Numbers (8×42,10×42,10×50, etc.)
The first figure 8x, 10x, etc. is the magnification. Thus, the subject is larger or appears closer by the magnification used. The second number is the diameter of the objective lens (the lens furthest from the eye). The size of this lens gives a good indication of the size, weight and light transmission of the instrument. Dividing magnification into the objective lens diameter gives a guide to light transmission by what is known as the exit pupil, the diameter of the beam of light projected into the eye.
8×20 = small compact exit pupil 2.5mm
8×32 = smaller full size exit pupil 4mm
8×42 = medium full size exit pupil 5.25mm
8×50 = large full size exit pupil 6.25mm
This is only mathematics and gives no indication of the quality of image.