A lot of factors influence binocular cost, including size or aperture, magnification, type of prisms, types and quality of coatings, body material, lens design and quality. Overall, optical quality is usually the main difference. In an inexpensive, low-quality binocular, the image is very small and dark, could be hard to focus, and only crisp near the middle of the image. In an expensive, high-quality binocular, the image is large and bright, easy to focus, and has a crisp, high-resolution image.