Need Source for Equivalent f-stops, pls

privatebydesign

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How obtuse can you be?

How can you not see that throwing away over half of something and then comparing that portion to the entirety of a different thing is not keeping all else equal?

'All else being equal' means in the end result, framing, dof, and noise, an identical image from different sized sensors.
 

stevelee

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Jul 6, 2017
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It is handy to think in terms of "equivalent" as long as you realize "in what way." Us old guys, even if like me one has never owned a "FF" DSLR, tend to think of focal lengths in terms of 35mm film cameras, like we used for 40+ years.

And it is also helpful, as a couple of us have emphasized, to remember in what ways they are not equivalent.

I remember fondly the 85mm f/1.8, I think it was, for my FT-QL, and for my first prime for my Rebel I wanted to get an "equivalent" lens. I had to figure out what I wanted to be equivalent, the optical characteristics of an 85mm lens, the right distance for portraits, or what? I finally decided that the latter was what I wanted, so I got the 50mm f/1.4, an 80mm f/2.24 (?) "equivalent."

A full-frame DSLR is not really equivalent to a 35mm film camera, but it is convenient to think of it in those terms in many ways.
 

unfocused

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stevelee said:
It is handy to think in terms of "equivalent" as long as you realize "in what way." Us old guys, even if like me one has never owned a "FF" DSLR, tend to think of focal lengths in terms of 35mm film cameras, like we used for 40+ years.

And it is also helpful, as a couple of us have emphasized, to remember in what ways they are not equivalent.

I remember fondly the 85mm f/1.8, I think it was, for my FT-QL, and for my first prime for my Rebel I wanted to get an "equivalent" lens. I had to figure out what I wanted to be equivalent, the optical characteristics of an 85mm lens, the right distance for portraits, or what? I finally decided that the latter was what I wanted, so I got the 50mm f/1.4, an 80mm f/2.24 (?) "equivalent."

A full-frame DSLR is not really equivalent to a 35mm film camera, but it is convenient to think of it in those terms in many ways.

No argument here. Talking about focal length and "equivalent" is a handy shortcut that makes sense to most people. It's easy to do the math, as you have done with 50mm and 80mm "equivalency."

But, I have seen too many people confused and misled when there are discussions of "equivalent" f-stops. As you correctly stated earlier, exposure is not in the least bit affected by sensor size -- a scene that requires an exposure of f8 at 1/250 of a second, will require f8 @ 1/250 no matter what the size of the sensor. Depth of field changes, but not because of any inherent characteristic of the sensor. It is because the photographer must either change positions or change focal length in order to get comparable framing, and in either case, you are changing the relationship between the subject and the background.
 

neuroanatomist

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unfocused said:
neuroanatomist said:
unfocused said:
The exposure is not only identical, but the truth is, the depth of field is as well if you shoot from the same distance to subject using the same focal length. All you are doing is cropping a portion of the full frame image in-camera instead of when processing the image. I know I won't win this argument, too many people are too invested in their own concepts to accept the reality, but I still have to try.

You won't win the argument...becuase you're wrong. It has nothing to do with people being invested in their own concepts, and everything to do with optical physics.

For clarity, and for the benefit of those who can understand and want to accept reality...if all else is equal and the only thing that differs is the sensor size, the DoF will be shallower with the smaller sensor. The exact same thing would be true if you crop the image in post, instead of using a smaller sensor. Note that 'all else being equal' assumes the other factors affecting DoF (output size, viewing distance, visual acuity) are also held constant. Granted, the magnitude of the difference is relatively small compared to the difference you'd see changing focal length or subject distance to match framing, and note that the difference is in the opposite direction from that latter situation (where smaller sensors are said to have deeper DoF). But there is a difference, and thus your statement that they are identical is wrong.

So please, stop trying...it would be unfortunate if you were to convince people of something that is not real.

Just to be clear. You are stating that if someone places a 5DS and a 7DII on tripods next to each other, places a 200 mm lens on both cameras, focuses on a target 50 feet away, shoots both images at f8 and then crops the 5DS image to exactly match the cropping of the 70D, that there will be a discernible difference in the depth of field?

I'm not afraid to be proven wrong, I'd just like to see the proof or a reliable source.

No, I'm not saying that. In fact, I explicitly stated that cropping the FF image to APS-C size and using an APS-C sensor would be the exact same thing.


unfocused said:
...if all else is equal (except it really isn't equal, because we are either changing the position of the camera or the focal length of the lens) and the only thing that differs is the sensor size (except that it isn't the only thing that differs, because we are changing positions, lenses and or cropping), the DoF will be shallower with the smaller sensor...

No, I'm definitely not saying that. For one thing, the scenario you describe (which mimics the 'common' understanding) would mean the smaller sensor (or cropped image) would yield deeper DoF — not shallower — because to compensate for the smaller sensor, you'd either be further away or using a shorter focal length. But that wasn't my point, at all.

The point is, if you keep all the physical factors the same (distance, focal length, aperture) and the assumptions the same (output size, viewing distance, observer visual acuity), and compare FF to a crop sensor or a cropped image, the DoF won't be the same...it will be shallower with the smaller sensor. I thought it would have been obvious, but in that scenario the framing wouldn't (couldn't!) be the same; I probably should have stated that explicitly, though.

Although it may seem esoteric, it's relevant in macro shooting. Consider using a 100L at 1:1 – should you use crop or FF? At 1:1, the distance is fixed, and so is the focal length. Many say crop, and that's often good if you want more pixels on target (although the 5Ds/R narrow that gap quite a bit). But from an optical standpoint, the FF sensor will give you a wider FoV and deeper DoF, the latter being something that benefits most macro shooting. That's why I generally shoot macro with a FF camera.

Sorry if you thought I was arguing that a crop sensor and a cropped image would be different, I didn't even think that was questionable.
 

Talys

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Oh my God. How can there be so much debate over something that is easily observable? :D

I took a focus calibration target and photographed it with a 50mm STM @ f/1.8 on an 80D and 6D2 from the same tripod. I would have used a 1.4, but my USM ring is stuck for the bazillionth time, and I haven't bothered to fix it.

There may be a distance variances of a millimeter or two because the tripod plate I used allows the camera to be positioned on it. This won't give you scientific proof of anything, but it should be good enough to know whether the DoF difference is enough to care about.

Here are both JPGs untouched. They are photographed at 1/1600 f1.8 ISO 100 with a remote trigger at close range. I manually focused using liveview zoomed 10x, and tried to keep equal amounts above and below the 0 rule on the left.

80D: http://talys.icxi.com/doftest/80D.jpg
6D2: http://talys.icxi.com/doftest/6D2.jpg

To compare DOF, I normalized the sizes; ie reduced the APSC output by 1.6, and put it side by side:

comparison.jpg


I'm sure there are more scientific ways to do this, but I think this does the trick for an easy exercise? Draw conclusions as you will!
 

unfocused

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Talys said:
...I took a focus calibration target and photographed it with a 50mm STM @ f/1.8 on an 80D and 6D2 from the same tripod...
...I'm sure there are more scientific ways to do this, but I think this does the trick for an easy exercise? Draw conclusions as you will!

Thanks Talys. I appreciate that you took the time to set up a real life demonstration.I would like to think that this would put this whole issue to rest once and for all, but somehow I doubt it. At least now there will be a reference image to use when the debate inevitably surfaces again.
 

privatebydesign

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unfocused said:
Talys said:
...I took a focus calibration target and photographed it with a 50mm STM @ f/1.8 on an 80D and 6D2 from the same tripod...
...I'm sure there are more scientific ways to do this, but I think this does the trick for an easy exercise? Draw conclusions as you will!

Thanks Talys. I appreciate that you took the time to set up a real life demonstration.I would like to think that this would put this whole issue to rest once and for all, but somehow I doubt it. At least now there will be a reference image to use when the debate inevitably surfaces again.

How, by illustrating a point nobody disputes?

In the context of equivalence there are three common scenarios, this illustrates the 'focal length limited' scenario. Anybody that understands either equivalence or common sense knows the two are the same.
 

hne

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Jan 8, 2016
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neuroanatomist said:
unfocused said:
neuroanatomist said:
unfocused said:
The exposure is not only identical, but the truth is, the depth of field is as well if you shoot from the same distance to subject using the same focal length. All you are doing is cropping a portion of the full frame image in-camera instead of when processing the image. I know I won't win this argument, too many people are too invested in their own concepts to accept the reality, but I still have to try.

You won't win the argument...becuase you're wrong. It has nothing to do with people being invested in their own concepts, and everything to do with optical physics.

For clarity, and for the benefit of those who can understand and want to accept reality...if all else is equal and the only thing that differs is the sensor size, the DoF will be shallower with the smaller sensor. The exact same thing would be true if you crop the image in post, instead of using a smaller sensor. Note that 'all else being equal' assumes the other factors affecting DoF (output size, viewing distance, visual acuity) are also held constant. Granted, the magnitude of the difference is relatively small compared to the difference you'd see changing focal length or subject distance to match framing, and note that the difference is in the opposite direction from that latter situation (where smaller sensors are said to have deeper DoF). But there is a difference, and thus your statement that they are identical is wrong.

So please, stop trying...it would be unfortunate if you were to convince people of something that is not real.

Just to be clear. You are stating that if someone places a 5DS and a 7DII on tripods next to each other, places a 200 mm lens on both cameras, focuses on a target 50 feet away, shoots both images at f8 and then crops the 5DS image to exactly match the cropping of the 70D, that there will be a discernible difference in the depth of field?

I'm not afraid to be proven wrong, I'd just like to see the proof or a reliable source.

No, I'm not saying that. In fact, I explicitly stated that cropping the FF image to APS-C size and using an APS-C sensor would be the exact same thing.


unfocused said:
...if all else is equal (except it really isn't equal, because we are either changing the position of the camera or the focal length of the lens) and the only thing that differs is the sensor size (except that it isn't the only thing that differs, because we are changing positions, lenses and or cropping), the DoF will be shallower with the smaller sensor...

No, I'm definitely not saying that. For one thing, the scenario you describe (which mimics the 'common' understanding) would mean the smaller sensor (or cropped image) would yield deeper DoF — not shallower — because to compensate for the smaller sensor, you'd either be further away or using a shorter focal length. But that wasn't my point, at all.

The point is, if you keep all the physical factors the same (distance, focal length, aperture) and the assumptions the same (output size, viewing distance, observer visual acuity), and compare FF to a crop sensor or a cropped image, the DoF won't be the same...it will be shallower with the smaller sensor. I thought it would have been obvious, but in that scenario the framing wouldn't (couldn't!) be the same; I probably should have stated that explicitly, though.

Although it may seem esoteric, it's relevant in macro shooting. Consider using a 100L at 1:1 – should you use crop or FF? At 1:1, the distance is fixed, and so is the focal length. Many say crop, and that's often good if you want more pixels on target (although the 5Ds/R narrow that gap quite a bit). But from an optical standpoint, the FF sensor will give you a wider FoV and deeper DoF, the latter being something that benefits most macro shooting. That's why I generally shoot macro with a FF camera.

Sorry if you thought I was arguing that a crop sensor and a cropped image would be different, I didn't even think that was questionable.

Yes, the DoF in the object space gets shallower if you take half the picture and enlarge by a factor two. How does that help you in your macro shooting? Are you limited by the too long focal length of your macro lenses? Do you have an obsession using the maximum magnification available of your lens?

It seems very esoteric, indeed. Could you please describe a setup or even better show an image that would have suffered from having been shot with a crop camera with a slightly shorter focal length, correspondingly lower magnification and larger aperture, with output size, viewing distance, and observer visual acuity fixed?
 

okaro

EOS 90D
Oct 10, 2015
134
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stevelee said:
It is handy to think in terms of "equivalent" as long as you realize "in what way." Us old guys, even if like me one has never owned a "FF" DSLR, tend to think of focal lengths in terms of 35mm film cameras, like we used for 40+ years.

Most people do not own horses and yet people talk about horse powers. When there is an established standard it is goof to use it. The equivalent aperture is less useful than the equivalent focal though but it is good to understand that f/2.0 on one inch sensor does not give same results as f/2.0 on FF. Mostly when you choose so small a format you accept limitations in some things like managing the depth of field.
 

Talys

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hne said:
It seems very esoteric, indeed. Could you please describe a setup or even better show an image that would have suffered from having been shot with a crop camera with a slightly shorter focal length, correspondingly lower magnification and larger aperture, with output size, viewing distance, and observer visual acuity fixed?

Yes, it's very esoteric and not a very useful from a practical perspective, because, most people with a crop camera will not stand in exactly the same spot as they would with a full frame camera, using a lens set at the same focal length.

Instead, with a crop camera vs full frame, they will change FL with zoom, or move nearer or further away as they would with a full frame, or use a different lens, to frame the same shot. The crop factor -- 1.6 -- is just too a huge change in composition. And as you change distance to subject and/or focal lengths, your depth of field is going to change.

The whole thing is mostly academic.
 

privatebydesign

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Talys said:
hne said:
It seems very esoteric, indeed. Could you please describe a setup or even better show an image that would have suffered from having been shot with a crop camera with a slightly shorter focal length, correspondingly lower magnification and larger aperture, with output size, viewing distance, and observer visual acuity fixed?

Yes, it's very esoteric and not a very useful from a practical perspective, because, most people with a crop camera will not stand in exactly the same spot as they would with a full frame camera, using a lens set at the same focal length.

Instead, with a crop camera vs full frame, they will change FL with zoom, or move nearer or further away as they would with a full frame, or use a different lens, to frame the same shot. The crop factor -- 1.6 -- is just too a huge change in composition. And as you change distance to subject and/or focal lengths, your depth of field is going to change.

The whole thing is mostly academic.

It isn't academic.

Say I took a scouting shot with my phone or P&S. I can work out what focal length and aperture I'd need for a bigger camera.

Say I saw a particular image on line of a place I was going and wanted to get a very similar shot. If I understand equivalence I can reverse engineer what I could retake that perspective with from the EXIF data of the image even if it was a phone.

Say I have the 'wrong' camera with me or my main fails and I have to finish a session with a different sensor size, if I understand equivalence I can maintain consistency throughout the shoot.

Say I am working a crop and ff camera at the same time. If my battery dies or the flash stops working I can instantly understand what I need to do to get the 'same' image from my other camera.

It isn't academic for many shooters; it is for many others. But core understanding and knowledge can be applied in so many areas it is very good to have. Conversely all the incorrect 'knowledge' out there leads to so much confusion it is farcical.
 

Talys

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privatebydesign said:
Talys said:
hne said:
It seems very esoteric, indeed. Could you please describe a setup or even better show an image that would have suffered from having been shot with a crop camera with a slightly shorter focal length, correspondingly lower magnification and larger aperture, with output size, viewing distance, and observer visual acuity fixed?

Yes, it's very esoteric and not a very useful from a practical perspective, because, most people with a crop camera will not stand in exactly the same spot as they would with a full frame camera, using a lens set at the same focal length.

Instead, with a crop camera vs full frame, they will change FL with zoom, or move nearer or further away as they would with a full frame, or use a different lens, to frame the same shot. The crop factor -- 1.6 -- is just too a huge change in composition. And as you change distance to subject and/or focal lengths, your depth of field is going to change.

The whole thing is mostly academic.

It isn't academic.

Say I took a scouting shot with my phone or P&S. I can work out what focal length and aperture I'd need for a bigger camera.

Say I saw a particular image on line of a place I was going and wanted to get a very similar shot. If I understand equivalence I can reverse engineer what I could retake that perspective with from the EXIF data of the image even if it was a phone.

Say I have the 'wrong' camera with me or my main fails and I have to finish a session with a different sensor size, if I understand equivalence I can maintain consistency throughout the shoot.

Say I am working a crop and ff camera at the same time. If my battery dies or the flash stops working I can instantly understand what I need to do to get the 'same' image from my other camera.

It isn't academic for many shooters; it is for many others. But core understanding and knowledge can be applied in so many areas it is very good to have. Conversely all the incorrect 'knowledge' out there leads to so much confusion it is farcical.

When I say that it's academic, I'm talking about the specific, and very narrow case that I took a test shot of -- depth of field of a FF and a crop sensor, both with identical lenses, taken from exactly the same focal distance. On the full frame image, the depth of field of any part of the image available on the crop is identical to the depth of field on the crop image.

But the only scenario that this would be useful for is if you had an 80D set up to take shots on a tripod, and it died; then you could swap in a 5DSr and get exactly the same shot, and just take the crop area. You'd get essentially identical depth of field in the images. The reverse is more likely, and even technically true but it's totally useless, because if you swap out a 5DSr with a 80D without moving and using the same lens, you'd lose 40% of your image, and one assumes that's not desirable.

I certainly don't mean that the understanding of focal length, aperture, distance to subject as it relates to depth of field is academic. It most certainly isn't!
 

okaro

EOS 90D
Oct 10, 2015
134
15
Talys said:
[When I say that it's academic, I'm talking about the specific, and very narrow case that I took a test shot of -- depth of field of a FF and a crop sensor, both with identical lenses, taken from exactly the same focal distance. On the full frame image, the depth of field of any part of the image available on the crop is identical to the depth of field on the crop image.

You get less DoF on a smaller sensor if you use same lens. If you later crop the FF image then you will naturally get identical results.

But the only scenario that this would be useful for is if you had an 80D set up to take shots on a tripod, and it died; then you could swap in a 5DSr and get exactly the same shot, and just take the crop area. You'd get essentially identical depth of field in the images. The reverse is more likely, and even technically true but it's totally useless, because if you swap out a 5DSr with a 80D without moving and using the same lens, you'd lose 40% of your image, and one assumes that's not desirable.

I certainly don't mean that the understanding of focal length, aperture, distance to subject as it relates to depth of field is academic. It most certainly isn't!

One naturally would use a different lens in the crop sensor or open the zoom more.