APS-C vs FF Depth of Field - Solved!

So does a full frame camera produce shallower depth of field or not? I think we all know it does, or does it?

I shot the same scene with a 5D-III / 135L and 7D-II / 85. They give the same actual field of view within about 1%.

Both at ISO 200, 1/100, f/2.5. As you can see the full frame camera gave noticeably shallower depth of field.

Then I shot the scene with the 85mm lens on each body and cropped the 5D image in Lightroom to match the 7D image.

Well how about that. Different bodies, same lens, same depth of field.

So it appears that it is really the lens after all that makes the difference.

You are treading old ground, and going In the wrong direction. The advantage a full frame camera has in producing shallow depth of field is caused by the standing closer to achieve the same framing as a crop camera.

This perfectly proves the point that the 1.6x crop factor "gives you more reach" is complete BS. Like saying that 250mm on crop is the same as 400mm on FF. It may frame the same, but it is not the same. Just like in this example 85mm = 85mm = 85mm, it doesn't matter the camera it is on.

gonzogolf wrote in post #17773928
You are treading old ground, and going In the wrong direction. The advantage a full frame camera has in producing shallow depth of field is caused by the standing closer to achieve the same framing as a crop camera.

This. ^

Bear in mind too that longer focal length lenses have more abrupt sharpness falloff from in focus to out of focus. So even when the DOF is the same in a calculator (as it likely would be for your first test) the OOF areas are going to exhibit more blur and a more abrupt transition.

I suppose it has been more than 120 years since depth of field was first figured out. Today it still confuses people. And I'm sure this will continue indefinitely into the future.

The standard rules of thumb for DOF work fine. It can be fun to do experiments to really see the effects. But the results are not news.

If we stand 101' from the subject and we photograph with 500mm lens on APS-C and 800mm lens on FF...we frame an area of 3' x 4.5' with both cameras, and...

1. 500mm f/5.6 gives us DOF zone of 2.16' on APS-C
   
2. 800mm f/5.6 gives us DOF zone of 1.34' on FF
   
3. 800mm f/9 gives us DOF zone of 2.16' on FF
   

He cropped the FF image...

Well the root question I was trying to answer is how does crop factor affect DoF in the focal length limited scenario. Stand in the same place, shoot with the same lens, crop the FF image because the lens is too short. Does FF still give less DoF. The answer is "no".

I am thinking the DoF advantage of FF diminishes as you crop and is gone once you crop the FF image by a factor of 1.6 as I did in my 2nd example.

gonzogolf wrote in post #17773928
You are treading old ground, and going In the wrong direction. The advantage a full frame camera has in producing shallow depth of field is caused by the standing closer to achieve the same framing as a crop camera.

Well, I am not going in the wrong direction but am exploring a corner case; a special case.

And if you stand closer you have changed perspective and all bets are off. Meaningful conclusions about DoF are nullified in that scenario.

The FF advantage is due to being able to use a longer lens thus reducing DoF and increasing background blur while capturing the same perspective.

n1as wrote in post #17773980
Well the root question I was trying to answer is how does crop factor affect DoF in the focal length limited scenario. Stand in the same place, shoot with the same lens, crop the FF image because the lens is too short. Does FF still give less DoF. The answer is "no".

I am thinking the DoF advantage of FF diminishes as you crop and is gone once you crop the FF image by a factor of 1.6 as I did in my 2nd example.

Certainly cropping a FF image is equivalent to using a crop frame. That's why they are called crop cameras.

Sometimes less DOF is an advantage, but I think most of the time MORE DOF is an advantage. Of course it depends on the objectives. But I tend to think in terms of a crop frame DOF advantage.

n1as wrote in post #17773985
Well, I am not going in the wrong direction but am exploring a corner case; a special case.

And if you stand closer you have changed perspective and all bets are off. Meaningful conclusions about DoF are nullified in that scenario.

The FF advantage is due to being able to use a longer lens thus reducing DoF and increasing background blur while capturing the same perspective.

If your objective is to have more background blur, then FF is your friend. FF has shallower inherent DOF, plus there are more wide aperture EF lenses available than EF-S lenses.

There is a useful rule of thumb that is worth noting, but which boggles the minds of many. The rule is that for equivalent framing, the DOF depends only on format (MF or FF or crop, etc) and aperture. Check it out. (Don't forget, though - equivalent framing!!)

The other thing useful to note is that longer lenses, even though they give the same DOF as wider lenses (with equivalent framing), give a more pleasing bokeh because of the perspective change.

As I said, after all these years, DOF still befuddles people.

gonzogolf wrote in post #17773928
You are treading old ground, and going In the wrong direction. The advantage a full frame camera has in producing shallow depth of field is caused by the standing closer to achieve the same framing as a crop camera.

This.

kawi_200 wrote in post #17773945
This perfectly proves the point that the 1.6x crop factor "gives you more reach" is complete BS. Like saying that 250mm on crop is the same as 400mm on FF. It may frame the same, but it is not the same. Just like in this example 85mm = 85mm = 85mm, it doesn't matter the camera it is on.

It still gives you more pixels on target with most cameras. Now that camera's like the 5DS and a7Rii are around it's less of an advantage, but those cameras also don't give you the same type of performance that a pro grade APS-C DSLR like the 7Dii would give you, thus the APS-C camera giving you "more reach" is still true and relevant imo.

n1as wrote in post #17773980
Well the root question I was trying to answer is how does crop factor affect DoF in the focal length limited scenario. Stand in the same place, shoot with the same lens, crop the FF image because the lens is too short. Does FF still give less DoF. The answer is "no".

I am thinking the DoF advantage of FF diminishes as you crop and is gone once you crop the FF image by a factor of 1.6 as I did in my 2nd example.

Once you crop the FF image to use a sensor that is exactly the same size as the 1.6X format you are comparing it to, the camera isn't (functionally) a FF camera any more. Put another way....a FF camera doesn't make a shallower DOF image just because there is a FF sensor in there someplace. You have to use the entire FF sensor and you have to fill that sensor with the same field of view by using a longer focal length lens (as compared to the 1.6X format body) and then you see the difference.

You just compared a 1.6X format camera to a 1.6X format camera and discovered that they make the same image. Don't slap yourself in the forehead too hard with this eureka moment.

Depth of Field under the SPECIFIED VIEWING CONDITIONS of any photographic image is actually only dependent on TWO factors. The first is the Absolute Reproduction Ratio, which is the ratio of the size of the subject (at the focused distance) in real life, to the size of the subject in the final image (usually specified as a print). The second dependency is to the absolute effective diameter of the aperture. The effective aperture diameter is the size the aperture appears to be from the sensor side of the lens. The important thing to remember is that we are using the actual physical diameter, not the f/number. In both cases, reproduction ratio, and aperture they are actually inversely proportional. So as both the reproduction ratio, and the aperture diameter get smaller, the DoF will get larger.

It is also important to remember that the specified viewing conditions are very important, probably more so than either of the two variable proportionalities. So changing the viewing conditions is likely to have a far more profound effect on DoF than changing either the reproduction ratio, or the aperture diameter. Also of note is the fact that all of the things that are usually considered when calculating the DoF, such as Sensor size, focal length, focal distance f/number etc, are just convenient ways of pre-defining the reproduction ratio, and aperture diameter, based on the equipment that is in current use.

Alan