Q: I am trying to choose between two variable-power scopes. One has a one-inch tube and 42mm objective , the other a 30mm tube and 50mm objective. Which scope would you choose, and why? Also, is there really all that much difference in light transmission and brightness between a scope with single-coated lenses and one with multi-coated lenses?
A: My personal preference is for a scope with one-inch tube and 42mm objective. There are two reasons for my choice. The threory that a 30mm tube passes more light than a one-inch tube is just pure hype. The only way to brighten your image is by increasing the size of the exit pupil, and the size of the exit pupil calls for a larger objective lens, not a fatter tube. While 50mm objectives do increase exit pupil diameters slightly over a 42mm objective, the quality of the lenses and their coatings have more effect on how much light enters the eye. Good quality single-coated lenses typically allow about 90- percent of available light to pass through, and multi-coated lenses allow up to 98 percent of light to pass through. This is why inexpensive scopes often transmit less than 80 -percent of light, so an El Cheapo with 30mm tube and 50mm objective is often dimmer than a smaller high-quality scope. Assuming the two scopes you are looking at are of equivalent quality, I’d still choose the smaller one because it can be mounted lower and will add less weight and bulk to your sporter.