You know the numbers….the 8×42, 10×32, that are printed on the binoculars….so what do they mean? Well, here we are to demystify the numbers.
The first number (e.g. the 8 in 8×42) refers to magnification. So if it says 8×42 then it means that the object (or bird or 
butterfly) will seem 8 times closer to you when you look through the binoculars.
The second number is the diameter of the objective lens (the far end of the barrell, not the end you look through) measured in millimeters. So if you have a metric ruler, you can check this out. Just turn your binoculars around and measure the end that is furthest from where you look through – the number you get will be the same as the second number that your binos are rated as.
So what does all this mean? Well with magnification there is a sweet spot. Too little magifiction and you may as well forget using binoculars, too much magnification and the shake of your hands will be too great to be able to enjoy yourself. Typically between 7x-10x is great for going out into the field for general nature watching and birding.
So that takes care of the first number. The second number (e.g. the 42 in 8×42) is important because the larger the diameter of the objective lens, the more light will be let in – letting you see more in low light conditions, such as dawn and dusk, as well as giving you more light in general to get more clarity and sharpness on your subject. The down side to the larger diameter is that it also means a larger (and often heavier) binocular. You’ll want to try out different makes and models so you can “weigh” the trade-offs. Of course with some of the newer lens coatings you can get very bright images with a smaller diameter objective lens but this may also impact the price.
Lots of tradeoffs here but at least you have one more piece of the puzzle.