The f/ratio or focal ratio of a telescope is the ratio between the telescope’s focal length and its aperture.

This seemed like a weird combination of specifications, but an f/ratio is a great way to quickly get a first impression of the capabilities of a telescope:

• A lower f/ratio indicates a larger aperture relative to the focal length, hence a brighter, sharper image with a higher resolution.
• A higher f/ratio indicates a longer focal length relative to the aperture, hence a narrower field of view, and a higher magnification of the image.

Speed

Another way astronomers are talking about f-ratio is in terms of speed:

• Faster up to f/5 focal ratios are generally best wide-field observing and deep space photography
• Slower f/11 to f/15 focal ratios are usually better for lunar, planetary, and binary star observing and photography
• Medium f/6 to f/10 focal ratios can be used for both.