Orthoscopic eyepieces have four lenses, of which two are biconvex curved and one biconcave. They offer a high degree of sharpness in the centre as well as at the edge. That's why they are interesting for observing planets and double stars. The advantage: they offer a flat field of view.
The low loss of light is another advantage of these eyepieces. The apparent field of view is approximately 40° – 45°.