Hi All,
Another quick question.
All things being equal (i.e.lense and coating quality) which has the greater effect on low light performance.. A larger objective lense or a lower power magnificaton? Essentially I'm looking at 7x50 or 8x56 and wondering which will be better in the twilight hours. Thanks!
Afterthought: What exactly determines a set of binoculars depth of field. It seems porro's have the lead where this is concerned and as usual it got me to wondering. Off topic I realize but I didn't want to flood the place w/ questions.
Best calculation I've seen (other than square of exit pupil) is square root of multiplication times aperture. So holding either constant and one variable should have equivalent diffs.
7x50 = 18.7 "twilight factor". 8x56 = 21.
However, that 8x is going to be heavier and the difference in 19 and 21 in "stops of light" will be small. I'd go for lighter and more compact in this decision. Also, age factors in. If over 40 or especially 50, you may only be able to take advantage of a 5 or 6 mm exit pupil.
This is one thing that despite all the fun calculations, only you can answer by experience. Hopefully others have more experience. My experience says go light. What you can hold you will use :-)
Matt
Lower power, regardless whatever else, almost never shows more. I think the "twilight factor" is pretty good. I like to watch owls as night falls. My favorite two binos for this are an 8x42 roof that only transmits 82%, and a huge 7x50 porro that transmits 95%. Sure, the 7x50 looks brighter, but the extra 1x brings the roof right neck and neck with the "night glass" until it is actually, bona fide, all the way "NIGHT". Then, the 7x50 is merely real bad, and the 8x42 is quite terrible, or, pretty much, who cares, time to go home.
Ron