Paul Furman wrote:

I purchased an old 300mm f/2.8 with a flawless front element (looks
beautiful). The seller had gone out & bought a new NC filter for selling
it because his old one was trashed and that's how he worked, he assumed
the buyer would want one. So I use it because it seems so exposed & hard
to protect without the monsterous lens shade attached... though I wonder
if it effects image quality.


No-one has ever produced comparison shots that showed any
deterioration in image quality when using a good quality filter. So
don't worry.

I have done my own careful comparisons with modern lenses and found no
discernible difference.

What I have seen is comparisons where older lenses (with poor coatings
by modern standards) were used with uncoated filters. The flare was
certainly significant.

Moral of the story: If you want to protect your investment in quality
modern lenses without having any discernible effect on your images,
always fit a top quality multicoated UV filter.

From a practical point of view, avoid Hoya HMC multicoated filters
because they are extremely difficult to keep clean. I strongly
recommend B+W, Heliopan and Nikon brand filters. They can all be
found discounted on eBay.