Exit pupil and low light
Exit Pupil Chart
Quick answer
Exit pupil equals objective diameter divided by magnification, so a 50mm objective at 10x gives a 5mm exit pupil, and the same 50mm objective at 25x gives only 2mm. This chart runs that division for every common objective from 20mm to 56mm across 3x to 24x, the full range a riflescope, spotting scope or binocular actually uses.
Exit pupil is the diameter, in millimetres, of the circle of light leaving the eyepiece, and it is the single best predictor of how bright an optic looks in the field: exit pupil equals objective diameter divided by magnification, a standard published optical formula with no approximation involved. A 40mm objective at 10x gives a 4mm exit pupil. The same objective at 4x gives a 10mm exit pupil, though a human pupil never opens that wide, so most of that light is thrown away rather than used.
A dark adapted human pupil runs roughly 5mm to 7mm depending on age, so an exit pupil larger than your own pupil at the moment you are looking through the glass is wasted, and one meaningfully smaller is a dim image. This page is the reference matrix for every combination worth knowing, plus how it stacks against real pupil sizes.
These figures are exact arithmetic from the published exit pupil formula, not a claim about how bright any specific scope looks compared to another with the same numbers.
Exit pupil in millimetres, objective by magnification
The working
Exit pupil (mm) = objective diameter (mm) / magnification. A 44mm objective at 16x gives 44 / 16 = 2.75mm.
Exact for every combination, with no correction for coatings, glass quality or eyepiece design, none of which change the raw exit pupil figure.
Find your objective down the left column and read across to your magnification. Every riflescope, spotting scope and binocular objective this site catalogs falls inside this range.
A 44mm objective gives a 2.75mm exit pupil at 16x, already below the roughly 5 to 6mm most adult pupils reach in low light, which is why high magnification and dim conditions rarely mix well on a smaller objective.
Sourced figure
| Objective | 3x | 4x | 6x | 8x | 9x | 10x | 12x | 16x | 20x | 24x |
|---|---|---|---|---|---|---|---|---|---|---|
| 20 mm | 6.67 | 5.00 | 3.33 | 2.50 | 2.22 | 2.00 | 1.67 | 1.25 | 1.00 | 0.83 |
| 24 mm | 8.00 | 6.00 | 4.00 | 3.00 | 2.67 | 2.40 | 2.00 | 1.50 | 1.20 | 1.00 |
| 28 mm | 9.33 | 7.00 | 4.67 | 3.50 | 3.11 | 2.80 | 2.33 | 1.75 | 1.40 | 1.17 |
| 32 mm | 10.67 | 8.00 | 5.33 | 4.00 | 3.56 | 3.20 | 2.67 | 2.00 | 1.60 | 1.33 |
| 36 mm | 12.00 | 9.00 | 6.00 | 4.50 | 4.00 | 3.60 | 3.00 | 2.25 | 1.80 | 1.50 |
| 40 mm | 13.33 | 10.00 | 6.67 | 5.00 | 4.44 | 4.00 | 3.33 | 2.50 | 2.00 | 1.67 |
| 42 mm | 14.00 | 10.50 | 7.00 | 5.25 | 4.67 | 4.20 | 3.50 | 2.63 | 2.10 | 1.75 |
| 44 mm | 14.67 | 11.00 | 7.33 | 5.50 | 4.89 | 4.40 | 3.67 | 2.75 | 2.20 | 1.83 |
| 50 mm | 16.67 | 12.50 | 8.33 | 6.25 | 5.56 | 5.00 | 4.17 | 3.13 | 2.50 | 2.08 |
| 56 mm | 18.67 | 14.00 | 9.33 | 7.00 | 6.22 | 5.60 | 4.67 | 3.50 | 2.80 | 2.33 |
All values in millimetres. A figure larger than roughly 7mm is delivering more light than any human pupil can accept, and the excess is simply unused rather than harmful.
How exit pupil compares to your own dark adapted pupil
Exit pupil only matters relative to how wide your own pupil actually opens, which shrinks with age and never reaches the population maximum in most lighting a shooter or hunter actually works in.
A dark adapted pupil runs from roughly 7mm in the 20s down to 5mm past 60, so an exit pupil built around a young shooter's eye is delivering unused light to an older one.
Shooter convention
| Age band | Typical dark adapted pupil | What a 5mm exit pupil does here |
|---|---|---|
| 20 to 30 | 7.0 mm | Fills 71 percent of the pupil |
| 30 to 40 | 6.5 mm | Fills 77 percent of the pupil |
| 40 to 50 | 6.0 mm | Fills 83 percent of the pupil |
| 50 to 60 | 5.5 mm | Fills 91 percent of the pupil |
| 60 and over | 5.0 mm | Fully used |
This is a population range from published ophthalmic data, not a specification you can look up for yourself without an eye exam. Individual variation inside each band is real, so treat these as a guide rather than a personal measurement.
Relative brightness: exit pupil squared
Relative brightness is exit pupil multiplied by itself, a published index that better reflects perceived brightness than exit pupil alone, since the eye's response to light is closer to the area of the pupil than to its diameter. A 50mm objective at 10x gives a 5mm exit pupil and a relative brightness index of 25, against a 4mm exit pupil and an index of 16 for a 40mm objective at the same 10x, a proportionally larger gap than the raw exit pupil numbers alone suggest.
A 50mm objective at 10x scores a relative brightness index of 25, the highest of the common combinations here, against 7.6 for a 44mm objective pushed to 16x on the same scope.
Sourced figure
| Objective x magnification | Exit pupil | Relative brightness |
|---|---|---|
| 20mm at 4x | 5.00 mm | 25.0 |
| 24mm at 6x | 4.00 mm | 16.0 |
| 32mm at 9x | 3.56 mm | 12.6 |
| 40mm at 10x | 4.00 mm | 16.0 |
| 50mm at 10x | 5.00 mm | 25.0 |
| 44mm at 16x | 2.75 mm | 7.6 |
| 56mm at 20x | 2.80 mm | 7.8 |
| 50mm at 25x | 2.00 mm | 4.0 |
Relative brightness is exit pupil squared, a comparative index rather than a measured lumen output. It says nothing about coatings or glass quality, only about how much of the raw light gathering advantage survives to a given magnification.
Sources
- Exit pupil and relative brightness, standard published optical formulas
- Dark adapted pupil diameter by age, published ophthalmic population ranges
Frequently asked questions
What is exit pupil and how is it calculated?
Exit pupil is the diameter, in millimetres, of the beam of light leaving the eyepiece, calculated as objective diameter divided by magnification. A 50mm objective at 10x gives a 5mm exit pupil. It is a standard published optical formula, exact for any combination, and the best single predictor of how bright an optic looks in dim conditions.
What exit pupil does a 40mm objective give at 9x?
4.44 millimetres, from 40 divided by 9. That comfortably fills most adult dark adapted pupils, which typically run 5 to 7mm depending on age, making a 40mm objective at moderate power a solid low light performer even though it is smaller than many long range objectives.
Is a bigger exit pupil always better?
Only up to the size of your own dark adapted pupil, roughly 5 to 7mm depending on age. An exit pupil larger than your pupil delivers light your eye cannot use, so it adds nothing beyond that point. What matters is exit pupil at the magnification you actually plan to use, not the largest number the scope can theoretically produce.
What is relative brightness and how does it differ from exit pupil?
Relative brightness is exit pupil multiplied by itself, a published index that tracks perceived brightness more closely than exit pupil alone, since the eye responds more to pupil area than pupil diameter. A 5mm exit pupil scores 25, while a 4mm exit pupil scores only 16, a proportionally larger gap than the raw diameters suggest.
Does tube diameter affect exit pupil?
No. Exit pupil depends entirely on the objective lens diameter and the magnification in use. A 1 inch, 30mm or 34mm tube built around an identical objective and set to an identical magnification produces exactly the same exit pupil, since the tube is a structural housing rather than part of the light path.
What is a typical human pupil size in low light?
Roughly 7mm in the 20s, narrowing to about 5mm past 60, a published population range rather than a figure you can look up for your own eyes without an exam. This is why the same scope and magnification can look noticeably dimmer to an older shooter than a younger one at last light.
Researched, not professional advice. This page is compiled from published manufacturer specifications, published optical and ballistic formulas, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are shooter convention rather than sourced numbers, and they are labelled that way wherever they appear. Ballistic figures come from a point mass model using the ballistic coefficient the maker publishes, so treat them as a starting point and confirm them on paper at a measured distance. Confirm your firearm is unloaded with the action open before you mount, level, torque or bore sight anything. Verify a zero only on a supervised range or a safe, legal backstop, know what lies beyond your target, and follow the law where you hunt and shoot, including rules on land access and permitted cartridges.