Binocular size comparison
Enter two sizes such as 8x42 and 10x42. You get each exit pupil, the twilight factor, which one shows a wider view, and the median view and weight for each size in the chart.
| Figure | 8x42 | 10x42 |
|---|---|---|
| Magnification | 8 | 10 |
| Objective lens, mm | 42 | 42 |
| Exit pupil, mm | 5.3 | 4.2 |
| Twilight factor | 18.3 | 20.5 |
| Relative brightness | 27.6 | 17.6 |
| Median field of view in the chart | 393 ft | 336 ft |
| Median weight in the chart | 24.2 oz | 24.9 oz |
| Models in the chart | 75 | 87 |
8x42 magnifies less, so it shows a wider view and shakes less in the hand. 8x42 has the larger exit pupil (5.3 mm against 4.2 mm), which helps at dusk.
A worked example: 8x42 against 10x42
The chart holds 75 models in 8x42 and 87 in 10x42. The median 8x42 shows a view 393 ft wide at 1,000 yards, and the median 10x42 shows 336 ft.
The 8x42 has an exit pupil of 5.3 mm, while the 10x42 has 4.2 mm. Both have 42 mm front lenses, so the lower magnification gives the larger exit pupil, which helps at dusk.
The twilight factor goes the other way. It is 18.3 for the 8x42 and 20.5 for the 10x42. ZEISS treats that figure as a limited guide, which is why the tool shows the exit pupil first.
Weight is close: the median 8x42 weighs 24.2 oz and the median 10x42 weighs 24.9 oz. The 8x42 against 10x42 comparison sets the two sizes out in full.
How the tool works it out
The first number in a size is magnification and the second is the objective lens diameter in millimeters. The National Audubon Society gives 8x42 as eight times magnification with 42 mm lenses.
The exit pupil is the objective diameter divided by the magnification. Nikon's own example is 42 divided by 8, which gives 5.3 mm. ZEISS and the RSPB give the same formula.
The twilight factor is the square root of the magnification times the objective diameter. Celestron prints 18.3 for an 8x42 and 20.5 for a 10x42, and Swarovski Optik gives the same formula.
For hand-holding, the tool reads magnification against two limits. The British Astronomical Association calls anything over 10x hard to hold, and Celestron says a tripod is necessary from 15x. Audubon adds that eight times magnification also magnifies hand movement eight times.
The tool shows a wider view and less shake for the lower magnification, and it names the size with the larger exit pupil. Models and medians come from the makers' documents behind the full binocular chart.
Reference: common sizes side by side
These figures are the exit pupil and twilight factor worked out from each size, with the median view and weight of the models in the chart. The count is the number of models of that size among the 554 in the chart.
| Size | Exit pupil, mm | Twilight factor | Median field of view | Median weight | Models in the chart |
|---|---|---|---|---|---|
| 8x32 | 4 | 16 | 408 ft | 18.5 oz | 28 |
| 8x42 | 5.3 | 18.3 | 393 ft | 24.2 oz | 75 |
| 10x42 | 4.2 | 20.5 | 336 ft | 24.9 oz | 87 |
| 7x50 | 7.1 | 18.7 | 367 ft | 36.8 oz | 24 |
| 10x50 | 5 | 22.4 | 315 ft | 29.3 oz | 40 |
| 12x50 | 4.2 | 24.5 | 277 ft | 29.2 oz | 36 |
| 8x56 | 7 | 21.2 | 375 ft | 40.9 oz | 9 |
| 15x56 | 3.7 | 29 | 234 ft | 45 oz | 11 |
What to do with the result
If the larger exit pupil belongs to the lower magnification, ask what you watch. The RSPB suggests 8x for woodland, close range or poorer light, and 10x for open country or large expanses of water.
A 7x50 has an exit pupil of 7.1 mm. Nikon puts the eye's pupil at about 7 mm in the dark, so that size suits dusk and night use. Swarovski Optik notes that the eye's opening shrinks with age.
For astronomy, the Royal Astronomical Society names 8x40 and 10x50 as sizes usually recommended for starting out. The British Astronomical Association adds 7x50. A 15x56 at 45 oz is a tripod size.
Once two sizes look right, open the page for 8x42 binoculars or 10x42 binoculars. You can also check one size alone with the exit pupil calculator or the field of view converter, or browse best binoculars compared.
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Questions and answers
Which has the larger exit pupil, 8x42 or 10x42?
The 8x42 does, at 5.3 mm against 4.2 mm for the 10x42. Both have 42 mm lenses, so the lower magnification gives the wider beam of light leaving the eyepiece.
What does the twilight factor tell me?
It is the square root of the magnification times the objective diameter. An 8x42 scores 18.3 and a 10x42 scores 20.5, but ZEISS calls it a limited guide and points to exit pupil instead.
Which size is easier to hold steady?
The lower magnification. Audubon says an eight times binocular magnifies hand movement eight times, and the British Astronomical Association calls anything over 10x difficult to hold.
Do two sizes with the same lens diameter weigh the same?
They weigh about the same in the chart. The median 8x42 weighs 24.2 oz and the median 10x42 weighs 24.9 oz.
What size do astronomy groups suggest for starting out?
The Royal Astronomical Society names 8x40 and 10x50. The British Astronomical Association names 10x50 and 7x50, and calls 8x40 the lighter choice.