snell@utzoo.UUCP ( ) (12/10/87)
Should have mentioned this in previous posting. (Also, no, I don't
own rubberized binoculars. I think they are foolish and unnecessary
for the reasons I stated).
Lovely Light. The more your binos take in, the better. The trade off
is that good light gathering plus magnification equals weight.
A simple formula will help you easily determine how good a set of binoculars
will be at this essential task. Weight and optical quality are separate
considerations.
Light Gathering Ability = Objective Size / Magnification
Thus Light Gathering of 12 X 30's = 2.5 (I've seen such at K-Mart).
" " " 10 X 30's = 3 (I've seen these at a K-Mart too).
" " " 9 X 40's = 4.44
" " " 7 X 35's = 5
" " " 8 X 40's = 5
" " " 10 X 50's = 5
" " " 7 X 50's = 7.14
" " " 3 X 30's = 10 (These are typical opera glasses)
" " " 7 X 80's = 11.43 (I saw such a pair in a Marine
Museum in Iceland. They were
beautiful... and huge)
The Light Gathering Ability of a `zoom' lense varies depending on
the magnification.
Obviously Leitz 9 X 40's do not come out as winners in the Light Gathering
War, but the optics are so much superior that they more than compensate.
__
Name: Richard Snell
Mail: Dept. Zoology, Univ. Toronto
Toronto, Ontario, Canada M5S 1A1
UUCP: {allegra,decvax,ihnp4,linus,pyramid,yetti,utai}!utzoo!snell
BITNET: utzoo!snell@utoronto.bitnetwillner@cfa.harvard.EDU (Steve Willner) (12/11/87)
in article <9084@utzoo.UUCP>, snell@utzoo.UUCP ( ) says: > Lovely Light. The more your binos take in, the better. The trade off > is that good light gathering plus magnification equals weight. > A simple formula will help you easily determine how good a set of binoculars > will be at this essential task. Weight and optical quality are separate > considerations. All true so far, but > Light Gathering Ability = Objective Size / Magnification Actually it's the square of this number times an efficiency factor to account for the transmission of the optics. However, the number given by the formula is important; it's the "exit pupil diameter". What's that, you ask? Very simple: It's the size of the bundle of light that emerges from the eyepiece. That bundle has to enter the pupil of your eye to do you any good. Try measuring your pupil under different lighting conditions; you probably won't find anything greater than 3.5 millimeters or so. (Among other things, if your pupils are bigger, it will be too dark to read a ruler.) Binoculars giving bigger exit pupils are _a waste_ for (most) terrestrial observations! For astronomy, it's another story. Young adults supposedly can have pupils as large as 7mm, hence the production of 7x50 binoculars for military night use and for amateur astronomers. (In reality, maximum eye pupil diameters of 5 or 6mm are more common, even after long dark adaption, and children and older adults usually cannot even achieve that.) For birding, large lenses on binoculars are just extra weight. You might as well use 7x25 's, or for low-price binoculars 7x35 since the exit pupil is often square at prices below $250 or so. (Diagonal of square exit pupil will be 5mm, fully filling a 3.5mm eye pupil.) For use in full daylight, 7x15 or 10x20 would be big enough! > " " " 3 X 30's = 10 (These are typical opera glasses) Opera glasses are usually 3x10 or 3x12, aren't they? > " " " 7 X 80's = 11.43 (I saw such a pair in a Marine > Museum in Iceland. They were > beautiful... and huge) Probably 11x80, for a 7mm exit pupil. There would be no point in making them with less magnification. > The Light Gathering Ability of a `zoom' lense varies depending on > the magnification. Zoom lenses have greater light losses because there are more optical elements, not to mention greater aberrations. Use them only if you absolutely need the zoom feature. > Obviously Leitz 9 X 40's do not come out as winners in the Light Gathering > War, but the optics are so much superior that they more than compensate. Exit pupil is 4.4mm and is almost certainly round in this price range. The lenses are likely to be highly efficient as well, so these have ample light gathering power for most uses. -- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa2 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA ARPA: willner@cfa.harvard.edu
dwj@mtx5d.ATT.COM (David W. James) (12/12/87)
In article <837@cfa.cfa.harvard.EDU> willner@cfa.harvard.EDU (Steve Willner) writes: >[Objective/maginification=exit pupil diameter] Steve is right, the formula gives the exit pupil diameter. I have seen a formula for light gathering ability that takes into account more than the optical efficiency and objective lens size, but have never seen it written unambiguously, or with the justification behind it. Another advantage of a large exit pupil is when the binoculars cannot easily be steadied (as on a boat.) A large exit pupil diameter makes it easier to keep the image on your pupil. This becomes more difficult in bright light as your pupil shrinks. david james