The only filters I tend to use are my Canon CPL and an x8 ND solid.  Other than that I have never thought it a good idea to cover good optical glass with maybe not so good filter glass.  I did relent when I purchased my 17-40L and my 70-200L IS though and purchased two Hoya HD Protector Filters, one for the 77mm lens and the other for the 67mm lens.  Why did I do this?  Well, to complete the weather sealing of the 17-40 it needed a filter (why can't Canon seal the whole lens without having to resort to a filter on the front end?) Can it really be that difficult to work out how to seal the whole lens, what with their level of technical expertise?   The 67mm filter was purchased to protect the front element which is frighteningly close to the end of the barrel.  And before you tell me I should always use the hoods for protection - I do!  And not just for lens protection, but for better colours, and deeper saturation.

Right, now on to the problem with the 77mm filter!  After a short trip out I decided the filter and lens should be cleaned and prepared everything for the task of doing so.  When I angled the filter at around 10 degrees from the plane to look for marks, there appeared a perfectly circular spot.  I then noticed a couple more and proceeded to clean the filter as one does.  They wouldn’t budge however.  On further investigation it turned out that they were tiny imperfections in the glass (air bubbles probably) and not on the surface at all.  Maybe they wouldn’t be noticeable on the image, but I couldn’t live with a filter like that myself. 

I checked the date of purchase and realised it was almost a year to the date.  Fortunately for me it was inside the warranty date by just 10 days and so the filter will be changed.  How lucky is that?

The moral of this story is to check our lenses more thoroughly than many of us tend to do.  And because we trust a particular manufacturer to provide top quality workmanship, we take it for granted that everything will be OK.  The truth is, some duds will always get through the net, at our expense if not vigilant, so keep an eye on your gear, and check it thoroughly after each trip out.