It decides whether the image is bright or dim, whether all the light gets into your eye, and where magnification stops being useful. This page defines it, works it out, and says what each value is for.
What it is, and how to work it out
A telescope collects light over the whole surface of its mirror or lens, and the eyepiece sends it to the eye as a narrow beam. The width of that beam is the exit pupil. You can see it: point the instrument at a daytime sky, move your head back thirty centimetres or so, and the small bright disc floating behind the eyepiece is it.
- From the magnification: aperture ÷ magnification. A 130/650 with a 25 mm eyepiece gives 650 ÷ 25 = 26× on 130 mm, an exit pupil of 130 ÷ 26 = 5 mm.
- From the eyepiece: eyepiece focal length ÷ focal ratio. The same 25 mm on this f/5 telescope gives 25 ÷ 5 = 5 mm, without going through the magnification.
- On binoculars it can be read off the format: 50 ÷ 10 = 5 mm for 10×50s, 50 ÷ 7 = 7.1 mm for 7×50s.
Why it decides the brightness
For an extended object (the Moon, a nebula, a galaxy) the brightness of the image per unit area follows the square of the exit pupil. More magnification spreads the same light over a bigger image: the object grows and fades.
There is a ceiling, and it surprises people: when the exit pupil equals the eye's pupil, a nebula looks at best as bright per unit area as it does to the naked eye, less what the mirrors and glass lose. The telescope does not light it up; it enlarges it enough for the eye to make it out. That is the conservation of luminance, not a manufacturing limit.
Stars are different: they are points, and their brightness follows the aperture, not the exit pupil. Magnifying darkens the sky background without dimming the stars, and brings out fainter ones, until turbulence smears them.
The two limits: your eye, and 0.5 mm
At the top, the limit is your own pupil. If the exit pupil is wider, the edge of the beam lands on the iris and that light is lost: you have paid for aperture you are not using. The site takes 6 mm by default when it works out the lowest useful power, not 7: 7 mm is a young eye in full darkness.
At the bottom, the usual limit is 0.5 mm. Below it, the image grows without showing one more detail, turns dull, and the floaters in your own eye start drifting across it. That is the rule of twice the aperture in millimetres.
What each value is for
- 5 to 7 mm: the widest field, for finding things and for big clusters and extended nebulae under a dark sky.
- 2 to 3 mm: the all-round deep-sky setting, for galaxies, globular clusters, small nebulae.
- About 1 mm: the Moon in detail, the planets, double stars, when the air is steady.
- Below 1 mm, down to 0.5 mm: only on nights of very good steadiness.
Seeing it at work
The site's simulator shows the exit pupil beside every magnification, and darkens the image as it shrinks. Slide the eyepiece from 25 mm to 4 mm on the same galaxy: it grows, then fades away. The exit pupil is what turns it off.
Questions people ask
How do I work out a telescope's exit pupil?Divide the aperture by the magnification, or the eyepiece focal length by the focal ratio.
A 10 mm on a 200/1200 (f/6) gives 120× on 200 mm, an exit pupil of 1.7 mm.
What is the best exit pupil?There is not one: about 1 mm for the planets, 2 to 3 mm for galaxies, 5 mm or more for wide fields.
A good set of eyepieces covers the range.
Why does the image get darker when I magnify?Because the exit pupil shrinks: the same light is spread over a bigger image.
Halving the exit pupil makes an extended object four times dimmer.
Is a 7 mm exit pupil a problem?
Only if your eye does not open that far: with age, a 5 to 6 mm pupil in the dark is common, and the extra light is lost.
What is a telescope's maximum magnification?About twice the aperture in millimetres, where the exit pupil falls to 0.5 mm: 260× on 130 mm, an exit pupil of 0.5 mm.
Turbulence often stops things well before.
Method and changes
What this page rests on and when it changed, computed from the catalogue — no date below is typed by hand. There is no price history yet, so this log says what was read and when, never whether it went up or down.
- First published
One price per model: the cheapest observed offer, with its date. Models are ordered by that price, never by what a link pays. How we choose.
Written and maintained by the person behind About.