From: "michael daly" <michael.daly@canrem.com>
Subject: binocular question
Reply-To: "michael daly" <michael.daly@canrem.com>
Organization: Canada Remote Systems
Distribution: sci
Date: 30 Dec 92 22:53:52 EST

DB>I believe we are having a difficulty in communicating.

Yeah, it seems that way. :-)  I think you're considering several things
at once and I tend to look at only one "variable" at a time (it's the
engineer in me).  The various parameters affecting the selection of
binoculars (exit pupil size, magnification, light grasp, "ease of use"
etc.) can be considered separately, IMHO.

DB>What I was trying to point out that if your eyes do indeed have 7mm
DB>dark light pupils, resulting in pupil surface area of 38.48 sq mm of
DB>surface area, and you illuminated them with a 3.125mm image, or 7.67
DB>sq mm, then you are using just 19.9% of your eye's pupil surface
DB>area.  Of course none of the incoming image is being wasted.  Your
DB>eyes would be capable of processing far more light than you are
DB>providing to them.

Not really.  The amount of pupil used is not the issue.  Your retina is
what counts and it gets all the light passed through the pupil,
regardless of how much of the pupil is illuminated.

If, in considering the advantages and disadvantages of various kinds of
binoculars, you vary only the exit pupil size (and therefore
magnification) keeping the objective size the same, as long as the exit
pupil is smaller than the eye's pupil, the amount of pupil illuminated
is irrelevant; all the light presented to the eye will be seen.  I don't
think that it's a good idea to cloud up binocular buying with a side
issue like how much of the pupil is being used; that's why I responded
to your post.

DB>Why then would a 16x80 provide a much brighter image than a 16x50?
DB>And given that both were designed to cover a 3.5 degree field.

When responding to your previous post, I was thinking only in terms of
the exit pupil size being varied and the objective lens being held
constant.  Under these circumstances, the light reaching your eye is
independent of the amount of the eye pupil illuminated and is only
dependent on the objective lens size.  All other things being equal, the
higher magnification yeilds a "smaller chunk of sky" and therefore a
dimmer image.  Sorry for the confusion.

Increasing the objective lens size increases the light available.  In
this case, going from 50 to 80 millimeters results in an increase of
(8/5)**2 = 2.56 times the light and the image brightness.

DB>Or if a 10x50 was compared to a 16x50 Wide Angle with both covering a
DB>5 degree field, which would be perceived as brighter?

They would both have the same amount of light (both 50 mm objectives)
but different apparent fields of view (Five degrees is the true field of
view.  The 10 power would have an apparent field of about 50 degrees and
the 16 power, about 80 degrees.)  If you observe a point object, they'll
appear equally bright.  An extended object will appear dimmer in the 16
power binocs (possible exception - a trained observer used to comparing
the brightness of extended objects under varying magnifications).

DB>And would a 25x80 provide approximately the same image brightness as
DB>a 16x50?

Again, the brighness is primarily due to objective lens size.  For point
sources, the 80 mm pair will be brighter.  For extended objects, since
the 80 mm binocs provide 156% more light with only 56% greater
magnification, the 80 mm will seem brighter.

DB>one has a dilemma.  It's nice to have those 50mm objectives for light
DB>gathering, yet it is difficult to hold binoculars above 7X steady
DB>(for many people).

Ain't reality a drag?  I've resigned myself to using higher
magnification binocs and a tripod.

 * DeLuxe2 1.21 #289 * 
--
Canada Remote Systems  - Toronto, Ontario
World's Largest PCBOARD System - 416-629-7000/629-7044