What is the exit pupil of my telescope or binoculars?
The exit pupil is the small disc of light that leaves the eyepiece, and the place your own pupil should sit. It is the aperture divided by the magnification, or equivalently the eyepiece focal length divided by the telescope’s focal ratio. This exit pupil calculator works it out for a telescope with any eyepiece and Barlow, or for binoculars from their markings such as 8×42.
It then compares the result with your eye pupil, which gets smaller with age, tells you how much light your eye can actually take in, and gives the lowest useful magnification for your aperture.
How to use the Exit Pupil Calculator
- Choose “Telescope and eyepiece” or “Binoculars or known magnification”.
- Enter the aperture in millimetres and either the telescope and eyepiece focal lengths or the magnification.
- Pick your age or type your own measured eye pupil.
- Read the exit pupil, the share of light your eye can use and the lowest useful power.
- Use the table to find the magnification or eyepiece for a target exit pupil.
Exit Pupil Calculator features
Two inputs
Telescope with eyepiece and Barlow, or magnification straight from the binocular markings.
Eye pupil by age
Six typical values from 7 mm for a young eye to 4.5 mm past 75, or enter your own.
Wasted-light check
If the exit pupil is wider than your pupil, the edge of the beam is lost.
Target table
Magnification and eyepiece focal length for exit pupils from 5 mm down to 0.5 mm.
Formulas and a worked example
Exit pupil = aperture ÷ magnification = eyepiece focal length ÷ focal ratio. Lowest useful magnification = aperture ÷ eye pupil.
Example: a 200 mm f/6 telescope (1,200 mm) with a 25 mm eyepiece gives 48× and an exit pupil of 200 ÷ 48 = 4.17 mm, which is also 25 ÷ 6. For a 65-year-old eye of about 5 mm that is fine. A 40 mm eyepiece would give 6.7 mm, so only about 56% of the light enters the eye. 10×50 binoculars have a 5 mm exit pupil.
Which exit pupil for which target?
- 5–7 mm: brightest, widest views; useful under dark sky, but only if your pupil opens that far.
- 2–4 mm: the all-round range for star clusters, nebulae and the Moon.
- 0.5–1 mm: high power for planets and double stars; the image is dimmer and shows floaters in the eye.
- Below 0.5 mm the view rarely improves; see the telescope magnification calculator for the 2× aperture limit.
Limits
Pupil size depends on the person and on how dark-adapted the eye is, and daytime pupils are much smaller, which is why 7×50 binoculars waste light in daylight. For binoculars see the binocular calculator for field of view and twilight factor.
Exit Pupil Calculator FAQ
How do you calculate exit pupil?
Divide the aperture by the magnification, for example 50 mm ÷ 10 = 5 mm. For a telescope you can also divide the eyepiece focal length by the focal ratio.
What exit pupil is best for stargazing?
Between about 2 and 5 mm suits most deep-sky viewing, as long as it does not exceed your own dark-adapted pupil.
What happens if the exit pupil is bigger than my eye pupil?
The outer part of the light beam falls on the iris and is lost, so the view is no brighter than with a smaller exit pupil.
What is the lowest useful magnification?
The aperture divided by your eye pupil. A 200 mm telescope and 7 mm pupil need at least about 29×.
How big is the eye pupil by age?
Roughly 7 mm in young adults falling to 4.5–5 mm in the elderly after dark adaptation. These are averages, so enter your own value if you can measure it.