theSuede
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"how apochromatic correction in a lens is achieved"...? :-)
I some cases, the explanation is really easy. Build a symmetrical lens (optical construction looks the same, only mirrored, on both sides of the aperture - and is generally well corrected for the usual abberations) - and only use it in 1:1 macro work! :-)
Otherwise, the "how" is not something that's explainable in a space as cramped as this, you'd need a good many chapters in a book to make the different ways even remotely explainable. As OlyAcme pointed to earlier (indirectly), the problem lies with all the other corrections the lens has to make. The amount of "bending" of the light has to be assymetrical in/out if you want to be able to focus on stuff thats not at an 1:1 distance from the lens...
Think of the lens (in the most simple, easy way) as two parts - as long as we're talking about primes! - one is leading IN to the lens from the front, one is leading OUT from the lens towards the sensor/film. A symmetrical 1:1 makro lens can be constructed with exactly the same amount of "bending" and surface curvatures on both sides. A lens that's meant to focus on a longer distance from the camera [than the lens - film plane distance] has to use assymetrical constructions. An assymetrical construction bends light LESS on IN-part (focus plane is further away), and MORE on the OUT part (distance to film plane is roughly the same as in the 1:1 example, but shorter!).
This makes the construction off-balance when it comes to many things, and among them is the colour dispersion.The colour dispersion effects (CA of all sorts and orders) are generally not the first on the list of "things to correct" for a lens designer. People generally want "general" sharpness and freedom from SA and distortion before the "almost perfect colour correction" arrives on the agenda.
To put things in perspective, I like to cite this little list of facts...:
*Different glass types will bend light of different wavelengths by a different amount.
*How far the colours are allowed to "separate" geometrically before hitting the next boundary (glass/air or air/glass) is determined by the distance it travels up to that next surface.
*geometries and distances are very constricted if you want "sharp" and "distortionless" pictures with a certain lens formula
*The 125F/2.5 lens has an apparent aperture area of almost 2000mm^2
*A normal pixel in a digital camera has an area of 0.000036mm^2
*The amount of "concentration" of light passing through the aperture then has to be 2000/0.000036 = 5.5 million times if you want a pixel-to-pixel sharp picture in the end...
The CO60 is not a "superachromat" AFAIK, as it only has three zero-passing points in the LoCA chart. The difference is mostly academical though, as the achromatic behaviour of the CO lens is so wide that the difference from "perfect" within the visible light spectrum can indeed be less than that of a "superachromat" lens.
I've only handled a superachromat once, a Zeiss 300F/2.8 for hasselblad. Probably the most expensive singular piece of photographic equipment that I've ever touched... :-) The owner and I jokingly had a conversation about how he would prefer the package to land on the back, and totally destroy the 35k$ HB DMF back, than that the Zeiss should even touch the ground... It's a LOT more expensive (and a lot more rare) than the HB3D it was mounted on.
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