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Message: <blockquote data-quote="OPTIC_NUT" data-source="post: 3371635" data-attributes="member: 121951">If I could only boil down the "can't ever calculate dof because """afocal!!""".... rebuttal.Try this:----joe has his vision correction on----he focuses on something 200 yds away (practically infinity)...say, with a 7x35----he (like me) then checks out posters with different size fonts    at 50ft, 75ft, 100ft, and 150ft  Scaling fonts for distances, he sees that the font that's supposed to see sharply at 100ft   is just starting to blur.  At 75ft, he can barely read "the quick brown fox" What can we say?1) In focusing sharply at 100 yds, he has not only formed a sharp image on his retina,2) He has created a sharp virtual image that exists regardles of whose corrected eyes look through.   (assuming he uses the sound practice of doing a 'longside-in' focus to reduce eyestrain)It is easy for Joe to see the de-focusing to a certain font at a certain range.It is simple to calculate at least a 2-lens approximation of when a point in the image   becomes a disk of a certain radial size in the virtual image.The way Joe is using the binoculars, they are quite focal in his eyes and at the virtual image plane.It's only 'afocal' if he keeps diddling the knob or his eye strains in the middle of its adjustment range.Sharp focus is what he paid for, what he focused for, and what he got.  Picking the relaxed-eye case reduces the system to a clearly focal instance.  If it didn't, Joe would demand a refund.It doesn't matter what you call the system. It must necessarily come to a sharp focus as a virtual imagein this case, and that can be calculated as to when a point on it will become a disk of a size you findtoo blurred (as you move in to closer targets without re-focusing).     Joe can clearly see a red herring, but because he uses binoculars, the smell is not too bad.He knows things are clear.</blockquote>

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