With monoculars and binoculars I STRONGLY suggest to look through them before buying them.
The reason is very simple : we see with different eyes..
That has to do with our vision (IE how you can read an eye-chart with either eye) as well as what we expect to see.
For example I have a Zeiss monocular that optically is good (for the size, 8x20, half of the bino discussed above ) however it is useless to me because I cannot spot a bird or something moving with it .
So in my case a bino works better and having spent 30 years selling them I cannot put up with crappy optics.
There are now several relatively light and decent binos of the Porro prism variety (that is the offset type ,not those roof prism ,the straight tube type) for example the Bushnell 7x26.
Aim for an exit pupil of at least 2.5mm , however the wider (bigger) the circle the better it will be in low light (up to 7mm )
Exit pupil is the cirle of light you see if you look through them holding them at a distance.
You get that by dividing the size of the front element (26mm) by the magnification (7x) so that 7x26 has an EP of 3.7 much better than the typical 8x20 (2.5)
Note as you increase the magnification, you need bigger lenses (10x25= 2.5 mm exit pupil, same as 8x20)