The eyepiece is an integral part of any compound visual system. Its purpose is to magnify the real image formed by an objective or intermediate relay system. For this reason, the eyepiece design affects the magnification, image brightness, field flatness, and aberrations of these systems. As in all optical systems, the designer trades one advantage for another, in an effort to optimize the eyepiece for specific applications. In choosing an appropriate eyepiece, one needs to consider the particular system being assembled. Since each eyepiece has individual advantages, a specific system may favor one eyepiece over another. For instance, the RKE® eyepieces are typically integrated into OEM industrial instruments because of their wide apparent fields, long eye-relief and resilient housing. A brief description of available designs follows:
Huygenian: This design consists of two plano-convex lenses with both plano surfaces facing the eye. These offer lateral color correction and diminished coma for a flat field. Not typically used with a reticle because the image plane is between the two lenses. Often used in economical systems.
Ramsden: These eyepieces consist of two plano-convex lenses with the plano surfaces facing away from each other. Lateral color is not fully corrected because the focal plane is external, but this does allow this design to be used with a reticle.
Kellner: This design improves on the Ramsden by replacing the eye lens with an achromat, leading to better color correction. These offer a very sharp, bright field at low/medium powers, but very short eye-relief at short focal lengths (high power). A substitution of a double-convex lens for the plano-convex lens is not uncommon.
Edmund Optics RKE®: Improving on the Kellner design, the RKE® eyepieces offer a wide sharp field, excellent aberration correction and longer eye-relief. They are manufactured in our own optical facilities and are made with the finest high-index optical glasses and anti-reflection coatings.
Orthoscopic: This design consists of a triplet field lens with a single element (plano-convex) eye lens. It offers excellent sharpness, color correction, and contrast. It also improves on Kellner's design by yielding relatively long eye-relief. Orthoscopic eyepieces have moderate apparent fields.
Plõssl: Also known as symmetric eyepieces, they are composed of two achromats (crowns facing each other). Plössl eyepieces offer long eye-relief and a superior field of view to that of the orthoscopic. They do fall short of the orthoscopic design in distortion, however.
Erfle: This design is extremely popular in wide-field applications. Erfle eyepieces usually consist of 5 or 6 elements and offer a long eye-relief but relatively short working distance. The distortion (for similar angular fields) is comparable to that of the orthoscopic.
Eyepiece Selection Guide
| Huygenian | Ramsden | Kellner | |
| Number of Elements | 2 | 2 | 3 |
| Apparent Field | • | • • | • • • |
| Eye Relief | • | • • | • • |
| Distortion Correction | • • | • | • |
| Sharpness | • | • | • • |
| Lateral Color Correction | • • | • | • • • |
| Reticle Adaptable | No | Yes | Yes |
| Common Application | Econ. Systems | Econ. System with Reticle | Binoculars/ Instruments |
| Overall Average | 1.2 | 1.4 | 2.2 |
| Price | Economical | Economical | Average |
| • • • • Excellent | • • • Very Good | • • Good | • Average/Poor |
| Edmund RKE® | Orthoscopic | Plössl | Erfle | |
| Number of Elements | 3 | 4 | 4 | 5 / 6 |
| Apparent Field | • • • | • • | • • • | • • • • |
| Eye Relief | • • • | • • • • | • • • • | • • • • |
| Distortion Correction | • • • | • • • • | • • • | • • • • |
| Sharpness | • • | • • • • | • • • • | • • • |
| Lateral Color Correction | • • • • | • • • • | • • • | • • • • |
| Reticle Adaptable | No / Yes | Yes | Yes | Yes |
| Common Application | Telescopes/ OEM Instruments |
Instruments | High-End Instruments | Wide-Field Inst./Systems |
| Overall Average | 3.0 | 3.6 | 3.4 | 3.6 |
| Price | Average | Expensive | Expensive | Expensive |
| • • • • Excellent | • • • Very Good | • • Good | • Average/Poor |
(Note: For reference only. Data based on general prescription trends and may vary for particular designs.)
If you would like to see other topics covered or more detailed information, let us know. We invite you to discuss any suggestions or specific application requirements with our engineering department at 800-363-1992 or by contacting us online.
