# How could I translate a field of view value into a magnification value?

When I zoom in with Stellarium, it indicates a field of view (FOV) value in degrees, but most binoculars and telescopes are advertised with value like "nX magnification power."

How could I translate this value so I get an idea of what I will see with a telescope or binocular?

For example, I if got a 30X telescope, how much should I zoom to get similar view?

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Different telescope and binocular eyepieces have different fields of view, so that there is no direct relationship between magnification and field of view.

Eyepieces range in apparent field of view from 30° to 110°, typically being in the range of 50° to 70°. For any given eyepiece, you can calculate the actual field of view by dividing the apparent field of view by the magnification. Thus a 30x eyepiece with a 60° apparent field of view will show you an actual field of view of 60°÷ 30x = 2°.

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The magnification on a telescope can be calculated using its focal length divided by the focal length of the eyepiece you are using, this means, using a 150mm focal length scope with a 5mm eyepiece would give you roughly 30x but also a 1200mm FL telescope with a 40mm eyepiece will.

If you can get the values of aperture, focal length and the eyepiece focal length, you can use this values in the plugins section of stellarium to configure it to the approximate FOV you will experience, but the image in stellarium will never be exact to what you see in a telescope or binocular, as the view depends on many factors, as light gathering, optical aberrations, design of the telescope, collimation and atmospheric conditions.

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There is no direct translation between FOV (Field Of View) and Zoom value (or magnification, in other words). In the astronomy lab I used to teach, we had the students focus their classroom telescope first on a parking sign in the distance. I explained to them that magnification was how big a particular letter on the sign was, while Field of View is how many letters can you see.

For a given telescope, the FOV and the magnification are inversely proportional- if you make each letter twice as big, of course you're only going to be able to fit half as many inside your field of view. But for different telescopes, or even different eye pieces that aren't specifically designed to give the same viewing area*, there's no way to compare, other than a general guess that a really big magnification means a really small FOV.

By the way, never ever buy a telescope that has its magnification advertised as its primary selling point. They're selling cheap merchandise gussied up with improper accessories. It's like the Christmas trees that they spraypaint green. Go for light-gathering power, or aperture.

*Viewing area? That means not how many letters can you see in the image, but something more like, can you cover the image with a penny, nickel, or quarter?

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This answer is a bit vague and uses imprecise terminology like "zoom" and "magnitude." – Geoff Gaherty Jun 17 '11 at 23:56
Zoom is a direct quote from the original wording of the question, and the magnitude was a typo- now corrected. The asker is clearly a beginner, so I deliberately avoided technical terminology that would most likely be lost anyway. What's the problem? – Andrew Jun 18 '11 at 1:51
I removed my down vote, as per your edits. – Geoff Gaherty Jun 18 '11 at 3:27