Originally posted by peterh337 A small distance indeed, but equally this implies that an AF camera is going to have a very hard time achieving optimal focus, and is relying on the depth of field to get away with it.......
I think you are correct and that the tolerance on perfect focus likely relates closely to "depth of focus" (the equivalent of "depth of field" at the image plane.) I think I recall the total depth of focus is something like 4 times the Airy disk diameter*.
This implies that the total tolerance on focus would be similar; for a 50mm lens, this is about
tolerance ~ (+/-) 2*2.44*(F-Number)*0.55 micrometers for average light
At F:8 this is:
Tolerance ~ (+/-) 21 micrometers..... still quite small*.
Dave
EDIT *I've also seen references say the total depth of focus is 2 times the Airy disk diameter so the tolerance would be half as large.. Gaussian optics theory seems to give yet different results (depth of focus changes with f-number squared I think.)
RE-EDIT The above is related to how precise the focus can possibly be and not to what is "Good Enough" for practical photographic purposes, in which case "Circle of Confusion", should be substituted for Airy disk diameter. With this change,
focus position tolerance ~ (+/-) C*(F-Number)
For an APS-C sensor, C ~ .020mm, so for F:8,
focus position tolerance ~ (+/-) 0.02*(8) = (+/-) 0.16mm ..... quite sloppy in comparison, but still only .006 inch.
I apologize for dragging this out with informal, sloppy math & no references but it was interesting to me and I thought I'd use this opportunity to record & report the results. Reporting will help me remember & maybe elicit responses which can help us better understand the situation.