Originally posted by beholder3
In my cases the airy disk is always smaller than the maximum tolerable CoC. The formula is meant to calculate a resulting max tolerable CoC by subtracting the airy disk's area from the original CoCs area.

The whole diffraction effect thing also was part of a huge debate on a German forum. There was no real consensus how to calculate it. The current subtraction concept to me seemed to be the best fitting - some other guys calculated it slightly differently. Same with the constant of 2,44 ("2,44 is the diameter of the middle maximum of the bessel function until its first zero point"). After reading through lots of page on this I just took and used it. The guys on the forum at least didnt object to the formula I proposed (and which is now foudn in the spreadsheet).

If you want to delve deep into this stuff, I suggest you take a look at

Depth of field and diffraction at the bottom of the page. You need to use some Bessel functions. At the latest that was when I got lost.

I'm pretty familiar with these topics.

I can't see that subtracting the Airy disk's area from the CoC has useful meaning. CoC is by definition the display area too small to resolve within (reduced in size by the ratio of image:display.) Therefore, referred back to the display, calculated CoC or Airy disk, whichever is greater should be used as the blur. Simply put, if the Airy disk is larger than the CoC, that's what will be shown on the display. Some function like MAX(Airy, CoC) might be useful.

CoC is a parameter derived from the characteristics of the viewer and display. It seems to me that all your calculations could be made without reference to CoC by starting with the diffraction disk diameter -

*ie* independent of the viewing system.

Much of the previous work I've seen and done on out-of-focus point diameter has used the lens' absolute aperture as an important parameter because at infinity the image of the disk on the image plane is (subject magnification * aperture diameter)

I like your approach better. Best might be to derive it all without reference to CoC until the very end.

Dave in Iowa