The formulas for these are really complicated and contain several times the focal length.
Surprisingly, despite the formulas given there:
in other words :
Because the F# is the same, also the NA (Numerical Aperture) is the same.
This is because .
As , also the angle doesn’t change.
Our definition of sharpness, the allowed circle of confusion, of course shall not change.
So if we have a close look at the location where the light gathers for one pixel, we see ..
The red disk shows the smallest Airy disk Possible, according to Rayleigh – controlled only by Wavelength and . (See Rayleigh Criterion). The diameter is (according to Rayleigh) twice the lens’ resolution.
The green disk is the user’s definition of “is still focussed”. As the angle is constant, so is , which is the opening angle of the double cone at pixel level.
As angle and slimmest part of the double cone are the same, so is the upper and lower point where the Green disk touches. The distance of upper and lower max. point of the green disk is the image side Depth of Focus. The object side DOF (depth of field) is Depth of Focus / Magnification , which is constant, because the FOV is constant.