OBJECTIVE LENS > The larger (front lens) the brighter the view. Thus the small 25 mm. lenses are much less bright than 40mm lenses.
LENS COATING > The best are "Fully Multi-Coated" (all lenses coated on all surfaces)GOOD lens coatings are color corrected so birders, for example, can see colors accurately.
WATERPROOFING > The standard of industry is fully nitrogen purged and sealed optics. Thus NO inside condensation.
EYE CUPS > Twist-out eye cups with several tactile detent stops are far better than flip out rubber eye cups.
PRISIM TYPES > Old style PORRO PRISM binocs heve "L-shaped" tubes. Aside from their bulkiness they are the best style, optically simpler, that is,& require less color correction.
New style ROOF PRISM binocs have straight tubes and much more complex prisms to get the image to your eye. They require the BEST lens coating for color correction. They are also nicely compact.
In a nutshell, for equal money the "L-shaped" PORRO PRISM binocs have the best image and color.
Basically, if the binocs are less than $200. you may be disappointed in the end.
ME? I carry an 8 power by 25 mm objecive lens monocular when backpacking. Small & light.
But birding & hunting I carry a $700. 8X42 Pentax DCF-SP and suffer with the weight for the needed clarity. For the money they are amazing. The rest of Pentax optics are only just "OK".
Oh, yes, go to the "Better View Desired" website for great, unbiased optics evaluations.
Eric