dub@pur-phy.UUCP (Dwight) (11/22/84)
Speaking of binoculars, could anyone explain to
me what most funny numbers like 3X7 on binoculars mean?
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PAX /\ o ::: Dwight Bartholomew
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UUCP:{ decvax, icalqa, ihnp4, inuxc, sequent, uiucdcs }!pur-ee!pur-phy!dub
{ decwrl, hplabs, icase, psuvax1, siemens, ucbvax }|purdue!pur-phy!dubjans@mako.UUCP (Jan Steinman) (11/27/84)
In article <pur-phy.1526> dub@pur-phy.UUCP (Dwight) writes: > > Speaking of binoculars, could anyone explain to >me what most funny numbers like 3X7 on binoculars mean? > The numbers "pXf" (read "p by f" as in the piece of wood known as a 2X4) stand for the magnification power and the focal width, respectively. Higher values of "p" make things look closer up, higher values of "f" make things brighter. The really important brightness spec, however, is "f/p", which yields the diameter of the exit pupil, which is proportional to the square root of the available light at the eye. For bright lighting, as in open sun or bright overcast in a meadow or open area, a p/f of 3 is adequate, corresponding to the "compact" binoculars, such as 7X21, 8X24, etc. For wooded areas, heavy overcast, or dawn or dusk viewing, a p/f of 5 is desirable: 7X35, 10X50. For night viewing and star gazing, use binocs with a p/f of 7 or more, such as common 7X50's or the monster 10X80's that Celestron makes. It doesn't make sense to get brighter binocs than you need. Brighter ones are >much< heavier as f/p increases, and the eye's iris closes down in brighter conditions, making the extra exit pupil diameter superfluous. Theoretically, you should have someone measure the size of your eye's pupil under the darkest typical viewing conditions, and buy a binoc with that f/p ratio. However, lens and prism quality, coating, and alignment also have quite a bit to do with percieved brightness. (But not with the actual exit pupil size.) Other magic numbers are usually field-of-view, usually expressed as the width of a viewed scene 1000 yards away: 385 is a common one for 7X35. This number also tends to be related to the magic "f/p" number, although wide angle binocs are available for a given f/p ratio. (This number is occasionally expressed in degrees.) Any other numbers are likely to be model numbers, serial numbers, etc. :-) More than you ever wanted to know? I hope this has been helpful. -- :::::: Jan Steinman Box 1000, MS 61-161 (w)503/685-2843 :::::: :::::: tektronix!tekecs!jans Wilsonville, OR 97070 (h)503/657-7703 ::::::