Weird Focusing issue

trolley

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?

MrFotoFool

I have had a B+W F-Pro clear filter on my 500PF since day one with absolutely zero focus issues. I have never heard of Kood until now. I assume this is an inexpensive brand, so maybe there is something in it that doesn't play well with the PF design? I am only speculating, but if I had to guess I would say that something (either slightly distorted filter material or lack of superior coatings) causes an issue with a PF lens that would likely not be an issue in a conventional lens. The reason I bought B+W is I figured there is no point skimping on the filter for a lens that cost me four thousand dollars (by the time tax was added).

Pistnbroke

I always found UV filters deteriorated the image ..maybe its a fake?

trolley

@Pistnbroke I don't see it as image deterioration - just out of focus. But the camera is convinced it's in focus

spraynpray

trolley said:

@Pistnbroke I don't see it as image deterioration - just out of focus. But the camera is convinced it's in focus

In focus or in best available focus?

Pistnbroke

Send it back ..dont waste your time just use a deep lens hood.

trolley

spraynpray said:

trolley said:

@Pistnbroke I don't see it as image deterioration - just out of focus. But the camera is convinced it's in focus

In focus or in best available focus?

I don't know - I haven't asked it

Pistnbroke

Its not weird at all its just a bad filter ..even a good one will deteriorate the image just send it back and relax ..chill ..thats life

spraynpray

trolley said:

spraynpray said:

trolley said:

@Pistnbroke I don't see it as image deterioration - just out of focus. But the camera is convinced it's in focus

In focus or in best available focus?

I don't know - I haven't asked it

So if you zoom in on the back of the camera and focus it manually, can you get it focussed?

trolley

@spraynpray - no.
I tried it over the weekend (still lousy light), and when I look at the images at 100% on ViewNX they still look 'unclear'. Maybe rather than a focussing problem, the filter softens the image?
Also, the lens seems to hunt with the filter in place - it focuses when I hit the AF-button, then jumps off & then back on again

Pistnbroke

I would have thought you would have smashed it to bits by now or sent it back. Its alwasys been my experience that they degrade the image on close examination.

spraynpray

trolley said:

@spraynpray - no.
I tried it over the weekend (still lousy light), and when I look at the images at 100% on ViewNX they still look 'unclear'. Maybe rather than a focussing problem, the filter softens the image?
Also, the lens seems to hunt with the filter in place - it focuses when I hit the AF-button, then jumps off & then back on again

Must be a bad copy. Bin it.

Pistnbroke

Its nice to see that the moderator and myself are in tune ...long may it continue ..ha ha

Paliswe

What about focus when you take the filter off? Are you satisfied with the speed? Hunting? No "unclear" picture?
Once I had a D7000 with a 24-200mm with a clear filter. It focused in front of the object. I had the camera and lens controlled by Nikon but they found no fault. I removed the filter and the problem disappeared. From that day I have never had any filter on my lenses.
The front glass on Nikon lenses are extremely durable. Have never had any issue with not "protecting" the lens other than using the lens hood.

trolley

Everything's fine without the filter. I just traded a lens in & the front element had a lot of fine scratches - which reduced the value a bit. So I thought I'd try & protect this one and make a bit more if/when I sell it

spraynpray

Sounds like your cleaning routine could be the problem. My philosophy is every time I clean the lens I reduce its quality so exercise extreme care and gentleness. Start with blowing then using a clean cloth flick gently , blow again and then use a little lens cleaning solution from a pump finally another clean, dry cloth. I discard the cloths and don't re-use them.

trolley

A few weeks ago I was in a hide with windows when a Marsh Harrier drifted across in front of me. Try as I might I couldn't get anything in focus properly (AF).
And on a birding holiday recently I found a similar thing when trying to shoot AF through a car windscreen.
It seems that my 500PF doesn't like anything in front of it!

Paliswe

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