What should you pay attention to?
The formula for calculating the exit pupil quickly provides clarity here. Exit pupil = lens diameter : magnification.
The aim is to produce the brightest and clearest possible image when using the 8x binoculars in the evening or at night.
Negative example: With 8x25 binoculars, the formula (8 : 25) would result in 3.125 mm. With an exit pupil of 3.1 mm, a particularly bright image cannot be expected.
Positive example: If you take a larger lens, e.g. 8x56 binoculars, the result is a large exit pupil of 7 mm.
Note: In order to achieve a bright image, you need a larger lens. Of course, the image brightness also depends on the quality of the binoculars' optical system.