Cap’n’s right.
A long while ago bought some 8×42 Opticrons, and went for walk at dusk along the canal. My partner had her brother’s cheap bins. We both looks at a wader, then a duck, I was marvelling at the colours, she just saw a silver silhouette! Treated herself to a set after that. It’s at the poorer conditions when you notice how good bins are.
General rules being;-
8×40 = ‘8’ is the magification in this case 8 times.
40 = size of end lens, 40mm.
Divide 40 by 8 = 5, to get a ratio ( 5 ).
Your eyes work at a ratio of 6 ( or is it 7?) and so the better bins are 7×42’s and gather as much light as your eye’s can use.
Tiddly 8×20’s fit into your pack well but suffer image quality if not in really good light.
Birders also don’t generally use anything over x 10 magnification as you then start to pick up noticable hand shake, so the image gets blurred. Also as said above, try different makes, though the magnification might be the same, they will feel noticably different, some will feel good and others not quite so good. Only you can then choose what’s good for you!
cheers
Q