They offer the greatest transmission of light, but are also more expensive. Eyepieces are also important to a good set of binoculars. Their optical design determines eye relief and field of view. Eye relief is a measure of how far from the eyepiece an observer's eyes must be to see the whole field of view. Short eye relief makes using a pair of binoculars difficult for most eyeglass wearers.
Most manufacturers supply an eye relief measurement in the binocular's technical specifications.
You can also measure it by aiming the binoculars at a bright source of light and projecting the exit pupil onto a card. Move the card back and forth until the exit pupil comes into sharp focus. Now take a millimeter rule and measure how far the eyepiece is from the card to get eye relief in millimeters. A more practical way is to simply look through the binoculars with your glasses on. If you can comfortably see a full field of view, eye relief is adequate.