Don,
Good question here. I am a bit late to the party and I had only read a few of the responses. First off, these two bins equate the same exit pupil (20/8 or 25/10) so there is absolutely no difference in light transmission. Secondly, I travel with both of these items in my show stock and any difference in resolution is directly related to sample variance. Which means the difference is minor (not major as some others indicate) from binocular to binocular.
I am not sure how some of these other birders are coming to some of these conclusions, but I always stay with the 8's. That way you keep your field of view in tact.
I hope this thread in it's entirety has helped you out. Best of luck!
Best,
Mike Freiberg
Nikon Birding Market Specialist
Light transmission might be the same but the twilight factor of the 10x25's are superior making them better performers in dim light.
Twilight Factor: Exit pupil, relative brightness, and "relative light efficiency" comparisons are interesting and often useful, but they're not the best judges of how well binoculars perform in low light. For example, 8 x 32mm and 20 x 80mm binoculars both have 4mm exit pupils, a relative brightness of 16, and a "relative light efficiency" of 24 - but the 80mm binoculars are much better in low light due to their 625% larger light-gathering capacity and higher "twilight factor."
The twilight factor is a more useful judge of a binocular's low light performance than its exit pupil, etc., as it takes into account both light gathering and magnification. Both factors affect how much detail you can see - and seeing detail is what binoculars are all about.
Simply put, the larger an image, the easier it is for you to see details in that image. By the same token, with a smaller image, the brighter it gets, the easier it is for you to see the same details clearly. So, within reason, if magnification goes up, brightness can go down without affecting resolution, and vice versa.
It's like reading a newspaper in the light of a 3-way lamp. If the lamp is at its low 50 watt setting, you have to hold the paper closer (making the image larger) to read the fine print. If the lamp is turned up to 100 or 150 watts, you can hold the paper further away (making the image smaller) and still read the same fine print. In other words, small bright images can show you as much detail as large dim images.
The twilight factor allows you to compare different combinations of aperture and magnification to determine the one that best balances an increase in power against a decrease in brightness (or vice versa). The larger the twilight factor, the better a binocular is in low light.
Twilight factors of 17 and higher are best for twilight or early morning use.
A binocular's twilight factor is found by multiplying its objective lens diameter by its magnification, then finding the square root of that product. An 8 x 32mm binocular, for example, has a twilight factor of 16, while a 20 x 80 has a twilight factor of 40, explaining the better low light performance of the latter despite the identical exit pupils and relative brightness.
As with relative brightness, the twilight factor is a mathematical relationship. It does not take into account light transmission differences between binoculars due to differences in optical coatings, so small numerical differences in twilight factors may not be visible in real life.
And just because an inexpensive binocular and a premium model have identical twilight factors, you cannot assume that their optical performance will be the same. Light transmission differences, distortion, and optical flaws in the less expensive binocular can severely compromise its sharpness and clarity.
So, while it's a useful figure, don't let the twilight factor be your only guide to choosing a binocular. Other factors can be just as important as the ability to make out details in dim light. For example, at first glance, a 10 x 40 might seem better than a 7 x 42 in low light because of its higher twilight factor (20 versus 17.1). But if most of your observing is done in dim wooded areas, the wider field of view and greater depth of field of a 7 x 42 are better for locating birds than the narrower and shallower field of a 10 x 40 - and finding the bird is half way to identifying it.
But, keeping these cautions in mind, the twilight factor still remains a more reliable guide to low light performance than exit pupil or relative brightness.
