If you’ve spent any time around telescopes, you’ve heard someone say something like “I picked up an f/6 refractor” or “this f/10 SCT is incredible for planetary.”
The Simple Version
F-ratio = focal length divided by aperture. A telescope with a 1000mm focal length and a 100mm aperture is an f/10 scope. A telescope with a 600mm focal length and a 150mm aperture is an f/4 scope.
What the Number Means
A low f-number (f/4, f/5) means a fast telescope — wide field of view, better for deep-sky astrophotography. A high f-number (f/10, f/12) means a slow telescope — narrower field, excellent for planetary detail.
Why Astronomers Have Opinions
F-ratio is treated with the same intensity that audiophiles treat cables. The deep-sky crowd will tell you anything above f/6 is a liability. The planetary imagers will tell you anything below f/10 is a toy. Everyone has a position and reasons for it.
“The ‘What the F-Ratio?!’ shirt is a play on ‘what the f***’ — pretty much every astronomer’s reaction the first time someone explains focal ratio. I run an f/6 myself. It’s a comfortable middle ground: fast enough for wide, bright deep-sky views, still slow enough to behave on the Moon and planets.”
Why Is This On a T-Shirt?
“What the F-Ratio?” is one of the great astronomy double-entendres. It is a legitimate question and also an expression of exasperation. Clouds rolled in right before the peak of a meteor shower? What the f-ratio. You drove an hour to a dark sky site and forgot the eyepiece case? What the f-ratio.
Keep exploring: funny astronomy shirts, and the best astronomy gifts for 2026.
Frequently Asked Questions About Telescope F-Ratio
What does f-ratio mean on a telescope?
F-ratio is focal length divided by aperture, written as f/number. It describes how “fast” the optical system is. A short focal length over a large aperture gives a low (fast) f-ratio; a long focal length over a small aperture gives a high (slow) f-ratio. It’s the same concept as f-stops in photography.
Is a lower or higher f-ratio better?
Neither is universally better — it depends on the target. Low f-ratios (f/4–f/6) excel at wide-field deep-sky viewing and astrophotography because they deliver brighter, wider images. High f-ratios (f/10 and up), like a Celestron f/10 SCT, favor high-magnification planetary and lunar detail and are more forgiving of budget eyepieces.
What is a “fast” telescope?
A “fast” telescope has a low f-ratio, typically f/4 to f/5. The name comes from astrophotography: a faster scope gathers light onto the sensor more quickly, so exposures are shorter. Fast scopes also give a wider, brighter field of view, which suits large nebulae and galaxies like M42 or the Andromeda Galaxy.
What f-ratio is best for planets?
Higher f-ratios (f/10 and above) are generally best for planetary and lunar observing. They naturally produce higher magnification for a given eyepiece and tend to deliver sharper, higher-contrast views on small bright targets like Jupiter, Saturn, and the Moon. Schmidt-Cassegrains at f/10 are a popular planetary choice.
What f-ratio is best for astrophotography?
Fast optics in the f/4–f/6 range are preferred for deep-sky astrophotography. They shorten exposure times and capture faint nebulae and galaxies more efficiently. Many astrophotographers use f/5 or faster refractors and reflectors, sometimes adding a focal reducer to make a slower scope faster.
How do you calculate f-ratio?
Divide the telescope’s focal length by its aperture, using the same units. A 750mm focal length on a 150mm aperture is 750 ÷ 150 = f/5. If you don’t know the focal length, multiply aperture by the f-ratio: a 100mm scope at f/7 has a 700mm focal length.
The What the F-Ratio?! shirt turns the most-argued spec in amateur astronomy into an inside joke — see it in the shop.