Explain bibocular & monocular numbers to me

I want to get a friend either inexpensive binoculars or a monocular that is as good as what I use for my own cheap binoculars, which is a Bushnell 8 x 23 (365 feet at 1,000 yards). Now, I can quote those numbers at you, but I don’t know what they mean. This makes comparison shopping difficult. Would someone please explain how to interpret these numbers, how I might compare them to monocular ratings, and recommend binoculars or a monocular under $50 that’s similar or better?


8 x 23 means that the optics magnify objects by 8x and the objective (front) lens has an aperture of 23 mm. 365 feet at 1000 yards is the field of view, that is, how much you can see. byt themselves, these numbers don’t mean much. In general, the larger the aperture, the better. Magnification, on the other hand, doesn’t tell you very much on it’s own. The resolving power of any optical system is dependant on its focal ratio, the ratio of focal length to objective aperture, however, binocular specs don’t usually call this out. When shopping for binoculars or any optics, the very best thing to do is to actually try them out under the kinds of conditions you expect to use them under.

If you’re looking for cheap, Bushnell is as good as any, probably.

The 8 = magnification size.

The 23 = objective aperture size.

Combined they tell you how much light enters into the binocular. The higher the light the better ( a ratio of 7, i.e. as is 7 x 49 or 50, is often considered optimum ). However the higher the magnification, the harder it is to keep a pair of binoculars steady, as at higher magnifications the effect of your hand twitching will be magnified when looking through the binoculars. So for really up-close viewing, where more powerful magnification is desired, folks will switch to a tripod-mounted monocular scope, which has the disadvantage of beiung bulky, but the advantage of having a better, more steady image. All of this can be confounded by optics quality and build, as for example better quality binoculars w/ speicial stabilizers can be had at a price.

What you want to use the glasses for, will decide what you want. 8 x 23 really isn’t optimum for dedicated birdwatching because of the light issue - for that you’d be better off with 8 X 40 or better. But they’re compact and a reasonable compromise if size and weight is an issue ( as with backpackers, perhaps ).

But try them out yourself and see which you think would suit you. ALWAYS try them out, anyway, as very occasionally you get manufacturing flaws, especially with less expensive brands. A slightly warped lens will give you a heck of a headache, believe me - I speak from experience.

  • Tamerlane

There’s more to binoculars than just objective diameter (the 23) and the magnification (the 8x). Monoculars will be the same- they’re just basically one half of a binocular.

Objective diameter is a rough guide to the amount of light the binoculars can gather- this will be important in low-light and night conditions.

Binocular design and materials is important as well. Porro-prism binoculars give the best image quality for cheap binocuars, but roof-prism ones are lighter and more compact for the same objective diameter and magnification. Materials play a big role too- the prism glass type is important- BAK-4 glass is what you want- it has nearly total internal reflection, giving brighter images than BK-7 glass. Another construction detail that is very important is the type and degree of antireflection coatings applied to the optical surfaces. Since every bare air-glass surface reflects about 4% of the light hitting it, this can build up pretty quick in binoculars, causing lower light transmission and washed out images. Manufacturers apply coatings that reduce this phenomenon- “coated” optics usually have magnesium flouride, which reduces it to 1.5% per surface, and “multicoated” have a mix of compounds that reduce it to something like .25%. There’s also a difference in how many surfaces are coated. Basically there are 4 types:

“Coated”. This means that some surfaces are coated with magnesium flouride.
“Fully Coated” This means that all surfaces are coated with magnesium flouride.
“multicoated” This means that at least one surface is multicoated, and (probably) the rest are single coated.
“fully multicoated” This is the best- all air-glass surfaces are multicoated.

There’s also exit pupil diameter, which is basically the objective divided by the magnification. This is the diameter of the shaft of light coming out of the eyepieces. So for 10x50 binoculars, the exit pupil is 5mm, and for 7x35 it is 7mm. This is important mainly for astronomical binoculars, because under lower light levels, your pupils open up to about 7mm, and you want to get as much light into your eyes as you can, hence the popularity of 7x50 or 11x70 for astronomical viewing. Exit pupil can also have some implications on daytime viewing- generally, you won’t need a huge exit pupil because your pupils will be constricted so that you won’t need much of an exit pupil- 8x23 binoculars are more than adequate for bright daytime viewing since your pupils will probably be around 2mm anyway.

And finally, there’s eye-relief. This is how far from the eyepiece lens that you can see a full-width focused image. If you don’t wear glasses, this is not much of an issue, but for glasses-wearers, this can be very important, since short eye-relief glasses are tough to use with glasses.

I have Bushnell 7 x 35 Wide Angle that I have had since about 1978 and it is a fine binocular.

Thanks, you all. Very helpful.