Weird Focusing issue

trolleytrolley Posts: 190Member
So I bought a Kood slimline clear filter to protect the front element of my 500 f5.6PF.
I had some issues with getting the lens to focus sharply, but Tom Hogan's method using auto fine tune was great.
I was still not convinced that the focus was sharp, but the weather's been really crap here recently & not good light. On Sunday I took some pictures of a Little Egret on a canal & even as I pressed & held the back focus button, I could see in the viewfinder of my D850 that it was way off focus. So I took the filter off & reshot from the same position. Lo & behold it was fine! Distance was about 30-40m
Why would a clear filter affect the ability to focus?
And in case you want to know, yes it was fitted correctly & screwed right in.
And no, it wasn't fitted when I did the AF fine-tune.
Anything to do with the PF element?

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