The main goal of any eyepiece design is to get all the light rays to form a sharp image. The difficulty of pulling it off depends on the f-ratio of the telescope. Telescopes with low f-ratios require more highly corrected eyepieces because the cone of light entering the eyepiece is converging more sharply. If you wear glasses, you probably need at least 15mm and preferably 20mm of eye relief to see the entire field of view. Finally, the optical design determines the size of the field of view you see.
Eye relief is extremely short and the apparent field is small.
It gives sharp, bright images at low to medium powers. They're good, low-cost performers, far superior to simpler Ramsden and Huygenian designs. A 40mm Kellner is an inexpensive way to get very low power on most telescopes. Orthos have excellent sharpness, color correction, and contrast, and longer eye relief than Kellners. They are especially good for planetary and lunar observing.