Focal ratio (same as f-stop for a camera) = focal length/aperture
Power (aka magnification) = focal length of telescope/focal length of eyepiece
Field of view = Apparent field of eyepiece / Magnification
Apparent field of view is specified on the eyepiece
The 25mm eyepiece that comes as part of the Celestron NexStar 5SE telescope has an apparent field of view of 50 deg.
Exit pupil:
The circular image or beam of light formed by the eyepiece of a telescope. To take full advantage of a scope’s light-gathering capacity, the diameter of an eyepiece exit pupil should be no larger than the 7mm diameter of your eye’s dark-adapted pupil, so that all of the light collected by the telescope enters your eye. (The eyepiece exit pupil diameter is found by dividing the eyepiece focal length by the telescope focal ratio.) Your eye’s ability to dilate declines with increasing age (to a dark-adapted pupil of about 5mm by age 50 or so). For those in this age group, eyepieces with exit pupils larger than their eyes can dilate to simply waste their telescope’s light-gathering capacity, as some of the scope’s light will fall on their iris instead of entering their eye.
Celestron NexStar 5 SE
Aperture: 5 inches (125mm)
Focal length: 1250 mm
Focal ratio: 10
Limiting Stellar Magnitude: 13
With a 25mm eyepiece Magnification: 50x
Highest useful magnification: 295x (4mm eyepiece)
Lowest useful magnification: 17.86x (70mm eyepiece)
Useful links:
Astronomy Terms
Scope Math
New Yorker article on light pollution and the Bortle Scale