It is the focal length divided by the aperture, and it decides field of view and how demanding a telescope is of its eyepieces. For imaging it also decides how fast signal builds; for visual use it decides very little about brightness.
The focal ratio is written with an f and looks like a property of the optics, which it is — but the thing it governs is not the thing most buyers expect.
The arithmetic
Focal length divided by aperture. A telescope of a given aperture with a long tube has a slow ratio; the same aperture folded or shortened has a fast one.
What it changes for the eye
Field of view, mainly. A given eyepiece in a fast telescope gives lower magnification and a wider field than in a slow one. It also changes how forgiving the telescope is: a fast mirror shows coma at the edge and rewards better eyepieces.
What it changes for a sensor
Exposure. A fast ratio spreads the same light over fewer pixels, so signal builds more quickly and a given exposure goes further. This is where the photographic vocabulary of fast and slow comes from, and it is the only place it means what it sounds like.
What the ratio leaves untouched: the aperture
The aperture. A fast telescope and a slow one of the same diameter collect the same amount of light; they distribute it over different areas. Nothing about the ratio adds photons.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the distinction this page keeps: a magnification is not a property of a telescope. It is the focal length of the tube divided by the focal length of whichever eyepiece is fitted, so it changes every time the eyepiece does, and a figure printed on packaging is a statement about a supplied eyepiece. This page publishes both focal lengths and the aperture, and reproduces no packaging magnification as a specification.
The questions that keep arriving
Is a fast telescope brighter to look through?
Not at the same magnification. Visual brightness depends on the exit pupil, which comes from the eyepiece and the ratio together. What a fast ratio genuinely gives the eye is a wider field for a given eyepiece.
Why do fast telescopes need better eyepieces?
Because light arrives at a steeper angle, and simple eyepiece designs handle that badly at the edge of the field. Below about f/5 the difference between a cheap eyepiece and a good one becomes obvious.
Last reviewed 17 September 2026