Just don't confuse Circle of Confusion with diffraction and the Airy
disk. Both may be considered optical "spots" produced by a lens. But
the Airy disk is a measure of the smallest spot a lens is capable of
producing at a given focal ratio... where the limitation is imposed by
diffraction.
The CoC is more a measure of the human eye's ability to resolve detail
See the illustration at top right of this page.
<http://en.wikipedia.org/wiki/Circle_of_confusion>
When a "point" is slightly out of focus it becomes blurred into a small
circle larger than the smallest size the lens is capable of producing
(which is the Airy disk). If, after magnification to make a print or
other display, this out-of-focus "point" is still smaller than the human
eye is capable of resolving at normal reading distance (10" or 250mm)
the image still appears sharp. It's the size of the CoC at a given
focal ratio and distance from the subject that is the basis for depth of
field calculations... which also make some assumptions about human
visual acuity and degree of final print magnification (usually about 8X)
If one makes prints with magnification beyond about 8X one would think
the a smaller CoC is required to maintain apparent sharpness. That's
true for something that might be inspected at reading distance but we
don't normally stand 250mm from wall size prints or billboards. So, in
the end, the standard 8X assumption for CoC usually works pretty well
even for large prints.
Chuck Norcutt
Fernando Gonzalez Gentile wrote:
> But I don't understand the acronym COC, ..............
>
> This becomes more difficult to me than to follow the diffraction limited
> resolution thread,
--
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