JonPB Offline Upload & Sell: Off
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I think it helps to know that, by default, lenses produce chromatic aberrations. Consider your basic prism, which takes white light and distributes it into a pretty rainbow. That's the result of dispersion, and all transparent materials that I'm aware of demonstrate some level of dispersion within their range of transparency. Color correction in a lens is a matter of using two or more lens materials that counterbalance each other. In theory, you could have one element that takes white light and makes a rainbow, then a second element that takes the rainbow and turns it back into white light; a single element will never do, but two or more might. Color correction is all about balancing lens types in order to minimize the amount of dispersion exhibited by the system as a whole.
Of course, dispersion is only theoretically independent from refraction, which is how strongly a lens element will redirect light that passes through it. Refraction is what makes a thicker or thinner element attractive, or one with more or less curvature. In practice, available glass types have only a limited range of combined dispersion and refraction characteristics. Use a new glass type to improve its dispersion characteristics and you'll likely change its refraction as well, requiring a whole new calculation for the lens.
But let's play the theory and say that you could retain the refraction characteristics of each lens element while changing their dispersion characteristics. Here, a given lens -- e.g., Zeiss 100 MP -- could retain the same style but have improved chromatic performance. And that'd be great; same rendering style, same optical signature, but with less CA. Even if that's possible, though, the glass will likely cost several orders of magnitude more. If the end product is going to cost two, five, or ten times as much, chances are that other adjustments can be made to the lens that are more cost effective, which could make for a better lens overall but with still some residual CA and with a different optical signature.
Which is to say:
1) Definitely not just adding one element.
2) Theoretically, you don't have to add any elements at all, just swap existing ones with glass types that perform better. This would allow the designer to recalculate for dispersion/chromatic effects only while retaining the same refraction/rendering "stamp."
3) Practically, doing so would be rarely cost effective (if it were, the original lens would have incorporated that design), and a cost effective redesign would change the entire calculation, including both for chroma and ultimately for rendering.
Note that I'm not an optical designer; I didn't even study physics in college, much less optics, much less exercise those theories in practice. I try to teach myself about the subject from time to time, but everything here should be taken with a slab of salt. I hope this helps, though.
Cheers,
Jon
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