Can any Leica user kindly explain what are chromatic and monochromatic abberations?

[thirdwheel]

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Thanks in advance !

[John Z. Goriup]

I quote verbatim from page 82 of the Leica Lens Book, a truly useful and informative brochure published by Leica A.G. in 2003: Brochure order Number (in English) is #91154)....no harm checking to see if it's still available.

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APO-color abberation correction - Every image forming glass element, like lens elements for example, bend light of different colors at different levels of refraction. This results in the fact that not all the light rays from a multi-colored subject point are re-united as a point in the image.This leads to color errors, or chromatic abberation. An outstanding color error correction (apochromatic) is achieved by means of the use of special optical glasses with anomalous partial dispersion, which results in increased sharpness of the image. At Leica, APO furthermore means that, already at full aperture, imaging performance is especially high and uniform across the entire picture area..

 

Pictorial examples: Color errors appear primarily as color fringes (Note added: cyan, magenta, purple) around dark objects in front of light backgrounds.With APO correction such color effects are no longer discernible.

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Optical glasses with anomalous partial dispersion - Light rays of different colors are refracted at different rates at a lens surface. This effect is called dispersion and different types of glass have it at different levels of strength. Most types of glass behave in a typical "normal" manner. Special glasses with "anomalous partial dispersion", however, have characteristics for certain ranges of color that deviate from this typical behaviour that permit a special color error correction that cannot be achieved with normal glasses. These glasses are used for the enhancement of imaging quality, which is needed in particular with APO systems

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The brochure also goes into detailed explanation of the term APO, and talks about ASPH, or aspherical lenses as used almost universally by Leica in their latest generation of M lenses. Aspherical lens elements, as defined by the brochure,are described as having "a surface that has a curvature at the edges that is different from that at the center. They make it possible to achieve several correction goals at the same time with a single lens element".

 

Hope this helps somewhat. Incidentally, these are common optical problems besetting all manufacturers of photgraphic lenses, and as you might expect, are far more common in inexpensive, mass produced lenses. The use of polished aspherical elements in Leica lenses is also partially responsible for their cost.

 

JZG

[thirdwheel]

Thanks John, this is helpful. Regards.