When speaking about eyepiece designs most astronomers classify the eyepieces in “old but good” designs like the Plössl eyepiece (field 45°) and the “new and large” designs with large apparent field of view, from 60° (a “small” large) to… 120° (definitively a “large” large).

I was surprised, and I bet most of the readers will also be, that a very large eyepiece design existed in WWII for submarine periscopes with an apparent field of view of 120°! The design of this eyepiece (called Koehler eyepiece) was available after WWII and look like this:
Eleven lenses are used inside and the apparent field of view is 120°. The happy customers who found one in WWII surplus and tested it on their telescope are all enthusiasts.

But the Koehler design is still a classical design. Before computers existed and was available to a large public, opticians did calculations by hand. It was a consuming task so the opticians calculated one “formula” for an eyepiece (let’s say 20mm) and other eyepieces were enlargement (40mm) or reduction (10mm) of the “master” 20mm eyepiece.

A classical eyepiece design is a design were all eyepiece share exactly the same design, scaled to the intended focal. By example a 20mm Plössl is exactly the double than a 10mm Plössl, except for the apparent field of view that stays constant. If the 20mm Plössl has an eye relief of 16mm and internal lenses of 18mm; the 10mm will have an eye relief of 8mm and internal lenses of 9mm.
Classical designs have one important drawback: for short focal eyepieces the eye relief is so short that observations are difficult: you can not keep eyeglasses and staying in position so close to the view lens is a tremendous and rapidity tiring task impacting quality of observations.

A modern eyepiece design
involves computer assisted design. Each eyepiece focal length uses an adjusted design, different from all other eyepieces in the same group. Calculating and adjusting the design for each focal length will be a time-life task without a computer.

Eyepieces based on a modern design share the same field of view -but unlike the classical design- keep the eye relief constant. A group of eyepieces based on a modern design will be about the same diameter, with the shorter focal eyepieces being the longer.


As eyepieces based on modern design share the same eye relief short focal eyepieces are very comfortable to use (user can keep its eyeglasses) at the expense of a large and quite heavy construction.