High Index Plastic Lenses: Abbe Number and Color Aberration
Abbe number is a very important and often-overlooked lens quality that is particularly relevant for high index plastic lenses.
Abbe Number and Color Aberration
A general rule of thumb when it comes to lens materials is the higher the index, the denser the lens. As lenses become denser and more efficient at bending light (which is what high index means), you start to get some undesirable side effects of the dense material. One is reflections, which is why it is important to get anti-reflective coating on high index lenses. The other is color aberration, which is the extent to which the lens blurs or “rainbows” colors, especially in the periphery.
- Abbe number is a lens’s rating for color aberration. The lower the Abbe number, the worse the aberration is. Most lenses have an Abbe number between 30 and 90, with high index lenses generally falling between 30 and 60.
- Color aberration is the extent to which a lens blurs colors. We’ve all seen the rainbows that are produced when white light passes through crystal and diamond. These rainbows are a form of color aberration; the crystal or diamond is so dense that it bends light in slightly different ways depending on the wavelength, or color, of the light. This happens to a lesser extent with many high index lenses.
- In general, you can assume that, the higher the lens’s index, the lower its Abbe number will be, and therefore the worse its color aberration will be.
- High index glass is infamous for having the worst color aberration properties, to the point that many people find they cannot wear glasses with high index glass material.
- The high index plastic that is generally agreed to have the best balance between refractive index and color aberration is high index 1.70. This plastic’s color aberration is relatively low and its index is high enough to make most prescription lenses thin and light.
- Aberration is something that some people can get used to and others can’t. It is a form of blurriness that streaks colors as opposed to lines.
- Aberration is always worse in your peripheral vision than when you’re looking straight ahead.
- Aberration in high index plastic is most common with 1.74 high index.
- The thicker the lens is (the stronger your prescription is), the worse your aberration will be. Higher index lenses do make the lens thinner, but their aberration will still be worse than slightly thicker, lower index lenses.
Most people who get regular high index plastic lenses notice little to no color aberration. It is most common in 1.74 and all glass high index. If you are considering getting a very high index lens (above 1.70), it is worth considering whether you can deal with aberration. In extremely high prescriptions, color aberration can get bad enough in certain lenses (especially high index glass) that the optical clarity and usefulness of the lenses is compromised. If you are unsure which index is right for you, read some of our other articles, and hopefully you will find the lens you’re looking for!