Depth of field - APS-C vs Full Frame

[Cubsprout19]

If I use my RF100mm macro on the R6 at f/11, and then swap the body to the R7 at f/11- keeping subject distance the same, what is the change in DOF please? Is it the new DOF equivalent to the R6 DOF*1.6. I've looked online and there seem to be contradictory views on this, adn I'm confused. I always thought that an APS-C camera had a greater DOF at a given focal length at the same distance and aperture than a Full frame camera.

Any help very gratefully received

 

[Archibald]

I would compare the two for the same composition - the same framing at the subject. That is how we would normally shoot. And then FF gets about the same DOF (and the same diffraction) at one stop narrower, compared to APS-C.

I shoot my macro bugs at f/11 on my R7 and go to f/16 using the R5.

Subject distance is a problematic measure because it does not easily convert to magnification or framing at macro distances.

 

[JustUs7]

Download a depth of field calculator. And app called Digital DOF is free and easy to use.

In your circumstances, DOF would be shallower with APS-C.

Now if you moved back for identical framing, APS-C would have greater depth of field. Or if you changed focal length to a 62.5mm lens and kept the same subject distance, APS-C would also have greater depth of field.

But the same lens from the same distance at the same aperture with the only variable being the sensor, then APS-C actually has shallower depth of field. Consider what would happen on full frame if you used a 160mm lens from that spot.

 

[Archibald]

Download a depth of field calculator. And app called Digital DOF is free and easy to use.

In your circumstances, DOF would be shallower with APS-C.

This is correct if the uncropped images are compared. However, the subject would look smaller in the R6 photo, so would need to be cropped afterwards on the computer to give the same image. And when that is done, the DOF is the same.

DOF is a very tricky subject because the answer depends on the settings and assumptions.

 

[David Berry]

I always thought that an APS-C camera had a greater DOF at a given focal length at the same distance and aperture than a full frame camera.

For the example given, I think that the size of the sensor is irrelevant:

• same lens + same aperture + same distance to subject = same depth of field

… David

 

[David Berry]

If I use my RF100mm macro on the R6 at f/11, and then swap the body to the R7 at f/11- keeping subject distance the same, what is the change in DOF?

Question : What is the change in DOF?
Answer : None, as far as I can tell.

The centre two-thirds of a full-frame image has the same dimensions (22.2 x 14.8 mm) as an APS-C image, resulting in identical DOF for that area as the image from a crop sensor. Why would there be a difference?

… David

 

[Archibald]

For the example given, I think that the size of the sensor is irrelevant:

• same lens + same aperture + same distance to subject = same depth of field

… David

LOL, I just said DOF is tricky because of settings and assumptions.

It's true that the image on the sensor is the same for both cases. But with the R6, if the image is not cropped, and the final images are presented at the same size, then the R6 image of the subject is smaller. So the R6 DOF is deeper.

If you crop the FF image to match the R7 image, then indeed DOF is the same.

 

[AntonLargiader]

I don't see the situation where the DOF changes by cropping the image.

 

[Archibald]

I don't see the situation where the DOF changes by cropping the image.

It is in how DOF is defined. DOF is the zone where sharpness is acceptable, usually defined by the circle of confusion (which in turn comes from assumptions about the final image size, viewing distance, and human visual acuity).

Cropping magnifies blurriness. Uncropped, the near and far limits are slightly blurry, but acceptable. Enlarging those areas makes the blurriness greater and no longer acceptable. New boundaries need to be placed where the sharpness is just acceptable. So the DOF shrinks.

Don't believe me? Few do. There are DOF calculators online that can give you the answers. They will show you that Stephen is right about APSC being shallower and that David is wrong about them being the same.

BTW, there is an analogy in noise. It's baked in to the image, one might say, but cropping makes it appear worse. Does cropping or enlarging make the image noisier? How is that possible? But it appears to.

DOF was understood a hundred years ago. We have the equations and can calculate it to many decimal places. But today most still don't understand it.

But those DOF equations and calculators are approximations, actually. They assume that the lenses are thin, but they are not. DOF is affected by the pupil magnification, which is ignored because it is an annoying complication. A simple flat lens has the same pupil size looking at it from the front or the back. But it is different with most complex modern lenses. Try it - take the lens off your camera and look at it from the front and then the back. Pupil magnification affects DOF, because DOF is the result of the exit pupil size.

Plus, viewing conditions vary all over the place. Smart phone pics look so sharp. The same pic can disappoint on a big monitor.

DOF equations and calculators are still useful because they give good approximate results that work in many situations, and can tell us about trends and principles.

 

[Cubsprout19]

I would compare the two for the same composition - the same framing at the subject. That is how we would normally shoot. And then FF gets about the same DOF (and the same diffraction) at one stop narrower, compared to APS-C.

I shoot my macro bugs at f/11 on my R7 and go to f/16 using the R5.

Subject distance is a problematic measure because it does not easily convert to magnification or framing at macro distances.

Thanks for taking the time to reply, much appreciated. As soon as I'd posted the question I thought that I could try to take the same shot of the same subject - probably a ruler, from exactly the same place with R6 then R7 and just eyeball the results. I'm happy that the R7 image will make the subject look larger in the frame, but was curious about the DOF.