December 17, 2017, 03:01:42 PM

Author Topic: Need Source for Equivalent f-stops, pls  (Read 10080 times)

privatebydesign

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Re: Need Source for Equivalent f-stops, pls
« Reply #15 on: August 23, 2017, 04:24:32 PM »
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 he isn't, but then the comparison is not equivalent. Output size is part of the equivalence equation and as such throwing away over half one result isn't equal, is it?
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.

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Re: Need Source for Equivalent f-stops, pls
« Reply #15 on: August 23, 2017, 04:24:32 PM »

Mikehit

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Re: Need Source for Equivalent f-stops, pls
« Reply #16 on: August 23, 2017, 05:40:06 PM »


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.

But that is not what you do.
You would use the appropriate lens on both bodies to give the same framing from where you are standing. And you would use the appropriate aperture to give the DOF you want.

unfocused

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Re: Need Source for Equivalent f-stops, pls
« Reply #17 on: August 23, 2017, 06:11:29 PM »
...if all else is equal and the only thing that differs is the sensor size, the DoF will be shallower with the smaller sensor...

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?

No he isn't...

How is that not what he is saying?  Read his statement.

privatebydesign

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Re: Need Source for Equivalent f-stops, pls
« Reply #18 on: August 23, 2017, 06:27:17 PM »
...if all else is equal and the only thing that differs is the sensor size, the DoF will be shallower with the smaller sensor...

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?

No he isn't...

How is that not what he is saying?  Read his statement.

Good god!

If you crop then all else isn't equal, you have thrown away half your image. Reproduction ratio is a key element of equivalence, you are reducing the ff camera to a crop camera and upping the reproduction ratio so there is no equivalence calculation, they are the same, but that is not the same as all else being equal.
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.

unfocused

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Re: Need Source for Equivalent f-stops, pls
« Reply #19 on: August 23, 2017, 08:25:31 PM »
...if all else is equal and the only thing that differs is the sensor size, the DoF will be shallower with the smaller sensor...

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?

No he isn't...

How is that not what he is saying?  Read his statement.

Good god!

If you crop then all else isn't equal, you have thrown away half your image. Reproduction ratio is a key element of equivalence, you are reducing the ff camera to a crop camera and upping the reproduction ratio so there is no equivalence calculation, they are the same, but that is not the same as all else being equal.

Oh, I get it now.

Quote
...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...

privatebydesign

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Re: Need Source for Equivalent f-stops, pls
« Reply #20 on: August 23, 2017, 08:43:48 PM »
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.
« Last Edit: August 23, 2017, 08:58:44 PM by privatebydesign »
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.

stevelee

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Re: Need Source for Equivalent f-stops, pls
« Reply #21 on: August 23, 2017, 08:55:27 PM »
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.

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Re: Need Source for Equivalent f-stops, pls
« Reply #21 on: August 23, 2017, 08:55:27 PM »

unfocused

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Re: Need Source for Equivalent f-stops, pls
« Reply #22 on: August 23, 2017, 10:11:42 PM »
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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Re: Need Source for Equivalent f-stops, pls
« Reply #23 on: August 23, 2017, 11:58:15 PM »
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.


...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. 
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unfocused

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Re: Need Source for Equivalent f-stops, pls
« Reply #24 on: August 24, 2017, 12:22:26 AM »
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.

No problem. This wouldn't be the Canon Rumors forum if there weren't arguments by people who are actually in agreement with one another.

Talys

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Re: Need Source for Equivalent f-stops, pls
« Reply #25 on: August 24, 2017, 02:42:49 AM »
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:



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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Re: Need Source for Equivalent f-stops, pls
« Reply #26 on: August 24, 2017, 09:40:25 AM »
...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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Re: Need Source for Equivalent f-stops, pls
« Reply #27 on: August 24, 2017, 09:51:25 AM »
...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.
« Last Edit: August 24, 2017, 09:55:49 AM by privatebydesign »
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.

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Re: Need Source for Equivalent f-stops, pls
« Reply #27 on: August 24, 2017, 09:51:25 AM »

hne

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Re: Need Source for Equivalent f-stops, pls
« Reply #28 on: August 24, 2017, 03:33:49 PM »
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.


...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?
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okaro

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Re: Need Source for Equivalent f-stops, pls
« Reply #29 on: September 09, 2017, 11:17:45 AM »
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.


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Re: Need Source for Equivalent f-stops, pls
« Reply #29 on: September 09, 2017, 11:17:45 AM »