What Does Focal Ratio Mean In Telescopes? The Simple Formula

The term focal ratio comes up quite often in talk about telescopes and astrophotography and for good reason. Focal ratio is an important metric when gauging optical scopes. But what exactly is it? Focal ratio sounds complicated and scientific but it is actually quite simple. By the end of this brief overview, you will know everything you need regarding this concept.

At its core, the focal ratio is a simple relationship between the focal length of a telescope and its primary aperture. By dividing the focal length by the width of the aperture (in the same unit of measure) you get a number that is the representation of the focal ratio.

We went over this numerical representation, the f-number, in another post, so if you want to know more about that nomenclature, check that piece out.

The formula is quite simple. The formula for the focal ratio is:

Focal length in mm / Aperture diameter in mm

This, normally, gives us a very neat number to use for the focal ratio. This is because telescopes, and other optical instruments, are usually designed with specific focal ratios in mind which give us nice numbers to use rather than odd fractions or strings of digits.

For example, if we have a telescope with a focal length of 800mm and an aperture diameter of 80mm you would get a focal ratio of 10. Easy enough right?

But why does the focal ratio matter in the first place? Why do people care about such an equation when it comes to their telescopes?

The Usefulness Of The Focal Ratio

The focal ratio is used to determine the “speed” of a telescope. The closer the focal ratio number is to 1 the “faster” the telescope is. For example, if you are looking at a distant nebula through a telescope with a focal ratio of 4, you will see it much faster than through a telescope with a focal ratio of 8 but the image will be smaller and less defined.

You will not notice this difference in speed with your eyes but if you are focusing on distant objects or have the intent of photographing deep space objects, this speed is going to make a difference.
A lower focal ratio is going to display more objects in a single shot due to the field of view but it will not focus in on singular objects as well. A higher focal ratio is going to produce cleaner more defined images at the expense of a wide field of view.

For example, if you are standing on a vista overlooking a mountain range and a valley you can look at it in a few different ways. If there is a distant mountain peak that is particularly beautiful to look at, a higher focal ratio is going to allow you to zoom in on that specific part of the scene and display its finer details in higher contrast.

The flip side, if you wanted to see the whole scene, the valley, the multiple mountain peaks (including the one beautiful one) you will want to use a scope with a lower focal ratio. This will encompass more of the scene with less detail.

This works the exact same way when looking at the night sky. If you want to see something in more detail with less in the frame, such as a planet, you are going to want a high focal ratio. If you want to see a larger scene as a whole, like a star cluster, you are going to want a lower focal ratio.

Focal Ratio and Astrophotography

While not as important for casual viewing, the focal ratio can have a profound impact on astrophotography. Getting good shots of different objects is going to require different focal ratios. If you want to take a picture of a binary star system in high detail, you are going to need a high focal ratio. Trying to take the same picture with a low focal ratio just won’t work for photography.

It may not seem like that big of a deal but in terms of getting good photographs out of your telescope, focal ratio plays a large and critical role.

Next time focal ratio is brought up in conversation, just remember, it is a simple relationship between focal length and aperture size which determines what kind of contrast you can get out of your telescope.