Nomercy448

Tube body diameter doesn't affect the light transmission. Transmissivity at the micro scale is a factor of the glass quality and the coatings. The light transmitted to your eye, at the macro level, is dependent upon the exit pupil diameter vs. your actual pupil diameter. Any time you have an exit pupil smaller than 2mm (about 3.5mm for my personal eye), you'll notice the image in the scope darkening. Exit pupil is objective diameter divided by magnification setting - so say you're looking at a 6-24x40mm, at 24x, that's 1.67mm exit pupil, and your eye can't restrict down small enough to fit the light being sent to it.