Go step by step:
Diameter of sleeve
Eyepieces are available with three different sleeve diameters. At your telescope you can use only eyepieces of the same or, via an adapter, of smaller diameter. Please check the insertion diameter at your telescope:
24,5mm (0.96 inch)
31,8 mm (1 ¼ inch)
50,8 mm (2 inch).
24.5mm diameter are to be found normally only on cheap telescopes. It is hard to find good eyepieces of this diameter.
1 ¼-inch is standard on most telescopes. You can choose between hundreds of different eyepieces.
2-inch eyepieces are used only for long focal lengths, as they offer a wider field of view at small magnifications. For small and medium focal lengths use an adapter for 1 ¼“-eyepieces.
Quality of the telescope
The quality of the telescope itself will have an effect on the selection of your eyepieces.
If you own an apochromatic refracting telescope for some thousand Euros than you should not start to save money on buying your eyepieces. Two optical parts come together, and they should fit.
The majority of amateur astronomers owns Newtons, Schmidt-Cassegrains or refracting telescopes of the Fraunhofer-type. Here you can use, as a standard, our Ploessls. They perform very well, and they are not to expensive. If you are willing to give a few Euros more, select a wide angle, may be in 2“. However, also for these telescopes a high-end eyepiece makes sense - but do not expect a big difference at the first look.
Are you owner of a very cheap discounter telescope? Try to exchange your simple eyepieces against a 25€-Ploessl! Possible, that you will say “Oops - never seen before in that way!” when looking on Saturn the first time. Your cheap telescope may be not to bad, but the eyepieces are for sure.
Type of telescope
Here we talk especially about the focal ratio of the telescope. As faster as the telescope is (F/6 - F/5 - F/4), as harder for the eyepiece especially near to the edges. A F/10 refracting telescope will be fine with nearly every eyepiece type, whereas a F/4 Newton needs high-priced, complex eyepieces at least if you look for wide angle eyepieces at longer focal length. Systems with F/8 or F/10 additionally need only eyepieces of longer focal length to achieve the maximum magnification, and also this is an advantage, as eyepieces of very short focal length mostly have problems with contrast and comfortable eye relief.
On our product pages we have added a comment especially on wide angle eyepieces, if they are suitable for a specific focal ratio.
There is no eyepiece, which is suitable for all observations. E.g. to make projections of the sun the best suited eyepiece is a Huygens type. Optical simple, but good for projection, and there are no lenses cemented together - so it is robust against heat.
For planetary observers and viewers of double stars there is no need for a big field of view, a Ploessl or a special planetary eyepiece will be the best choice. The same is valid for condensed deep-sky objects.
For nebula, galaxies and star clusters eyepieces with a big field of view and good contrast are recommended. If you find 70° apparent field of view sufficient or if you would like to have more than 80° - I can not tell you. For this I recommend to have a view to such eyepieces, may be at telescope meetings.
How much money do I have to spend for eyepieces? Hopefully we can ease this decision with our test results as given on the product pages. We recommend to look for cheaper eyepieces if you need longer focal length and small field of view. Such eyepieces are generally of good performance, and there is not really a need for expensive ones. You would like to have a 30mm wide angle for a F/4 Newton? Sorry, but take the Pentax. No way out.
If you are willing to abandon add-ons like rubber eye cap or similar, you can use simple Ploessls instead of Super Ploessls without optical compromises. What you should not do is buying 8€-stuff without reasonable coating. You will spend double money, because the need for better eyepieces will come - for sure.