Archived posting to the Leica Users Group, 2007/01/19
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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! _______________________________________________ Leica Users Group. See http://leica-users.org/mailman/listinfo/lug for more information