the same distance. Clearly it is not now in acceptable focus. The only thing that has changed is the subject magnification. We have increased the CoC to such an extent that it no longer holds true, we can clearly differentiate between a point and a circle. To bring it back into acceptable focus we all we need to do is increase our viewing distance, step back from your monitor, across a room, and the zip will become sharp again.

Cool isn't it? :)

Thanks for the example! Now that I've seen the example, I definitely understand where you're coming from.

Practically speaking, I know you're right about the depth of field being dependent upon print size, but from a mathematical perspective, isn't depth of field still the same?

In other words, if we were to define a specific "sharpness" level as a minimum threshold for being considered in focus, wouldn't the two images, no matter what size, have the same areas of the image being "in focus?" Without the limitations of our vision, technically the images have the same depth of field, even if we can't discern the difference, right? Please correct me if I'm way off here.
 

neuroanatomist

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ecka said:
This is only an illusion of sharpness. The truth is what really matters (the information). Looking at the print from far away only proves that human vision is very limited. At close-up you can see all the information captured by your camera, both sharp and blurry parts. So, sharpness = information. Then from the distance you see much much less information despite that it looks sharper. This kind of sharpness ≠ information. This trick is about the CoC of your eyes, DoF has nothing to do with it.

Sorry, but DoF has everything to do with it. I suggest you acquaint yourself with the definition of depth of field, and note that part of that definition includes output size and viewing distance. There is no such thing as an 'objective' DoF. Assumptions can be made, and must be made (although they're not always stated) for DoF calculators, etc. - those include an arbitrarily chosen output size and viewing distance, without which a determination of DoF is meaningless.
 

privatebydesign

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ecka said:
privatebydesign said:
Wild said:
privatebydesign said:
Also here, http://www.josephjamesphotography.com/equivalence/ for a very detailed insight into sensor sizes and their interaction with focal length, dof, aperture and iso. Yes, even iso has a crop factor!
Privatebydesign, thanks for the great link. Lots of fun explanations in there. However, I'm still not sure how print size and viewing distance affect DOF. Is there another explanation somewhere, or maybe some examples?

Btw, I've tried just resizing some of my photos on my monitor and seeing if they seem to have more/less DOF and can't really tell a difference...maybe I'm doing it wrong.

Here is an example. I took this image for an artists show, it was printed to 46"x31". As a 700px web image most would agree the zip picture left, by her right cheek, is within acceptable focus, at f7.1 with a 100mm lens it is well within a dof calculators range. The second image is what that zip looks like when I printed it at 46" and viewed from the same distance. Clearly it is not now in acceptable focus. The only thing that has changed is the subject magnification. We have increased the CoC to such an extent that it no longer holds true, we can clearly differentiate between a point and a circle. To bring it back into acceptable focus we all we need to do is increase our viewing distance, step back from your monitor, across a room, and the zip will become sharp again.

Cool isn't it? :)

This is only an illusion of sharpness. The truth is what really matters (the information). Looking at the print from far away only proves that human vision is very limited. At close-up you can see all the information captured by your camera, both sharp and blurry parts. So, sharpness = information. Then from the distance you see much much less information despite that it looks sharper. This kind of sharpness ≠ information. This trick is about the CoC of your eyes, DoF has nothing to do with it.

DoF is only an illusion of sharpness.
 

privatebydesign

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Pi said:
privatebydesign said:
Now you could argue that I could replicate my 4/3 shot with the ff camera at f11 and iso 1600,

Consider that I said it. :)

and you'd be correct, but the 4/3 is going to cost much less, be much smaller and lighter etc etc. That is why it was sound advice for him to get a 4/3 camera.

I did say that it was probably a good advice. But the fact remains: smaller formats do not allow you to get deeper DOF.

As I wrote in my first post in this thread, the second post on page one, depending on how you make your comparison a crop sensor can be shown to demonstrate more, the same, or even less dof than a ff sensor.

The truth is, two photographers standing next to each other, one crop sensor, the other ff sensor, with exactly the same camera settings and the same framed image (meaning the crop owner has a wider lens), if they both take an image then the crop sensor image has more dof. You can argue the semantics and explain it is the lens that is making the difference, and that is fine (we have done a lot of that in this thread), but in practical terms to many actual photographers the difference is most readily associated with sensor size.

Or, 'If I take this framed image from a set point with these settings how much dof do I get with this sensor/lens combination?' that is a question many photographers would ask and the smaller sensor/wider lens combo will realise more dof.

Technically a smaller sensor displays narrower dof, after all for a standardised output the CoC must be smaller as it needs more enlargement, however, for same framed images the difference in focal length and apparent aperture far over power the CoC differences.

Or, consider the corollary of your statement "But the fact remains: smaller formats do not allow you to get deeper DOF.", that would be, larger formats do not allow you to shoot narrower dof. Show me a narrow dof image from a P&S, or a phone, you are just arguing semantic points, not embracing the way we actually understand and use the equipment available to us. Sure if somebody made a sub f1 P&S we "could" take a narrow dof image, but they don't. Similarly you can't shoot an image that replicates a 200 f2 on a ff body with a single shot from a crop camera (but you can the other way around).
 

ecka

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ecka said:
... I thought that DoF is the thickness of the sharp focus plane your camera can capture (how is it called then?)...
... the thickness in reality, not in the picture.

neuroanatomist said:
...There is no such thing as an 'objective' DoF...

Do we need a new definition here? Because I'm pretty sure that OP was asking about THAT THING, not the CoC.
 

privatebydesign

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Wild said:
Thanks for the example! Now that I've seen the example, I definitely understand where you're coming from.

Practically speaking, I know you're right about the depth of field being dependent upon print size, but from a mathematical perspective, isn't depth of field still the same?

In other words, if we were to define a specific "sharpness" level as a minimum threshold for being considered in focus, wouldn't the two images, no matter what size, have the same areas of the image being "in focus?" Without the limitations of our vision, technically the images have the same depth of field, even if we can't discern the difference, right? Please correct me if I'm way off here.

Yes, the same area, as a percentage of the total image area. That is where your enlargement and viewing distance come in. Enlarge a point and it gets bigger and more obvious until it is a circle, but it stays the same size in relation to the image as a whole. CoC is just about setting a standard about the distance at which a sharp point becomes a blurred circle to a humans eye, you cannot take the human eye out of the equation because the very definition of DoF contains the words "acceptably sharp" or "beyond the resolution of the human eye". That is a subjective element that is generalised to 0.2mm at 25cm in the final output. Obviously if we stand back our eyesight becomes the limiting factor so we can increase the CoC.

Maintain a reproduction size and viewing distance ratio such that the CoC (point at which you can't see the difference between a point and a circle) and you can go as big, or as small, as you'd like.
 

privatebydesign

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ecka said:
ecka said:
... I thought that DoF is the thickness of the sharp focus plane your camera can capture (how is it called then?)...
... the thickness in reality, not in the picture.

neuroanatomist said:
...There is no such thing as an 'objective' DoF...

Do we need a new definition here? Because I'm pretty sure that OP was asking about THAT THING, not the CoC.

The plane of focus has no depth. Imagine it as a sheet of the thinnest paper, only much thinner. Everything in front of, and behind, that sheet of paper is less sharp than whatever is on the sheet of paper. Because of limitations to our eyesight something very close to the paper might look in focus, but it isn't, at some point as you move towards the paper things become more obviously out of focus, you have now surpassed your DoF/CoC criteria, but, step back and you again can't see the differences because your eyesight can't resolve it.
 

ecka

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privatebydesign said:
ecka said:
ecka said:
... I thought that DoF is the thickness of the sharp focus plane your camera can capture (how is it called then?)...
... the thickness in reality, not in the picture.

neuroanatomist said:
...There is no such thing as an 'objective' DoF...

Do we need a new definition here? Because I'm pretty sure that OP was asking about THAT THING, not the CoC.

The plane of focus has no depth. Imagine it as a sheet of the thinnest paper, only much thinner. Everything in front of, and behind, that sheet of paper is less sharp than whatever is on the sheet of paper. Because of limitations to our eyesight something very close to the paper might look in focus, but it isn't, at some point as you move towards the paper things become more obviously out of focus, you have now surpassed your DoF/CoC criteria, but, step back and you again can't see the differences because your eyesight can't resolve it.

I agree that the focus plane of an optical image projection is thinner than it looks like. That's what the CoC thing is all about. However, the sensor resolution is limited and it has it's smallest possible dot size which is a pixel and which is a constant for a given camera.

privatebydesign said:
Maintain a reproduction size and viewing distance ratio such that the CoC (point at which you can't see the difference between a point and a circle) and you can go as big, or as small, as you'd like.

No, you cannot do that. It could only apply to a camera with an infinite number of pixels. If a pixel is too big, then it becomes a square. If it's too small, then it disappears.
In all my statements I assumed that both FF and APSC sensors had the same pixel pitch. Otherwise, even the same format cameras (same sensor size) with different megapixel numbers (like 12 vs 36) should have different DoF/CoC characteristics.
 

privatebydesign

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ecka said:
privatebydesign said:
ecka said:
ecka said:
... I thought that DoF is the thickness of the sharp focus plane your camera can capture (how is it called then?)...
... the thickness in reality, not in the picture.

neuroanatomist said:
...There is no such thing as an 'objective' DoF...

Do we need a new definition here? Because I'm pretty sure that OP was asking about THAT THING, not the CoC.

The plane of focus has no depth. Imagine it as a sheet of the thinnest paper, only much thinner. Everything in front of, and behind, that sheet of paper is less sharp than whatever is on the sheet of paper. Because of limitations to our eyesight something very close to the paper might look in focus, but it isn't, at some point as you move towards the paper things become more obviously out of focus, you have now surpassed your DoF/CoC criteria, but, step back and you again can't see the differences because your eyesight can't resolve it.

I agree that the focus plane of an optical image projection is thinner than it looks like. That's what the CoC thing is all about. However, the sensor resolution is limited and it has it's smallest possible dot size which is a pixel and which is a constant for a given camera.

privatebydesign said:
Maintain a reproduction size and viewing distance ratio such that the CoC (point at which you can't see the difference between a point and a circle) and you can go as big, or as small, as you'd like.

No, you cannot do that. It could only apply to a camera with an infinite number of pixels. If a pixel is too big, then it becomes a square. If it's too small, then it disappears.
In all my statements I assumed that both FF and APSC sensors had the same pixel pitch. Otherwise, even the same format cameras (same sensor size) with different megapixel numbers (like 12 vs 36) should have different DoF/CoC characteristics.

Forget pixels, they have absolutely nothing to do with this conversation. If you have a sensor with very few pixels (none of us do) then it would be incapable of rendering our plane of focus without jaggies/steps. Similarly if you had a 100mp ff sensor it would just resolve your CoC better, it wouldn't make anything "sharper" or "more in focus" or change the size of the CoC. The DoF and CoC calculations do not care if your equipment is up to the task. Just like they don't need lens resolution capabilities to calculate their figures.

The DoF and CoC figures are just as applicable when you look through your viewfinder, where there are no pixels, as when you look at an output image.

Of course you can do that! That is the whole point. Go look in a TV store at the 1080 HD displays, stand 4 feet away from a 60" screen, now do the same to a 24" screen. The 24" screen looks sharper. Stand back so the 60" screen appears as big as the 24" screen did from 4 feet, bang, the 60" screen looks as sharp as the 24" screen did. The 60" screen has much bigger pixels so you need to view it from further away so it appears as a seamless image not a collection of dots.