[Leica] Filters part 2

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

There has been some interest in the effect of filters in degrading the
rendition of very fine details of pictures taken in more normal and not so
critical situations. On the assumption that the filter is really good, the
degrading effect is quite small, but not alway negligeable. The problem here
is that there are so many different circumstances and lenses and filtes etc,
tat any atttempt at generality would have to fail. Let me give you personal
experience. I made many testpictures from a street scene on tripod with
several apertures and lighting conditions. In itself a very boring test:
take a picture, add filter take picture, remove filter adjust aperture take
picture etc. Then I looked at the set of pictures and could not see any
difference that would be valid given the statistical variance of the
conditions. Projection tests showed a small drop in the rendition of very
fine detail, but again, small exposure variations could easily account for
this difference too in a number of pictures.
To quantify, but do not see this as the last word on the topic, but as a
intermediate result, I would say that the drop in resolution is less than
10%, so in stead of 77 linepairs/mm, you get 70. Or to use another
comparison: if you overexpose by more than 1/2 stop from the ideal exposure,
the very fine details are gone too.
I have tried quite earnestly to find situations where the degrading effect
could be easily visualized (apart from the conditions mentioned i my earlier
posts). On axis it is almost impossible and in the field  you may see it
when using the wider angle lenses.
I keep trying to find a simple setup to visualize the effects in a
repeatable way, but till now I hav not found it.
Theory predicts a degradation, but in practice it is not so easy to see it.
Shooting obliquely into a strong light source with a filter is easy and
gives the desired proof: flare and secondary images etc. and loss of
definition.
In other situations the effect is much more subtle.


Erwin