The two numbers in 8x42 are the whole specification, and everything else on the page is derived from them. Divide the objective diameter by the magnification and the answer is the exit pupil, the disc of light the eyepiece delivers to the eye: Celestron publishes 5.25 mm for the Nature DX 8x42 and 4.2 mm for the Nature DX 10x42, which is 42 divided by 8 and 42 divided by 10. The same 42 mm objective therefore delivers a wider disc at 8x than at 10x, and exit pupil is the first figure that falls as magnification rises. The condition that changes which pair to read is eyeglasses: the 8x42 is listed at 17.5 mm of eye relief and the 10x42 at 14.0 mm, and eye relief is what a spectacle lens spends.
Two powers, one objective, four differences
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| Specification, as published | Celestron Nature DX 8x42 | Celestron Nature DX 10x42 | Bushnell R5 8x42 |
|---|---|---|---|
| Exit pupil | 5.25 mm | 4.2 mm | 5.3 mm |
| Eye relief | 17.5 mm | 14.0 mm | 17.5 mm |
| Field of view at 1,000 yd | 388 ft (7.4°) | 304 ft (5.8°) | 390 ft |
| Weight | 22.2 oz (629 g) | 22.2 oz (629 g) | 24.7 oz |
Celestron lists the 8x42 at 388 ft of linear field at 1,000 yards and the 10x42 at 304 ft, a loss of 84 feet, and 3.5 mm of eye relief with it. Both weigh 22.2 oz and both focus to 6.5 ft, so the extra power costs field, eye relief and exit pupil while changing nothing else. Bushnell's R5 8x42 publishes 5.3 mm of exit pupil, 17.5 mm of eye relief and 390 ft of field — the same arithmetic on the same aperture, rounded one decimal differently.
Relative brightness and twilight factor are sums, not glass quality
Two more figures on the page come straight out of the first two. Relative brightness is the exit pupil squared: Celestron prints 27.56 for the 8x42, and 5.25 squared is 27.5625. Twilight factor is the square root of magnification times objective: the same page gives 18.33, and the square root of 336 is 18.33. For the 10x42 the published figures are 17.64 and 20.49, which are 4.2 squared and the square root of 420.
Neither number contains any information about the glass. Coatings, prism type and prism glass are the specifications that actually separate two instruments with identical arithmetic, and those are named rather than measured on a consumer page: Celestron states BaK-4 prisms, phase coated, with fully multi-coated optics, while Bushnell states BaK-4 roof prisms with dielectric and phase coatings. A higher twilight factor on a 10x instrument does not mean a brighter view, because the same change that raises it lowers the exit pupil to 4.2 mm.
Bushnell states an IPX7 sealed chassis for the R5 8x42, which is a code from IEC 60529, Degrees of protection provided by enclosures, in its 1989 edition with the 1999 and 2013 amendments. Celestron states that the Nature DX is waterproof and nitrogen filled without printing a code, and the nitrogen is the part that stops internal fogging rather than the part that stops water. Those are two different claims about the same housing, and only one of them names a test.
Field of view converts between degrees and feet
Example, using the published pair. One degree subtends about 52.4 feet at 1,000 yards. Celestron's 7.4° angular field on the 8x42 therefore works out to 388 ft, and the 5.8° on the 10x42 to 304 ft, matching both published linear figures. That conversion is the reason a European specification given only in degrees and an American one given only in feet are the same measurement.
The figure has a practical floor. At 304 ft of field per 1,000 yards, a bird 40 yards away sits in a window about 12 ft wide, and every hand tremor is magnified ten times rather than eight. That is also where weight stops being neutral: 22.2 oz held steady differs from 24.7 oz held steady, and the same is true of any instrument carried on a strap alongside a headlamp and a full hiking backpack.
Before you buy binoculars
- The exit pupil in millimeters, checked against the objective divided by the magnification.
- The eye relief in millimeters, since spectacle lenses consume it.
- Field of view in feet at 1,000 yards or in degrees, converted at about 52.4 ft per degree.
- The prism glass and coatings as named, since the brightness figures are arithmetic.
- The ingress code, and whether the maker says nitrogen purged as well as sealed.
- The close-focus distance in feet, which sets what is usable at arm's length.