I was recently rereading parts of "Choosing, Using & Repairing Binoculars" by J. W. Seyfried. In it he went over the whole thing about the 7-8mm maximum exit pupil of the human eye. And also, as one ages, the max exit pupil decreases, until, on the average, it is down to 4mm by your mid-50's.
Assuming this is correct, and assuming all other things being equal, would this mean regarding the differences between say, a 10x42 and a 10x50 binocular, for a person in their mid-50's:
In dim light, or at night*:
1. the 50mm would still provide a slightly higher resolution (not factoring in hand tremor etc.)
2. The 50mm would not provide increased light grasp.
* if i remember correctly during regular sunlit daylight, the max exit pupil is 2-2.5mm, no matter what ya using.
My own experience suggests that that oft-cited guideline is to be taken with a grain of salt. I'm 48 and I have no trouble at all seeing the difference between bins with exit pupils of 4 and 5 in dim conditions.
Larger exit pupils are also less critical of eye placement and thus often quicker to get on the view with.
The variance in those averages is quite large (plus or minus 1mm I've seen quoted).
So that can only be taken as a trend not an absolute (as Fireform points out).
As I've mentioned many times you can measure you own pupil size in many light conditions with a digital camera and a ruler held to your forehead for scale. Turn red eye off. Adapt to the lighting condition then take the photo. Try it in the dark and the bright sun to find your full range (and also in "dim" conditions like overcast).