Βρήκα αυτό το άρθρο στο Internet, πιστεύω να σε βοηθήσει : "A telescopes focal ratio is its focal length divided by the aperture. Where the focal length is the distance from the main lens (or mirror) to where the light converges to a focus. For example, if a telescope has a focal length of 60 inches and a six-inch aperture, then its focal ration is f/10. This might sound a bit confusing and what you probably want to know is what the focal ratio has to do with deciding on what telescope to buy. Unlike photography, the focal ratio does not have a direct meaning in terms of viewing quality. In most cases, however, a larger f/number will provide better quality at a lower cost. Here is a real life example that will make this easier to understand. If two telescopes have an equal aperture and one is f/4 and the other is an f/8, then the f/8 will almost always provide better quality than the f/4. The f/8, however, will be twice as long, making it more difficult to carry than the f/4. There are f/4 scopes that will provide outstanding quality but, the tight tolerances needed to produce such a quality instrument make it quite expensive. So, while a larger f/number does not always mean a better instrument, it does generally mean a better image for an equal amount of money." Μάνος