[Leica] binoculars - looking for assistance

[ricc at mindspring.com (Ric Carter)]

Is the exit pupil related to relief distance and eyeglass wearers?

Ric Carter


On Jan 19, 2007, at 8:30 PM, G Hopkinson wrote:

> Very interesting Marc. I am by no means expert. I was familiar with  
> the method of calculating the exit pupil diameter and the
> twilight factor performance as espoused by Zeiss. Please  
> immediately mail to me any Zeiss binoculars that you may have on  
> hand with
> large objectives as they are of no use to you ;-)
> Seriously, I've now dug out some Zeiss binocular literature here  
> and with the equation calculation, it also suggests that "the
> twilight performance factor only serves as a reference and does not  
> give an indication of the twilight efficiency of a binocular.
> This is determined by high transmission in the correct spectral  
> range, a low stray light component, high contrast and resolution"
> Hey, this starting to sound like why we like and appreciate good  
> German glass around here.
>
> I was pleased to hear that you found your K&E Log Log Duplex
> Decitrig hiding under some Ciro-Flex literature :-) ;-)
>
> Cheers
> Hoppy
>
> -----Original Message-----
> From: lug-bounces+hoppyman=bigpond.net.au@leica-users.org  
> [mailto:lug-bounces+hoppyman=bigpond.net.au@leica-users.org] On  
> Behalf Of
> Marc James Small
> Sent: Saturday, 20 January 2007 10:45
> To: Leica Users Group
> Subject: RE: [Leica] binoculars - looking for assistance
>
> 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!
>
>
>
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