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Sorry for the noob question but I don't seem to be able to find the answer on the internet.

I've been looking through some telescopes and binoculars and noticed that shops typically give different specifications for the two groups. For example, for telescopes I'll often see focal length, aperture ratio, or limit value, but I haven't seen any of those for binoculars; for binoculars, on the other hand, they'll mention exit pupil, field of view, or glass material, none of which I've seen given for a telescope. Why is that? Are the two devices so different as to be incomparable?

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    $\begingroup$ great question! $\endgroup$
    – uhoh
    Commented Jul 7, 2019 at 16:46

3 Answers 3

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With a binocular, all its optical components are fixed - the user can't change them. What's important for the user to know is the size of the front lens, which determines the brightness (and in theory sharpness) of the image, the magnification, and the field of view. These are all useful things to know.

A telescope has an interchangable component, namely the eyepiece. The choice of eyepiece determines:

  1. The magnification (= focal length of telescope divided by focal length of eyepiece)
  2. The field of view, which depends on magification and design of the eyepiece
  3. The exit pupil size (diameter of objective divided by magnification)
  4. and a few other things

Hence, based on a particular eyepiece type you can work out the same information as you get with binoculars.

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  • $\begingroup$ Why aren't binocular eyepieces interchangeable? Why aren't telescope parts other than just the eyepiece interchangeable? $\endgroup$
    – Vikki
    Commented May 2, 2020 at 22:22
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Binoculars tend to be mostly used for daytime observing (of birds, ships etc) Telescopes are mostly used for astronomical observing. The users of the two types of equipment want different information. You may find large, mounted, astronomical binoculars described more like a telescope. Similarly, small "mononcular" may be described in the same terms as binoculars.

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  • $\begingroup$ Thank you for the answer. But are you saying glass material or field of view are not very important for telescopes? With binoculars they keep raising them as almost the most important thing. Anyway, why not give all of the information for both? $\endgroup$
    – Kamil S.
    Commented Jul 7, 2019 at 12:44
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    $\begingroup$ Binoculars tend to have short focal lengths, hence thicker glass, and they have thick prism reflectors, quality transparent glass is required. Telescopes have longer focal length and so thinner glass objective lenses. Anyway most astronomical telescopes use mirrors, and never have prisms. Remember all this is advertising talk, so treat it as such. $\endgroup$
    – James K
    Commented Jul 7, 2019 at 13:05
  • $\begingroup$ There is a whole class of "astronomical binoculars" which are optimized for looking at things in the sky (the biggest compromise is that they may not focus close). Ads for them tend to give the same kinds of specs as the ones I would buy for bird watching. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jul 8, 2019 at 20:38
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You've been given good answers already. Let me just drive this point home:

When you buy a truck, you might be interested in things such as how much stuff it can carry.

When you buy a sedan for your daily commute, you might be interested in the gas mileage, and the comfort.

When you buy a race car, all you care about is top speed and acceleration.

Same with binoculars and telescopes. Manufacturers emphasize the things that are important for each category.

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    $\begingroup$ The other answers clearly state that binoculars and telescopes are used by people with different needs. Your lengthy analogy doesn't seem to add anything to that -- it's just another example of different products being designed for different things, and the vehicle analogy gives no insight into optics. $\endgroup$
    – David Richerby
    Commented Jul 8, 2019 at 14:26

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