monocular question

36 is the size of the front lens in milimeters. The bigger the better for letting in light and giving you a bright image.

The magnification power to front lens size will also affect the image quality. There is a formula for selecting optics for use in day or night. Off hand I don't remember what it is but the general rule is to have the aperature of the eye end no less than 7 milimeters at full magnification if you intend to use it in dark places. 7 milimeters is the size of a fully dialated pupil.
 
You use the formula that I can't remember to find the eye size. You should find it on the internet. I believe Mead telescopes and binoculars or Celestron has a guide on selecting binoculars that has the formula. It's a simple formula but I haven't used it or seen it in years so it's long forgotten.
 
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