Chromatic Aberration
Chromatic aberration is probably most easily understood as a prism effect. When light shines through a prism, you see that different colors of light bend (undergo Refraction) differently and create a rainbow effect. When light shines through any lens, including glasses and the natural lens of your eye, the same occurs to a lesser degree. The result of this is that the Focal Length of the lens is not exactly the same for all colors of light. For the human eye the focal plane of red light (long wavelength) and blue light (short wavelength) are about 1.5 Diopters apart due to chromatic aberration. This causes Myopic defocus to look different than Hyperopic defocus. This difference is acting as a signal to control emmetropization[1][2]. Notably, cameras are engineered with special lens arrangement to minimize chromatic aberration.
Duochrome test
KH | HK |
---|---|
EPO | OPE |
The chromatic aberration expected during a normal eye exam is about a half diopter difference between red and green. This can be a handy test to dial in a final prescription by looking at text on red and green backgrounds and seeing which is clearer.
External Sources
Lens Material
Different materials have different Abbe numbers. Materials with higher values are better, since they tend to have less chromatic aberration.
Here are some typical values:[3]
Material | Index | Abbe Value |
---|---|---|
Crown Glass | 1.523 | 59 |
High Index Glass | 1.60 | 42 |
High Index Glass | 1.70 | 39 |
Plastic CR-39 | 1.49 | 58 |
Mid Index Plastic | 1.54 | 47 |
Mid Index Plastic | 1.56 | 36 |
High Index Plastic | 1.60 | 36 |
High Index Plastic | 1.66 | 32 |
Trivex | 1.53 | 43 |
Polycarbonate | 1.58 | 30 |
MR-174 | 1.74 | 32 |
Tradeoff Between Chromatic Aberration vs Distortion
When choosing lens index, remember to take into account overall distortion at the edges of the lens. Too much overall distortion may make it difficult to view the edges of the lenses.
Red: Do You Need High Index Lenses?
The following vendor articles may be of interest:
Anecdotally from the forums, -3 is a balanced cutoff range for CR-39 where one can take advantage of higher abbe and competitive cost, without experiencing too much distortion at lens edges. Outside of that range, consider higher index lenses to minimize distortion.
References
- ↑ https://www.nature.com/articles/s41598-022-26323-7 Myopia: why the retina stops inhibiting eye growth
- ↑ https://www.sciencedirect.com/science/article/abs/pii/S0014483522002676?via%3Dihub Chromatically simulated myopic blur counteracts a myopiagenic environment
- ↑ https://www.allentownoptical.com/abbe-value-interpretation/