Re: [Leica] Degradation with filters

[Guy Bennett <guybnt@idt.net>]

>...Here I will keep it short and note that the image degrading effects of
>filters do concentrate around bright spots in the image (flare and halo
>phenomena) and will be stronger when using lenses of wider aperture and
>wider angle of view and when the object has high overall contrast and
>intensily bright spots.
>[snip]
>Provided the filter is really good and multicoated. Really good means really
>plane surfaces that are very smooth as any irregularitiesdisturb the passage
>of the rays.
>
>Erwin


this has been my exact experience with image degradation resulting from
filter use.

i had just acquired the 35 summicron asph for my new m6ttl, and was still
using a uv filter to 'protect' the front element. one afternoon i attended
a poetry reading in a small, dimly lit café and shot some frames of the
poet as he read his work, standing against a dark wall. the sole source of
light was a large bay window to his left (my right), and that light
reflected off a mirror hanging on the wall behind his head. the lens was
wide open, and i used a shutter speed of 1/30 to 1/15.

when i picked up the processed film from the lab i was very surprised to
see that the shots in questions were marred by low contrast exacerbated by
bright blurs appearing across the image plane. i honestly couldn't figure
out what the problem was, as the image quality of the other shots on the
roll was quite good. i later came to the conclusion that the light coming
in through the window reflected off the mirror into the lens, thus
backlighting the subject. some of the light further reflected off the
surface of the front element, bounced off the inner surface of the filter
(a b+w mc uv filter), then passed through the lens to expose the film. i
don't know if this is exact, but that was the (uninformed, perhaps naive)
explanation that i came up with.

whatever the real explanation may be, i can add that this experience, along
with discussions here on the lug, convinced me to not use uv filters, to be
particularly careful when shooting with other, colored filters, especially
with respect to backlit subjects. i have never had the same problem again.

guy