Originally Posted by DavidLGCrawford
8x42s must be pretty dark. Fine I suppose in bright sun, but I'd be worried using them in low light. They only have a brightness rating of 5.25.
The "standard" watch binoculars of 7x50 have a brightness of (a little over) 7. That would make me feel a lot safer in lower light situations.
For those who don't know, divide the lens diameter by the magnification for your brightness rating. So in the above two it's 42÷8=5.25 and 50÷7=7.14
It does depend on the size of your pupil.
The above equation gives the size of the exit pupil of the binoculars. So a 7x50 binocular has a 7.1mm exit pupil. If your own pupil size is say 6mm under dull illumination there is no extra light entering when going from binoculars with a 6mm exit pupil to 7.1mm.
In practice it pays to have a little leeway to compensate for some misalignment, but few adults have a 7mm pupil so a slightly smaller objective lens (or higher magnification) is fine.
You can measure your own pupil size.
The 7x50 rule
is good guide as to the ideal size for boating
. The 50mm objective is a bit larger than most people can use, but the extra weight and size is not so important for marine
use. However, you should not be put off 7x42mm say unless your pupil size is large.
The 7x magnification is a good compromise for non stabilised marine
use, but it sometimes quoted as golden rule
. We all sail on different sized boats with very different stability. There are also personal differences depending on how steady you can hold the binoculars.
There are a lot of optically much better binoculars than the typical marine offerings and often these are in non 7x50 models. My own binoculars are 8x50 so it pays to widen your search.
Personally I find 10 x too high, 7 is a bit low and about 8 is ideal, but your results will vary.