Field of View.
The field of view is important if you want to get an idea of the angular size of the object you are observing.
Three important values are:
1. The focal length of your telescope in mm.
2. The length of your eyepiece in mm.
3. Apparent field of the eyepiece in degrees.
Just enter the values below, click "Calculate the Field of View" and the calculator gives you your answer.
Reference:

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Focal Length:
Every telescope has a stated focal length, which is effectively the distance from the primary lens or mirror to the point at which it forms an image of a very distant object. 
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Length of the your Eyepiece:
Eyepieces have focal lengths, too — 25 or 10mm, for example. 
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Field of view:
Aside from the focal length, each eyepiece has an apparent field of view, measured in degrees (°). This tells you the apparent width of sky, in angular terms, that is presented to your eye — eyepieces with larger apparent fields take in greater gulps of sky than smaller ones. Simpler eyepiece designs tend to have apparent fields of about 45° ; widefield designs may be 60° or more.
What is sometimes more important to the user is the true field of view, which is obtained by dividing the apparent field by the magnification the eyepiece delivers. For example, consider a 10 mmfocallength eyepiece of 50° apparent field in combination with a scope of 1,000 mm focal length. The magnification will be 100x (that is, 1,000/10) and the true field will be 50°/100x, or 0.5° — so the full Moon would just nestle within the field of view.