A friend recently looked at some of the data developed to evaluate binoculars for US Gov't use. Allot of the data refers to immersibility, shock resistance etc. But of particular interest to me was the "light transmission efficiency" rating as a % of total wideband light projected through an aperture 2.5mm x 2.5mm. The ratings for the best binoculars were the Fujinon FMTR-SX and Nikon Prostar (I didn't have a Nikon model # so I don't know if this is the current ED version or previous model) at 97.4% and 97.5%. Then came many, many other binocular models like the Fujinon M-22 (96.2%). I guess the Sandia, Lawrence Livermore, Los Alamos, JPL mandate was to test just about every binocular they could find. But the highest rated roof prism model was the Swarovski SL 10x50 at 93.3%, with the Zeiss FL 10x56 T* right behind at 93.2%.
Sadly even the Steiner 7x50 Marine and Tasco Waterproof 322BW had better light transmission than the Swarovski and Zeiss (96.4% and 95%). There were many other Swaro and Zeiss models tested at magnifications ranging between 7x and 15x and none did as well as their 2 best rated models except the Zeiss 7x50 Marine BG*T at 96.6%. The Leica Ultravid 8x50BR was the best performing Leica at only 91.5% light transmission. There were dozens of binoculars that had greater light transmission efficiency than this and many of those retail at less than US$200.
Supposedly light transmission efficiency considers reflection, dispersion and diffusion in its measurement. To the extent that chromatic abberation causes light to be bent away from the direct light path it would also be a factor in light transmission efficiency, but the quality of the glass used, the fineness of the grit used in polishing, the clarity of the cement used to make cemented pairs or triplets, the number of lens surfaces and the quality and completeness of the multicoatings are the dominant factors.
I'm just wondering if this difference in light transmission would be noticible as greater brightness at dusk or dawn. There were dozens of porro prism models that rated better than the best roof prism models for light transmission. Is there a way for roof prism models to compensate for this with larger objectives? For example would the Zeiss 10x56 only be as bright as the Docter 10x50 T*? How much less useful are roof prism models on cloudy days when dawn dawns later and evening comes earlier? Obviously during the brightest sunlight of midday the lower light transmission might be unimportant, but why aren't porro prism makers mentioning this more often in their marketing, especially on models with smaller exit pupils? It would seem to be an important consideration. By the way, older model uncoated binoculars had been tested years ago and the best uncoated models had light transmission efficiency of less than 75%.
[QUOTE=ksbird/foxranch]
I'm just wondering if this difference in light transmission would be noticible as greater brightness at dusk or dawn.
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You have to remember that at the end of the optical system of binoculars you put your own eye. While in typical light your eye pupil has around 5mm diameter when it's dark it could be as large as 8mm. To be more precise this number is correct if someone is below his 40's and he has healthy eyes. Older people can't get pupil larger than 5mm so for them there's no point in buying and using night binoculars with exit eye pupil larger then 5mm.
The transmission is varying in scale of one percent. Light gathering power depends on objective size in square. Difference between 40mm and 50mm is really huge: it's like 1.56 to 1. Difference between 42mm and 40mm is 10% so in poor light conditions - when your eye has a chance to get bigger then 5mm - 8x42 get 10% more light then 8x40 if both has the same light transmission factor.
Taking into account that roof prism system has to have worse light transmission (as there is at least one mirror instead of total internal reflection) I'm almost sure that is the reason of tradition of putting bigger objectives in roofs then in porros (32 mm vs 30 mm, 42 mm vs 40 mm and so on).
All in all if you take 8x40 porro with 97% transmission and 8x42 with 90% transmission you get almost the same numbers concerning total light you see at the end of the binocular (97% vs. 110%*90% = 99%). But this numbers works only in bad light conditions and good eyes with eyes pupils larger then 5mm.