Binoculars are a product for which rigorous comparative reviews really matter. Specifications rarely tell the whole story, even for image quality, not to mention comfort and usability. For example, although some reviews use a specification called "exit pupil" as a guide to how bright the binocular image will be, the best binocular reviews note that optics quality matters much more. Another specification, eye relief, is a major factor in eye comfort, especially for people who wear glasses, but testers in comparative reviews evaluate "eyeglass-friendliness" by actual usage, often rating a binocular higher or lower than the eye-relief specification would predict.