[Leica] the filter is in the lens

["Duane Birkey" <dbirkey@uio.telconet.net>]

The obvious difference below is that the filter is in the middle of the
lens... and as such it is more part of the optical formula...  as opposed to
the real life situation of placing a filter on the front of the lens.

I had a Canon lens that had that type of internal filter... The instructions
said to always use it... and I had no reason to doubt their wisdom.... So I
never tried shooting without it.  I figured also figured doing so would be
the easiest  way to get all sorts of dirt, lint and stuff inside your lens
anyhow.

I am not certain that the filter had exactly parallel surfaces either...
which would throw another variable into the mix.

Duane

>Many long lenses take into consideration the presence of the filter, which
MUST be in place in such lenses (e.g., the >400 f/2.8. The effect is not
linear, but depends upon the focal length. Perhaps someone at Leica could
enlighten us.