BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="OPTIC_NUT" data-source="post: 3370744" data-attributes="member: 121951">"Being an afocal system, a binocular-telescope's depth of field (DOF) is mathematically undefined!! In contrast, the eye focuses on the retina, and has a well-defined DOF. When the telescope and eye are used in combination, standard lens equations reveal that the DOF of the combined system involves dividing the eye's DOF by M^2 (M = instrument magnification). This is an optical truth."I like this...sort of.   One hang-up:   the idea that you can cleanly divide a depth of field doesn't work, opticallyor mathematically.   If I can focus 2 feet to infinity at 1x (no bins), dividing infinity, 2 feet,  or the difference, at say, 7x makes no sense.   The image equations are extremely non-linear.1/d(obj)  + 1/d(img) = 1/Fl , etc...   It's no mystery for cameras.   The system with the eye is only afocal when you strain your eye  and don't place the image plane at a virtual infinity.  The image on your retina must always be  sharp, and the eye relaxed (mostly), so you are part of a focal system.  The lore about 'afocal'  is confusing compared to systems that concentrate but never form an image, like solar collectors   or, in reverse, light projectors.   The 'afocal' problem breaks down into 2 'focal' pieces:   ----the closest virtual image distance you can see   ----the farthest  ( a tall thing at near-infinity)          But are extremely 'focal', ending up projected onto the retina.   Or, if you like: your eye at highest and lowest Fl.   I will probably never convert the local lore, but the irnage over which you stay in a certain dot-size   focus can be calculated, and barring that, it can easily be tested and observed.  It starts with   the binoculars forming an virutual image at infinity, then at your close-point.  There is no overpowering    mystery, just a lack of home ray-tracing software.  ;-)</blockquote>

Verification