afocal lens (size lens)
Also called size lens
A lens with no refractive power that still magnifies the retinal image. It is a thick meniscus whose front curve and center thickness are chosen to enlarge the image by a set percentage, without changing the focus1.
Parallel light enters the lens and leaves it still parallel, but slightly spread. The lens works as a miniature Galilean telescope cut from a single piece of glass, with a plus front surface and a matching minus back surface, so its magnification is entirely shape factor2.
More detail
When prescribed this lens, which carries the patient's refractive power as well as the magnification, is called an iseikonic lens, and some dispensing texts use the two names interchangeably4.
Clip-on size lenses of stock magnifications are the trial devices for aniseikonia. Over the eye whose image needs enlarging it should relieve the symptoms, and over the other eye it should make them worse; that pairing confirms that aniseikonia is the cause before an iseikonic lens is made1,3.
Sources (9)
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 19, Aniseikonia
- Schwartz, Geometrical and Visual Optics, ch. 15
- Pickwell's Binocular Vision Anomalies, ch. 11
- Brooks and Borish, System for Ophthalmic Dispensing, glossary
- Chadwick Optical (2024) Aniseikonia Size Lens Kit. Available at: https://chadwickoptical.com/product/aniseikonia-size-lens-kit/ (Accessed: 4 May 2024).
- Edmund Optics (2021) Laser Beam Expanders. Available at: https://www.edmundoptics.com/knowledge-center/application-notes/lasers/beam-expanders/ (Accessed: 4 May 2024).
- Metcalfe, N. (2009) Afocal Deep Sky Astrophotography. Available at: https://astro.dur.ac.uk/~nm/astro/afocal.html (Accessed: 4 May 2024). secondary
- Rutstein, R., Daum, K. (1997) Anomalies of Binocular Vision: Diagnosis & Management, Mosby.
- Zemax (2021) How to design afocal systems. Available at: https://support.zemax.com/hc/en-us/articles/1500005488001-How-to-design-afocal-systems (Accessed: 4 May 2024).
Reviewed October 2026
Related terms
Nearby in the glossary
- adduction
- Adie’s tonic pupil
- adjustable suture
- afocal lens (size lens)
- afterimage test
- alternate cover test
- amblyopia