Should I use filters?
A hotly debated topic, this one. Any filter in front of the lens will add one additional airspace and two additional surfaces. So by definition image quality should be degraded. How visible will this be? One obvious case: when strong light sources are shining directly or obliquely into the lens + filter, severe flare and secondary (ghost) images will be detected. Even when we are taking pictures in situations where contrast between dark and light areas is very strong, some degradation can be expected.
These effects will also be stronger when we are using the wider apertures. Stopped down the flare will be less noticeable, but the ghost images will still be visible. If this is objectionable to you depends on subject matter and your own criteria.
A filter will be useful for protecting the lens surface. Leica front lenses are hardcoated, but not invulnerable to dust and chemical reactions. So I prefer to use a filter when I am sure image quality is not degraded by its use. In sensitive cases I just remove the filter. In low-contrast situations. landscapes, reportage etc, everywhere when the use of a filter is acceptable from an image quality view and helps to keep the front lens clean and protected I use a filter. When using B&W some filters must be used to get the correct tonal reproduction. (TechPan for instance).
You should realize that the degrading effect of a filter is much lower, in most situations,than using a shutter speed of 1/15 sec. Many Leica users feel no inhibition to use slow shutter speeds, but are afraid to use a filter, bcause ofiits impact on image degradation.
Stopping down to f/11, or using a speed of 1/30 or an inaccuracy of rangefinding aproduce more disaster than a filter (except the cases first mentioned of course). Let us keep the things in perspective and first attack the big causes of image degradation before going to the smaller evils.
(Erwin Puts)
By Joe