What the notation fixes
Two lengths in millimetres: the clear aperture of the optics, and the focal length of the telescope. Written with a slash between them, aperture first.
From that pair everything optical follows. The focal ratio is the quotient. The magnification a given eyepiece produces is the focal length divided by the eyepiece focal length. The exit pupil is the eyepiece focal length divided by the ratio. Three of the four figures that decide how a telescope behaves are derivable from two numbers in its name.
That is why the notation persists: it is the most compact honest description of an instrument available.
What it does not promise
Optical quality. Two telescopes of the same designation can differ in glass, in figure, in coatings and in how well the tube holds alignment, and none of that appears.
It says nothing about the mount, which decides as much as the optics do about whether an instrument is pleasant.
And it says nothing about what is in the box. A designation is not a specification of supplied eyepieces, a diagonal, a finder or a dovetail bar, and the absence of any of those changes the real price.
What it works with
Every telescope family on this site, and the guide scopes and smart telescopes that borrow the same convention. It is also the pair of figures this site publishes in place of any magnification claim, because magnification is a division that the reader should make with their own eyepiece.
The questions that keep arriving
Which number comes first?
The aperture, by habit: 70/700 is a 70 mm lens with a 700 mm focal length. The two are rarely close enough in value to be confused, and where they are, the smaller is the aperture.
How do I get the focal ratio?
Divide the second by the first. 70/700 is f/10; 150/750 is f/5. The ratio is a consequence of the pair rather than a third independent figure.
Why do American listings write it differently?
Apertures are frequently given in inches there — an eight-inch Dobsonian rather than a 200 mm one — and the focal length is often quoted separately. The quantities are the same.
Last reviewed 17 September 2026