From: bunner@vms.macc.wisc.edu (Dana A. Bunner)
Newsgroups: sci.astro
Subject: Re: binocular question
Date: 28 Dec 92 18:22:34 GMT
Organization: University of Wisconsin Academic Computing Center

In article <3220@devnull.mpd.tandem.com>, joe@woody.mpd.tandem.com (Joe Senner) writes...

>two, a 10x50 minolta and a 16x50 pentax. the 16x50's were significantly
>lighter than the 10x50's surprisingly enough, but did magnify the jitter
>considerably. both appeared to have excellent optics. all reflections
>from lenses were a rich green or blue and there was no obvious deformation

Was this the primary (first) reflection or did you shine a light down into
the lense and check several of the inner surfaces?  Most of the low-end
Pentax and Minolta, ie list of $250 or less, coat only the outside surfaces.  
For example the Pentax PCF and Minolta XL series only coat the outer
surfaces.  As most binoculars have in the neighborhood of 10-16 glass
surfaces and each uncoated surface reflects approximately 5% of the light
reaching that surface, coating just the outer surfaces does not help a 
great deal.  Also these glasses are usually heavily discounted.  My
local shop sells the Minolta 7x50XL which list at $194 for $109.  Pentax
10x50 PCF's list at $238, sell for $99.

Example light loss chart for a pair of binoculars with 10 air to glass 
surfaces (2 on objective, 2 for prisms, 6 for eyepieces):

10 uncoated a-g surfaces:   40.13%
 2 coated a-g:              35.64%  (using single layer Magnesium Fluoride)
 2 multicoated a-g:         33.99%
 4 multicoated a-g:         27.22%  Coating front & back of obj & eyepiece
10 coated a-g:              14.03%
10 multicoated a-g:          2.48%

This is using standard figures of 5% uncoated, 1.5% coated, 0.25% multi-
coated for light loss.  Many multi-coatings are closer to 0.5%, this 
would yield a loss of 4.89% for 10 air to glass surfaces, and 28.05% for
4 air to glass multi-coated & 6 uncoated surfaces.

Thus the standard ploy of applying single-layer coatings to just the 
front of the objective lense and the back of the eyepiece results in 
little reduction of light loss.  You will find few binoculars under
$60 which offer more than this.  This will also often employ cheaper
prisms which do not transmit light as well as the better BaK-4 glass models.

>I guess my only concern is, should we have opted for the heavier 10x50's 
>instead? if so, why? the 16's have a 3 degree field of view (narrow, I
>realize) compared to the 5 degree (?) of the 10's. the 16's are a little
>jittery when locating things, but once aligned on the tripod a good
>stable image is available. 

I'm not familar with these 16x50's however one other drawback, stability
is indeed a concern ... even 10X's exhibit a fair amount of jitter,
is the small exit pupil diameter.  A 16X50 will have an exit image size
of 3.125mm.  Most people in their 20's and 30's have a maximum eye pupil
size of 7mm.  Since surface area is a ratio of squares, the size of the
image will only illuminate about 20% of your pupil, wasting 80% of your
night sight capability.  16x50's are designed as daytime glasses, when your
eye contract to about 3 mm.  

Usually a minimum exit image is 5mm for night use.  You'll need a 5mm 
image to achieve even 50% image coverage on your pupils.  However if you
are over 35-40 years old, that maximum pupil size usually begins dropping
to the 5.5-6.0 mm range.  The standard night glass has a 7mm image.  Thus
the popular sizes are 7x50, 8x56, 10x70, and 11x80.  You'll find the 
images in these sizes to appear much brighter.  Note that Celestron does
not advertise the primary use of their 12x50 Pros as night glasses.  The 
smallest 16X models I've seen advertised for night use are the Fujicon 
16x70's and many consider them too dim for night use.

This is not to say they can't be quite useful at night.  I believe one
sci.astro poster routinely uses 20x60's.  However there is a marked
decrease in image brightness once one begins to drop below about 4.5 mm
exit images.

>I'm hoping that the optics have had the attention
>they need for good 16x magnification, and tend to belive so given the
>large price jump as compared to the 10's (10's were typically in the $100
>through $200 range).

How much did the 16x50's cost?

Dana
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