# Canon’s new 600mm and 800mm lenses

Started 3 months ago | Discussions thread
Plane of Best 'Focus'

bclaff wrote:

Jack Hogan wrote:
... I see that Alan has provided a list of available alternatives for criteria to be used.

I had a friend who worked for DARPA who used to say "the great thing about standards is you can pick whichever one you want".

Maybe the difference is minimal with well corrected lenses, though it seems that something of the order of one p-v wavelength OPD for rho^4 aberrations is somewhat normal for such at lowish f-numbers. The question is not trivial because it is a determinant of the plane of best 'focus' in the center. Which of course affects DOF, as Alan suggested (not to mention a lot of other stuff).

Where is best 'focus' from a photographer's perspective? W is p-v OPD in microns, N is f-number. The minimal geometric CLC occurs further than suggested by minimum RMS.

The plane of best 'focus' in the center of the field of view in practice is often taken to occur when the p-v Optical Path Difference caused by, say, SA3 is offset by defocus' ( W020 = -W040), based on RMS units*. So we can estimate the location of the plane of best 'focus' in either case.

I went through this thought process here (see around Equation 4):

https://www.strollswithmydog.com/dof-diffraction-image/

The choice W020 = -W040 in the center comes from reasoning in RMS units but it is also evident when looking at aggregate MTF, hence energy concentration, as shown there in Figure 1, and below in Figure 7

https://www.strollswithmydog.com/simple-model-for-sharpness-in-digital-cameras-spherical-aberration/

Jack

* Geometrically the plane would instead be further away, at W020 = -1.5*W040, as can be easily gleaned from the optical bench image above. Wyant is our friend for stuff like this.

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