An eyepiece is a multi-lens system that allows us to see things from both far away and close up. An eyepiece is therefore suitable for activities such as astronomy and medical research. The focal point of an eyepiece is miniscule, which helps make the picture parallel. This is vital for defined focus on planets and micro-organisms.
For any viewing device with an eyepiece, a maximum magnification factor applies. When this is exceeded you'll get a blurry result. As such, this factor plays a crucial role in getting an object into defined focus. The magnification factor can be calculated by dividing the lens' focal distance by that of the eyepiece. So if this is, say, 500/25, the factor will be 20. The magnification rule of thumb is that it must not be more than twice the diameter of the lens. So, if we have a lens measuring 50mm, we can maximally magnify 100x. When buying an eyepiece it is important to take this maximum magnification factor into account.
The number of lenses an eyepiece comprises determines its functionality and performance. As such, we differentiate between eyepieces with two (Huygens), three (Kellner), four (Plössl) or more lenses. The more lenses there are, the more definition can be spread over a larger field of view. Meaning several objects can be observed in the one picture. In addition, high quality lenses can correct image aberrations, distortions and colour errors. A 60 degree viewing angle is par for the course nowadays.
Rapid technological developments to eyepieces over recent years have resulted in many changes. One option with some eyepieces is to directly forward captured images to a hard drive or laptop, meaning they can be instantly saved or viewed. Brands that closely monitor such developments are, e,g, Bresser and Nikon.