To stack or not to stack

gardenersassistant wrote:

Pixel Pooper wrote:

With the same field of view, and the same entrance pupil diameter, you get the same DOF (and the same diffraction) regardless of magnification or sensor size.

I am a bit puzzled by this. Perhaps you can correct my thinking here please.

You got me. The statement above is true at normal distances, but since we are talking about macro I should have said exit pupil rather than entrance pupil.

Below 1:10 magnification we use the f/stop which depends on the entrance pupil to calculate DOF, but above 1:10 magnification we have to use the effective f/stop which depends on the exit pupil.

Suppose I photograph a scene at 1:1 with a full frame camera using a 100mm macro lens set to f/8. (To avoid a complication later, let's assume the f-number is set physically on the lens.) The aperture is focal length / f-number, which is 12.5mm.

Suppose that I now add a 2X teleconverter, without changing the f-number setting on the lens, and move away from the subject so as to get the same field of view as before, and hence the same magnification of 1:1. So the field of view is the same as before, and the entrance pupil diameter is the same because I haven't changed the f-number setting. But isn't the depth of field different now, roughly twice what it was before because the effective f-number is two stops larger, and the effective aperture two stops smaller? If that is so, depth of field does not depend solely on field of view and entrance pupil diameter.

In this example the field of view is not the same. The magnification at the focal plane is the same, but the lens without the teleconverter has a wider angle of view so the background has a wider FOV. In this case my previous statement applies:

"For a given sensor size, DOF depends only on magnification and effective f/stop but this doesn't work across different formats."