Filters. Can't live with them, can't live (with piece of mind) without them. I have always been a little sus concerning filters. The makers of lenses work hard to perfect their optical formula, then we whack a lump of glass in front and don't expect any degradation of quality.
On he other hand, plenty of tests have proven that they do little if any harm, even if they are cheap, even if they are stack a dozen at a time on the front of the lens*
The 25mm lens that I have been coming to terms with has always left me a little under whelmed. It has solid numbers, a great reputation and has produced for me some excellent images, but there is something un settling.
After a couple of images with strong sun in them coming out a bit halo-smeared, I thought to check the filter on the lens. Sure enough it was a bit dirty. I have cleaned it, but have not put it back on.
This troubles me as I have had a couple of issues with marks on the front of Olympus lenses from spots that would not move, cleaned firmly turning into shiny speck blemishes that look permanent.
Olympus used to be known for their soft lens coatings, but this was a bit of a surprise. All of my lenses promptly got the filter treatment, so taking one off is a bit unnerving.
Unfortunately I did not think to do a test before cleaning the filter, so I tried to "stress" the lens, naked of protective glass this morning.
The issue, if there is one, looks to be from strong sun deflecting off surfaces and edges. Possibly ideal conditions for a polariser? The image below was taken with the suspect filter on, showing little compromise in quality if off angel to the sun.
The big question to me is focussing distance. I have had lenses that excel at one end of the focus range and fail at the other, but not in Olympus. The 25mm has performed well at close distances as a rule, but maybe the issue is with infinity focus? More testing required.