Cameras and Lenses
Large exit pupils revisited
Since I got a Nikon 10x25 HGL, I've spent some time exploring its limits, both during the day and in low light in the evening, by comparing its performance against a Nikon 10x42 SE and a Hensoldt Fero-D17, a heavy 7x50 IF porro. I also used some other binoculars during the testing, including an old 8x50 and an 8x32, but I'll focus in my comparison on the Hensoldt and the two Nikons. My eye pupils still open to about 6 mm, so I should in theory benefit from the larger exit pupil of the 7x50 in low light conditions. All my testing was done in an open landscape with fields and meadows. Things will be somewhat different in dark forests.
What's become clear during my informal testing is that during the day, until about half an hour before sunset, both 10x binoculars yield a lot more detail than the 7x. The Hensoldt with its large exit pupils is easier to use in the field than either the 10x42 or the 10x25 and offers a more relaxed view, but the detail just isn't there because of its low magnification. Not surprisingly, the 10x42 is also easier to use than the 10x25, even in bright sunshine, and it seems to yield a bit more detail as well. The emphasis is on "a bit more detail" - there's a difference, but it's smaller than the difference between either of the Nikons and the 7x50.
From about half an hour before sunset the situation changes, with the 10x25 becoming increasingly more difficult to focus. The details just aren't there anymore, and colours are becoming more difficult to see. The 10x42, however, shows quite a bit more detail than the 7x50 until some time after sunset. About half an hour to an hour after sunset the 7x50 rules unequivocally. It doesn't show all colours anymore, but it does show a lot more detail. Especially against dark backgrounds like the edge of a forest I could still actually *see* details with the 7x50 neither of the Nikons showed. An example: With the 10x42 I *thought* I saw something moving in a meadow, with the 7x50 I could *see* it was a hare. With the 10x25 I didn't see anything, not even the movements of the hare.
So, large exit pupils are somehow nicer to use during the day - but you won't really see more with them. Even a good 10x25 will show you more detail during the day than, say, a 7x42 or an 8x56. The deciding factor is the magnification, not the size of the exit pupils. I found that result rather surprising, I would have thought the small exit pupil would have more influence on the amount of detail I could see.
The lower the light gets, the more important the size of the exit pupils becomes. But even in low light binoculars with 4mm exit pupils still work quite well. Any more is overkill, at least in my neck of the woods. Things may be different in the far North, where you'll do a lot of your winter birding in low light, or if you find it difficult to keep the entrance pupil of your eyes within the exit pupil of your binoculars. At night, however, you really want large exit pupils, that's where the 7x42's, 7x50's or 8x56's really shine and where they're worth the extra weight.
One more conclusion: A good 10x25 will show you *almost* as much detail as an equally good 10x42. Almost, but not quite. However, it's not as easy to use as a 10x40/10x42. But if the weight is really important (or if I want to do some "birding light" ... 
), I can get by with a 10x25. Plus a scope, of course, but then I always carry a scope anyway.
Hermann
Hi Hermann, Thanks for a very interesting post and for taking the time to do this. I have tried these low light tests using the resolution charts, but this was with a Nikon 10x42SE and a Fuji 10x50 FMT-SX. The 10x50 "pulled" away from the 10SE the darker it got until all the element on the chart could not be seen. I walked up to the chart and could read the chart's bigger elements from a few feet away with direct vision.
Nice catch, Hermann.
Other testers, however, have reached other results. Without doubt, a binocular with greater magnification has the ability to show smaller details than one with lesser magnification. And magnification is not linear, but square.
The area by which a object is represented in the image, will be about twice as big with a 10x than with a 7x binocular.
This is also where "twilight factor" comes in. A binocular with the same size of the exit pupil but greater magnification will have a greater twilight number because the details will be magnified with a square factor, not a linear.
So a 10x56 will have a smaller exit pupil than an 8x50 but the twilight factor will give it the edge at night.
I must oppose to your idea that you see "more" with a larger magnification. This is only partially true. You can see the small details better but the inevitably smaller FOV will mean that you lose overview.
And in a similar way to the squared area a detail is shown with in the image, the linear FOV will represent an area that increases not linear, but squared.
The FOV, again, is not only a line or an area, but rather a three-dimensional cone.
The volume of the cone will be much greater with a wide angle (almost always coming with lesser magnification) binocular so it will contain much more image information.
Last but not least, the depth of field, which is greater with a low-mag binocular, will decide how much of the above-mentioned FOV cone that is actually usable without refocusing.
A 10x will have vastly less usable "volume" than a 7x. Ask me, I tried to spot a flying Western Grasshopper Warbler this evening with my 10,5x Monarch. I never saw it, but it would have been easy with my 6,5x Fury.
So, does one really "see more" with greater magnification? Shouldn't think so...
Edit: I don't oppsoe to your findings on the exit pupil size, which was your subject in the original post. Sorry to be a little off-topic...In fact, I consider getting a decent 10x32 for daytime use.