focal ratio
f-number

A high focal ratio also means the field of view will be narrow.

They are available in short, medium, and long focal ratios.

They have very long focal ratios and are sold in blue aluminum tubes.

A short focal ratio can be more easily obtained, leading to wider field of view.

The combined focal ratio is about f/3, but the image quality of the two mirrors alone would be poor.

Even though the principles of focal ratio are always the same, the application to which the principle is put can differ.

The all spherical surfaces are much easier to manufacture, especially in short focal ratios.

The design is noted for allowing very fast focal ratios, while controlling coma and astigmatism.

Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.

The drawback is the system will have a high focal ratio, with a correspondingly dim image, and some vignetting.

The glass plate will usually break if bent enough to generate a curve for telescopes of focal ratio f/2.5 or faster.

The primary mirror is a concave hyperboloid with 4.1 m diameter and about f/1 focal ratio.

The Chinese Future Giant Telescope has a primary focal ratio of 1.2.

Low focal ratio primary mirrors can be combined with lenses that correct for coma to increase image sharpness over the field.

Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.

Telescopes with high focal ratios may use spherical mirrors since the difference in the two shapes is insignificant at those ratios.

Newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5.

It is the largest mirror of this shape and of such short focal ratio; polishing it took 2 years, which was longer than anticipated.

His first camera had an aperture of about 360mm or 14.5" in diameter, and a focal ratio of f/1.75.

The focal length of the telescope objective is the diameter of the objective times the focal ratio.

Their optical arrangement gives flexibility in placing the focal plane and changing the focal ratio by mere refocus.

This is of particular importance in fish, where the value may decrease from as high as 3.6 to 2.3, decreasing the focal ratio of the lens.

A higher focal ratio is thought to compensate for the relatively high Matthiessen's ratio brought about by constraints of small eye size during early development.

Schmidt-Newtonian telescopes offer images with less coma than Newtonian telescopes of the same focal ratio (usually about half).

Today's anti-reflection coatings make these usable, economical choices for small to medium aperture telescopes with focal ratio f/6 or longer.