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Brightness: Magnification, Objectives, Exit Pupil

Last updated 2010-01-18 - Send Feedback
Related: binocular, binoculars, optics, video

Brightness is a more complicated subject than one might think— it even involves your own eyes.

Magnification

Magnification is typically 7X, 8X, 10X. Generally speaking, the higher the magnification, the narrower the field of view. And to get the same brightness, higher magnification binoculars become much larger and heavier.

A magnification of 7X or 8X is relatively easy to hold steady. A magnification of 10X starts to get the “shakes”, for me at least; it takes some effort to hold things steady. Specific applications have specific needs, but for most users, a 7X wide view binocular with good eye relief will feel luxurious, so I would trend to that unless you really need 8X or 10X.

While 10X is very appealing in some ways, it can be too difficult to use for long, unless perhaps with image stabilized binoculars (not tested).

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Objective size — raw light gathering power

The objective size (in millimeters) determines the raw light gathering power of the binocular. This is the size of the entrance pupil, which is essentially the diameter of the front lens element of the binocular.

An 8 X 42mm binocular has 42mm objectives, 7X32 has 32mm objectives, etc. It’s an area thing, so 42mm objectives collect 72% more light than 32mm, and 50mm objectives collect 2.4 times more light than 32mm objectives. That’s about 1.3 stops in photographer-speak.

That extra brightness is of no consequence during the daytime (except perhaps for being too bright), because pupils are contracted. But it makes all the difference in low-light conditions, up to a point.

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Optics — transmission, color and contrast

Binoculars with the same specifications might in fact behave quite differently: the optical glasses used, along with the lens coatings can result in a quite different viewing experience.

Good optics are expensive, and indeed specialty glass such as fluorite needed to build top optics is itself very expensive. Only with the best optical glass will color, contrast and brilliance be high, and the optical path must be assembled with high precision.

Exit pupil — your age matters!

The exit pupil is a simple calculation based on the magnification and objective size:

exit pupil size = objective size (mm) / magnification

An 8 X 42 binocular has a 5.25mm exit pupil, a 7X50 has a 7.1mm exit pupil.

But there is a twist—

In other words, buying 8X56mm binoculars might result in an image no brighter than 8X42mm binocular for older users.

For example, if you’re 60 years old, your pupils might not be able to dilate more than 4mm or so. A binocular with a 6mm exit pupil is shining more than half the light onto the iris of that eye!

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My experience with brightness at dusk

My middle-aged (40's) eyes apparently can still dilate reasonably well. I tested the following binoculars at dusk:

The Leica 8X42 binoculars helped me see into the shadows much more easily than the Zeiss 8X32’s. This is expected, my pupils at my age should still be able to take advantage of a 5.25mm exit pupil.

The Fujinon 7X50 binoculars further improved the shadows over the Leica 8X42 Geovids, but the difference was not as great between the 8X42 and 7X50 binoculars. At night when the eyes make a further accommodation to dark, there might be more of a difference.

Based on this testing, I’d estimate that my pupils can dilate to around the 6mm range. But I suspect that my eyes do not dilate to the full 7.1mm afforded by the Fujinons.

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