Archived posting to the Leica Users Group, 2007/01/19

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Subject: [Leica] binoculars - looking for assistance
From: marcsmall at comcast.net (Marc James Small)
Date: Fri Jan 19 16:45:54 2007
References: <82c9dd70701180437w53164f0as5cdf7876ee4dc0c1@mail.gmail.com> <9b678e0701181822k1af49093j513fc3c8d90d4667@mail.gmail.com> <200701190336.l0J3aXUx089592@server1.waverley.reid.org> <9b678e0701191514q334daf02r9845b671a24a73cd@mail.gmail.com> <200701192327.l0JNRwUR010458@server1.waverley.reid.org> <001201c73c26$4d84a170$6501a8c0@asus930>

At 07:02 PM 1/19/2007, G Hopkinson wrote:
 >Marc, I hadn't thought of that regarding our pupil sizes as we get
 >older. It sounds plausible. However will not the larger diameter
 >exit pupil allow for more latitude in positioning your eyes against
 >the eyepieces?
 >Surely also, the brightness will vary? Subjectively, binoculars with
 >larger objectives are much brighter and more comfortable to use
 >to my older eyes.

Hoppy

Whether the entire cone of light can be used by 
moving the eyes about depends upon the design of 
the eyepieces but, for the most part, 
no.  Peripheral parts of the cone are optically 
less satisfactory than the central parts, as is 
the general case in photographic lenses.

Roy Bishop, a former President of the Royal 
Astronomical Society of Canada and a former 
Editor of their Handbook, has written a brief 
(four-page) article on the use of binoculars 
which appears annually in this Handbook.  He 
basically sets out three methods of evaluating 
binoculars through mathematical formulae:

RELATIVE BRIGHTNESS:  the square of the exit 
pupil.  This is a commonly used factor but can be 
misleading.  The RLE of a pair of 6x30 glasses 
would be the square of five, or 25, for instance, 
while an 8x40 would be the same, while a 6x42 
would yield an RLE of 49.  Trust me, in practical 
use, a 6x42 is not twice as utile as a solid pair of 8x40's.

Second, another common factor is magnification 
times diameter, keeping Imperial to Imperial and 
metric to metric.  A pair of 8x40's, thus would 
have a factor of 320 while a 6x42 would have one 
of 252, arguably a more useful figure.

Zeiss goes one step further by taking the square 
root of the MXD figure as the "Twilight 
Performance Factor".  Thus, those 8x40 would have 
a TPF of around 17.9 while those 6x42's would 
yield one of around 16.4.  (My K&E Log Log Duplex 
Decitrig was hiding under some Ciro-Flex 
literature but I finally located it.)  This is 
perhaps the best mechanism for mathematically 
determining the relative efficiency of high-end 
binoculars under low-light conditions.

In the end, if the eyes can only open to 5mm, 
then all glasses with a 5mm or larger exit pupil 
will have the same brightness to the 
viewer.  Everything beyond that is wasted weight and expense.

Marc


msmall@aya.yale.edu
Cha robh b?s fir gun ghr?s fir!



Replies: Reply from hoppyman at bigpond.net.au (G Hopkinson) ([Leica] binoculars - looking for assistance)
In reply to: Message from faneuil at gmail.com (Eric Korenman) ([Leica] binoculars - looking for assistance)
Message from don.dory at gmail.com (Don Dory) ([Leica] binoculars - looking for assistance)
Message from marcsmall at comcast.net (Marc James Small) ([Leica] binoculars - looking for assistance)
Message from don.dory at gmail.com (Don Dory) ([Leica] binoculars - looking for assistance)
Message from marcsmall at comcast.net (Marc James Small) ([Leica] binoculars - looking for assistance)
Message from hoppyman at bigpond.net.au (G Hopkinson) ([Leica] binoculars - looking for assistance)