Vintage Bausch & Lomb Custom 10×40 Binoculars: Reading the Specs and Using Them for Stargazing

The earlier post on this site was a used-sale listing for a pair of Bausch & Lomb Custom 10×40 binoculars marked “5.2 Degree Field” and “Made in Japan”. The seller noted the optics were free from fungus, the collimation excellent and the focus firm but smooth. We could not find a manufacturer source for this vintage model, so this rewrite does not make claims about its history. Instead it explains what its markings mean and how to use binoculars like these for astronomy.

What “10×40” means

Binocular specifications follow a simple pattern:

• 10x is the magnification. Objects appear ten times closer.
• 40 is the diameter of each front (objective) lens in millimetres. Bigger lenses gather more light.

Astronomy magazine notes that 7x or 10x is usually practical for hand-held use, and that magnifications above 10x with apertures over 50 mm generally need a tripod.

Exit pupil: 4 mm

The exit pupil is the width of the beam of light leaving each eyepiece. You calculate it by dividing the aperture by the magnification. For a 10×40 that gives 40 ÷ 10 = 4 mm.

Nikon explains that you can see the exit pupil as a bright circle in each eyepiece when you hold the binoculars about 30 cm from your eyes and point them at a bright light, and that a larger exit pupil gives a brighter view. A 4 mm exit pupil is a good all-round compromise between brightness and magnification.

True field: 5.2° is about 273 feet at 1,000 yards

The “5.2 Degree Field” marking is the true (real) field of view, the actual angle of sky you see. Older binoculars often quote field as feet at 1,000 yards instead. Both the British Astronomical Association and Astronomy magazine give the conversion: divide feet at 1,000 yards by 52.5 to get degrees. Working backwards, 5.2° × 52.5 is about 273 feet at 1,000 yards.

Astronomy magazine says most binoculars have fields of around 6° or 7°, with some higher-power models shrinking to 3° to 5°. So 5.2° is a normal field for a 10x binocular.

Apparent field: about 49°

The apparent field is how wide the view looks to your eye. Nikon gives the formula: apparent field = 2 × arctan(magnification × tan(half the true field)). For this pair:

• 2 × arctan(10 × tan 2.6°) is about 49°

That is a fairly standard apparent field, neither cramped nor ultra-wide.

Why collimation matters

The original listing stressed that “the collimation is excellent”. Collimation means both barrels point in exactly the same direction. If they do not, your brain struggles to merge the two images. The listing explained that poor alignment can lead to double vision, eyestrain and headaches, limiting viewing to a few minutes. To check a used pair, look at a distant straight edge such as a rooftop. The two images should merge instantly into one with no strain.

Stargazing with 10×40 binoculars

Binoculars are an excellent first astronomy instrument. Good targets include:

• The Moon: craters and dark seas are easy at 10x.
• Jupiter’s moons: up to four points of light beside the planet, if you hold the binoculars steady.
• The Pleiades: a beautiful star cluster that fits easily in a 5.2° field.
• The Andromeda Galaxy: a soft oval glow from dark skies.
• The Milky Way: sweeping along it reveals countless stars and clusters.

Tips for steady views

• Brace yourself. Lean against a wall or rest your elbows on a car roof.
• Use a tripod adapter if the binoculars have a threaded socket.
• Lie back on a reclining chair to view high overhead.
• Focus on a bright star until it is the smallest possible point, then adjust the right-eye dioptre.

Buying vintage binoculars: a checklist

• Look through the front lenses with a torch for fungus, haze or separation.
• Check for double images at a distant target.
• Make sure the focus wheel moves smoothly over its full range, as the original listing described.
• Check the hinge holds its setting.

Related guides on this site

• Budget binoculars for stargazing: how to read the numbers
• Vintage Nikon 8×24 binoculars explained
• Nikon MONARCH 5 10×42 stargazing guide
• Celestron SkyMaster 20×80 binoculars guide

The bottom line

A good vintage 10×40 with clean optics and accurate collimation is still a fine astronomy companion. Its 4 mm exit pupil and 5.2° field make it a versatile choice for the Moon, star clusters and sweeping the Milky Way.

Sources: Astronomy magazine: using binoculars; British Astronomical Association: binoculars for the travelling astronomer; Nikon binocular guide: choosing magnification; Nikon binocular guide: field of view.