Any and all filters will degrade lens optical performance very, very slightly. It does pay to buy the most expensive filter you can afford. Too many people make the sad mistake of slapping a $5.00 bottle top on a $3,000 apo lens and wonder how they could manage to "waste so much money on that lens!". Modern filter spectral coatings have reduced light transmission lost to around 1%, sometimes as low as 0.3%. None of this of course is visible in the end result.
If a lens has worn coating, scratches, fungi or whatnot, it's not a given that any particular filter will improve the optical performance; it can't: a filter is just there to alter the characteristic of the light entering the lens. A damaged lens may well produce much more flare with a filter than without because a filter is an additional layer of glass and thus an extra reflective surface.
At risk of introducing another variable into the conversation.
A good polarizing screen - granted difficult to use on your Leicas - will boost the color and BW contrast of any lens.
Most people will pick out the higher contrast photo as being the better photo.
Given that lower contrast is the primary effect of flare, a polarizing screen used with a lens hood should give you the best you can get out of any particular lens.
I am glad that I am not the only one to realize this.
Originally Posted by RalphLambrecht
also, if there are no stupid questions, then, the world must be full with inquisitive idiots.:laugh:
Originally Posted by Gerald C Koch
I have had photos with weird reflections from the filter. I didn't realize how often it happened to me until I went back and looked at hundreds of old pictures. It was over 20%.
I mostly take outdoor photos in sunny/daylight conditions. But I also had a lot of flash photos where it happened.
I also learned that I don't like polarizers either because they kill sparkly highlights like what you get on water.
I seldom use a filter now but I try to always use a lens hood.
Filters won´t reduce flair. For that purpose, a large lens hood or compendium shade is a better recommendation.