For your information, dispersion has nothing to do with color correction. Some glass has high dispersion, some medium, some low dispersion. Some glasses have high index, some medium, some low. None of these by themselves have any affect on color error.
What one needs to make any kind of lens is at least one high dispersion element coupled to one low dispersion element. generally the high dispersion also has high index, and the low dispersion has low index.
Color correction for any two glasses is usually going to be the same as any achromat.
The only way to get better correction is to use one glass which has a partial dispersion that is equal to or close in value to its mating element, so that the color error generated by the first element is neatly cancelled by the second element. Normal glasses cannot do this. It takes one glass that has abnormal dispersion to match a normal glass so that complete cancellation of color error can take place.
Therefore, it is the partial dispersions of the two glasses that is important, not whether they are low or high. Partial dispersion is the derivative of the dispersion, for those of you who are math nuts.
Look for marketeers to glom onto this term, "Partial Dispersion" in upcoming ads
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Rolando
