One of the things that initially surprised me when I bought my first telescope, the company was apparent from visual and real (or actual) field.
If you look through his telescope, is the area of ​​sky they see as the real (or real) field. The field of view (FOV) of the telescope is determined by the design of the telescope eyepiece.
To calculate the field you must first find the value of the field. Each eye has its own value of the field, provided by the manufacturer of the eye.
Next, calculate the magnification. To do this, divide the focal length value of the telescope by the focal length of the eyepiece. For example, if your telescope has a focal length of 650 mm (that is the case of a Sky-Watcher Explorer 130 P) and the eyepiece has a focal length of 10mm, the magnification of your telescope 650/10 = 65.
Finally, the apparent field divided by the increase in the actual field / get real. For example, if the eyepiece has an apparent field of 50 degrees and a magnification of 65X, the current field of view 50 degrees / 65X = 0.77 degrees.
If the eyepiece has an apparent field of 55 degrees and a magnification of 65X, the current FOV is equal to 55 degrees / 65X = 0.85 degrees.
Therefore, the larger the apparent field of view, the greater the actual field / real. Note also that the reduction increases, a wider angle to get further increases.
If you do not know what the apparent field eyepiece, you can estimate the actual field / property using the following formula:
42/Magnification
Therefore, for the example above, we 42/65 = 0.65 degrees.
This is only an estimate, but good enough, you get an idea of ​​how much of heaven will try to get through one eyepiece of the telescope.