Here's how to assess how much light a riflescope or binocular can (and should) deliver.
For most hunting you don't need a big exit pupil, just high light transmission. But that can come at a cost, so it pays to know what factors affect what you see.
If a little suits you, more is generally better. More money. More vacation time. More game. More dessert. Trouble is, more is seldom free. In optics, hunters seem to have embraced the notion that if a little power is good, more is better, so the bottom-end magnification of riflescopes and binoculars has climbed over the years. But extra magnification comes at little cost; that's not so for brightness.
Glass does not let all incident light exit. As much as 4 percent is lost to reflection and refraction at each surface, and it wasn't until the 1930s that optics engineers learned they could coat lenses to enhance light transmission.
Since then, coatings have improved. They've been applied to internal as well as external glass (fully coated optics). And they've been refined to target specific wave-lengths of light, for application in layers (multicoated optics). The more coatings (and the higher-tech the coating), the higher the price tag.
Realize, too, that a bright image is not always a useful image. You must also see sharp detail. A multi-coated scope or binocular that boasts a high percentage of light transmission may also show color fringing, excessive field curvature and fuzzy images. Bright images of low resolution do you little good.
Once you've applied state-of-the-art coatings on top-quality lenses, you've done all you can to ensure maximum light transmission, which is expressed as a percentage of the incident light passing through. But exit pupil--the diameter of the shaft of light reaching your eye--can also limit the amount of light you see.
Exit pupil is determined by dividing the magnification into the diameter of the objective (front) lens in millimeters. Thus, a 7x35 binocular has a 5mm exit pupil (35 divided by 7). So does an 8x40.
The higher the magnification, the smaller the exit pupil--unless you also increase objective size to compensate. You can, for instance, get a 5mm exit pupil from 10X glass if you make the front lens 50mm in diameter. Bigger lenses, of course, add weight and bulk.
For hunting, even in the poorest legal hunting light, a 6mm exit pupil gives you as thick a shaft of light as your eye can use. For perhaps 95 percent of the glassing and shooting I've done, a 5mm exit pupil would have delivered as bright an image.
With 5mm as your target exit pupil in a riflescope, you can crank your 3-9x40 up to 8X. If you insist on a 7mm exit pupil--the biggest shaft of light useful to a healthy human eye that's fully dilated--you'll need to stay under 6X or buy a scope with a 56mm front end.
As 6X is more magnification than I need in big game hunting, I lean toward scopes with smaller objectives, such as the 2.5-8x32 from Zeiss, Leupold's 2.5-8x36 and the new Nikon 2-8x32.
A 5mm exit pupil is harder to find in a handy binocular as the trend to higher magnification has put a lid of 4mm on the exit pupils of sub-40mm binoculars. I prefer 32mm objective lenses to bigger ones. So even at 8X, I must be satisfied with a 4mm exit pupil. And I am.
Lenses and coatings in even mid-priced binoculars these days are so good I get as bright an image with a smaller shaft of light. And unlike most binoculars with 40mm and 42mm objectives, I can endure a 32mm glass on a simple neck strap, which I prefer instead of harness.
In spotting scopes, you'll have trouble getting a 4mm exit pupil. A full-size scope with an 80mm objective gives you that only if you keep magnification at 20X At the top end of the popular 20-60X range, a spotting scope delivers a tiny 1.3mm exit pupil. I like the 15-45X range, which, with a handy 65mm objective, gives you a 4mm exit pupil at the low end--enough light even for dim conditions.
But how do you assess these qualities when you're shopping for glass in a store or catalog. In the store, you can be fooled by larger objectives and their larger exit pupils. The big lens may in truth seem brighter because the image is easier to find.
A big exit pupil allows you more latitude in eye placement. If you must search for the image in a smaller optic, your eye is spending more time fighting the shadows, and you perceive the instrument is not as bright as its big-eyed competitor.
Another thing to keep in mind when assessing optics is twilight factor, a number combining the effects of brightness and magnification. Twilight factor is the square root of the optic's magnification multiplied by its objective diameter. So if you have an 8x40 binocular, the twilight factor is the square root of 320 (8 times 40), or 17.9. For a 10x40, the twilight factor is 20, but the exit pupil shrinks from 5mm to 4mm. The extra magnification more than offsets the drop in brightness.
If you make the objective lens 25 percent bigger (50mm), the twilight for an 8X glass is 20. For a 10X it's 22.3. Once more, high magnification results in a greater twilight factor. You need a 25 percent gain in objective diameter to equal a 20 percent boost in magnification.
So while brightness is an asset, reducing magnification to make the exit pupil bigger will not necessarily give you a higher twilight factor or better resolution in dim light.