To get the most from your scope you need a quality eyepiece (EP). Decent views can be had with EP’s costing less than $100 though oculars in that price range won’t be much improvement on what got shipped with the scope. Factors to consider are field of view (FOV) - how much of the sky is shown; exit pupil - the diameter of the opening you look through; eye relief - how close to the lens your eye needs to be and; before all else, magnification.

     Magnification is the place to start because carefully planned increments of power can minimize the actual number of EP’s you need, and that equates to money you can spend elsewhere. Essentially, with low, medium, and high power capability you can cover most situations and, since many people own binoculars with nice wide FOV’s, I would suggest that low eyepiece magnification be 40x to 60x; medium would be 80x to 120x; and high is in 160x to 240x range. Though 240x may not seem that high seeing conditions, particularly turbulence in the atmosphere, have the tendency to degrade views magnified much more than 300x.

     To calculate the power of an eyepiece in a given scope simply divide the focal length of the scope by the focal length of the eyepiece.  If you don't know those numbers, offhand, read on to find out how to figure that out:

Eyepiece focal length is easy - it’s generally marked right on the side (ie) 21mm, but if the focal length of your scope is not inscribed somewhere it can be determined by measuring the distance light travels from the primary objective to the eyepiece - simple with a Newtonian or refractor but more difficult with a Cassegrain design. EP’s focal lengths are in millimeters so it’s best to use metric measurements for all your calculations. A good rule of thumb is that 1” = 25mm.

     Now, using my Newtonian reflector as an example:
• Light travels 48” from mirror to EP, so the focal length is 1200mm (48 x 25)
• It has a 6” primary mirror, so the objective diameter is 150mm (6 x 25)
• I give you the objective diameter in case you happen to know the focal ratio of your scope (ie) f8
• By multiplying 150mm by f8 (150 x 8) I know the scope’s focal length will be 1200mm
• So, with a low power EP of 21mm, I should get a magnification of 57.14x (1200 / 21)

     A good way to get more value from your ocular collection is to use a Barlow to effectively double (or triple) the magnification of any eyepiece while maintaining its lower power (larger) exit pupil. Though you should invest in a good Barlow, the cost will be much less than any $400 Pentax or Nagler you might buy.

     To finish this example I’ll tell you that in addition to binoculars I have a 21mm EP, a 7mm EP, and a 2x Barlow. This gives me the following power selection:

Binos = 10x
21mm = 57.14x
21mm + Barlow = 114.28x
7mm = 171.42x
7mm + Barlow = 342.84x (for those really good nights)

     As you can see, a wide range of power in regular graduations can be had for minimal investment with just a little planning and a little math.