[Leica] Internal focusing

["Erwin Puts" <imxputs@knoware.nl>]

Is colour fringing in out-of-focus areas the unavoidable artifact of
internal focusing was the question and one Lugger noted that this is BS,
without any argumentation. The topic, however, can not be discarded so
lightly. On the assumption that the original person used the lens in near
focus distance, as this is where internal focusing is designed to improve
optical performance, the question than becomes: is there a longitudinal
chromatic  error that can be increased in the unsharpness area, due to the
close focusing. As chromatic errors are not related to magnification in a
lens and only to the (apo)chromatic correction, it is doubtful that this
phenomenon can be increased by internal focusing. When the angles the rays
of light make are quite large, higher order aberrations however might
introduce some longitudinal chromatic error and if the cams along which the
internal lens group moves are not adjusted properly there again is a source
of error. Theoretically all of these options are to be discarded for this
phenomenon and practically too there is nothing to support the original
assertion. It is however an interesting question and while the straight
answer is a plain NO, you need some reflection to bring the arguments for
this NO.

Erwin