[Leica] Degradation with filters

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

I conducted a numberof experiments to see if and under what circumstances
the use of a filter could degrade image quality. In theory, when the
surfaces are perfectly plane, the effect would be very small. Note that the
Apo-Summicron-R 2/180 has a permanent built-in filter in front of the glass.
Here we have perfectly plane surfaces and the computation of the lens was
done with the filter effect incorporated.
Generally we can expect some stray light and obviously some secondary
reflections. These latter effects I will neglect for the moment. Stray light
and flare around specular highlights are the general degrading effects when
using filters. These effects are stronger  in situations with high overall
contrast  and strong light sources in the image  and when the lens angle is
greater as then the skew rays are more troublesome to correct.
In a coming issue of LFI there will be a lengthier article with comparison
pictures to show the effect with and without filter.
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.
On the other hand: a longer telelens at moderate apertures and objects
without bright small highlights in dull or overcast weather will not show
any effects at all.
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