Why do zoom lenses need more IR focusing correction at the wide end of the lens?

["Rolland Elliott" <rolland_elliott@xxxxxxxxxxx>]

Just a question out of curiosity. I'm not an optical engineer, but I am 
curious, Why do zoom lenses need more IR focusing correction at the wide end 
of the lens than the telephoto end?  I have about 5 or 6 zoom lenses and all 
of them have this characteristic. In fact the Sigma 70-210 f/2.8 APO lens I 
have has a slight focus shift at 70mm and by 210mm it doesn't even require 
an IR correction for focusing.

Anyone have any insight. I'm curious because compared to prime lenses the 
reverse seems to be true. Telephoto prime lenses require significant 
correction while wide angle prime lenses require very little IR focusing 
correction.

Peace Rolland
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