What does the “f ratio” Means in Telescope Talk The Easy Explanation

One of the most prominent specifications on any telescope spec table is always a number that looks like this f/8. It is dropped in so casually, so universally, so fluidly, into every single telescope review and description that one has to wonder, what does it mean? Is it important?

The F-number or focal ratio of a telescope is an important function that tells you about the viewing angle and strengths of a telescope’s optical suite. It is a simple character representation of the focal ratio that is presented to the reader in an easily digestible number to convey as much information as possible in as few characters as possible.

For example, if a telescope has a focal ratio of f/8, you do not have to write out the full words focal ratio nor do you have to do the equation to figure out the focal ratio for yourself. By displaying the number as f/x or f/8 or f/10.5 or whatever it may be, that framework tells you, the dear reader, that the number you are looking at is the focal ratio for the telescope.

That’s all great, but what exactly does it mean? What information does the f-number provide me?

Important Information Displayed

Without getting too far into the nitty-gritty details of optical science, it is easy to say that not all optical tubes are created equally. Some optical tubes excel at wide-angle viewing and others excel at more narrowly focused viewing. For example, some telescopes do much better looking at large starfields that take up a larger space in the sky while others like smaller, brighter, compact objects such as planets or singular stars.

The f-number, denoted as f/x or f/ratio, tells us which type of telescope it is in regards to the viewing angle.

A telescope with a higher f-number has a narrower viewing angle than one with a lower f-number. The f-numbers can go from 1 to, theoretically, infinity, but most casual telescopes have an f-number in the range of f/4 to f/14. Rarely, will you find telescopes who have denoted f-numbers outside of that range.

The Numbers Explained

There are three effective ranges for these numbers that will tell you where a telescope falls in terms of viewing angle.

Narrow Angle: f/9 to f/14>
Medium Angle: f/7-f/8.9
Wide Angle: >f/4 to f/6.9

The higher the f-number, the narrower the viewing angle. Inversely, the lower the f-number is, the wider the viewing angle will be. The numbers in the middle of our range have a middling viewing angle that can do both fairly well.

The f-number is a representation of the focal ratio, which is determined by the telescope’s focal length to aperture diameter. You can read more about that here, we have dedicated a whole post to simply the focal ratio.

This means that the f-number is determined by the measurements of your individual telescopes. Many telescopes with low f-numbers are short and squat reflectors and many telescopes with high f-numbers are long, narrow refractors. This is not always the case, however.

Wide Versus Narrow Angles

These numbers are useful because it is good to know what kind of viewing angles your telescope is going to be able to confidently handle. Each different angle excels at different kinds of viewing so picking a telescope that will compliment your style and goals as an astronomer is important which is why this denotation is so important.

If you want to get great views of the moon, Jupiter, Saturn, Mars, the moons of Jupiter, or Andromeda, you are going to want a telescope with a narrow field of view that best encapsulates singular objects rather than multiple objects.

If you want to try and get views of deep space, nebulas, distant galaxies and other large form celestial objects, you are going to want to get a telescope with a low f-number and a wide angle view that best encapsulates very large objects.

Or you can shoot for the middle and get one that does a little of both.

Either way, knowing which kind of angle and views your telescope is going to be able to provide is important and that is where the f/ratio moniker comes in. At a glance, you will be able to tell what kind of angles and views a particular telescope is best at without having to do any legwork.