Thread: Newbie: Suggestions for first binoculars?

Originally Posted by Cordite

Repeated, sustained observation requires comfortable binoculars.

Comfortable binoculars have a bright image, not too much perceptible shake, and are not too heavy.

Higher magnification like 10x requires thicker, heavier glass for their lenses to bend light that much, which steals some light. They have a smaller exit pupil which can punish at dusk, when our own pupils may dilate to 5+mm (young eyes 7-8mm!) but the 10x42s will only feed them with a cone of light of 4.2mm. And they have perceptible shake due to the magnification which makes prolonged (handheld) observation a strain.

Don't go above 8x magnification for a combination of all the above reasons.

This puts me squarely in the cheapskate category of course, but the considerations above are basic physics so can't be gotten around by manufacturing a 10x42 to a higher spec. The glass will still be thicker to gain that magnification, the magnification will still cause image shake, and its exit pupil (EP) will remain limited to 4.2mm.

A little table to show you I can do division on a calculator, though the easy ones I did in my head:

10 x 50 EP 5.0mm
10 x 42 EP 4.2mm
8 x 50 EP 6.25mm
8 x 42 EP 5.25 mm
7 x 50 EP 7.14mm
7 x 40 EP 5.7mm
7 x 35 EP 5.0mm
6 x 30 EP 5.0mm

Disclaimer: the best binoculars are the ones you have on your person when you need them, someone else on this forum said that.

Now to turn your argument on its head.

The light that enters the lens is determined by the objective lens size and the losses are pretty much the same for an 8x or 10x magnification, the difference in lens thickness is very little. So the exit pupil might be smaller, but it will be of higher intensity, so the amount of light getting through is the same. Now if your 8x42's have an exit pupil of 5.25 and your eyes at night have a 5mm pupil then you will get losses going into your eyes of at least 1- (5²/5.25²) = 9.3% and more if your line of sight is out by more than 0.125mm from the centre. With 10x42's with a exit pupil of 4.2mm your eye need to be more than 0.4mm from the centre of the exit pupil any light is lost.

Physics!