CPL filter messes up focus

[Bristolian]

Hi all,

I have a Hoya CPL filter (this one http://www.jessops.com/online.store/products/75694/show.html) which fits both my 18-105 and 70-300 lenses. Yesterday I noticed that when the filter is fitted to the 70-300 the image through the viewfinder looks out of focus and indeed the picture taken by the camera is OOF. It's quite possible this is the first time I've used it with this lens.

Today, just to be sure it's the filter causing the problem I checked the focus with and without the filter on both lenses. Weirdly the problem only manifests itself on the 70-300

Anyone have any idea why this might happen ?

 

[odd jim]

If I put my 77mm cpl on my siggy 70-200 it massively overexposes! I think it may have something to do with the setup of a longer lens, though I could be way off...

Or was it my nd grad, can't remember. Either way putting a filter of the longer lens messed things up!

 

[HoppyUK]

Long lenses only look through a small section in the centre of a filter, so any imperfections are greatly magnified. You would never notice it with a standard focal length or wide angle lens, but it's not uncommon with lower quality filters. I'm a bit surprised it should be so noticeable with even a cheaper Hoya though, unless you've got a poor sample (possibly even a fake).

 

[Bristolian]

If I put my 77mm cpl on my siggy 70-200 it massively overexposes! I think it may have something to do with the setup of a longer lens, though I could be way off...

Or was it my nd grad, can't remember. Either way putting a filter of the longer lens messed things up!

Oddly enough I only bought an el-cheapo CPL for my Nikon 70-200 but that's fine - lots of good shots with it yesterday

 

[Bristolian]

Long lenses only look through a small section in the centre of a filter, so any imperfections are greatly magnified. You would never notice it with a standard focal length or wide angle lens, but it's not uncommon with lower quality filters. I'm a bit surprised it should be so noticeable with even a cheaper Hoya though, unless you've got a poor sample (possibly even a fake).

Hmm, interesting thought. If this is a fake then Jessops will have some explaining to do Having said that, it wasn't a cheap filter and I read some good reviews of this one before I bought it. On the 18-105 lens - which it was initially bought for - it's absolutely fine.

What I find hard to fathom is that if I can see it's OOF in the viewfinder how does the camera's AF system think it's in focus?

I wonder how it focusses in live view? That'll be a test for tomorrow - watch this space

 

[HoppyUK]

If it's from Jessops, then very unlikely to be a fake but poor quality samples do get through. There have been examples posted on here, and I've tried them myself. I have also seen focus-shift issues, but that only shows up if you focus without the filter in place and don't refocus when you put it on.

The camera shouldn't have too much trouble focusing as it just goes for best sharpness, not necessarily perfect sharpness.

I would take it back.

 

[Bristolian]

I've just completed some testing and thought I'd share the results with you. Throughout the camera was mounted on a tripod and focussed on the same object about 2m from the camera focal plain. All tests were done in aperture priority mode at 70, 135 and 300mm focal lengths and f/8 - shutter speeds varied accordingly.

1. without any filter - focus if good
2. with the Hoya filter - boo hiss :razz: rubbish :thumbsdown:
3. with Cokin filter mounted in ND holder - focus is good
4. with Hoya filter using live view - same as 2 above :thumbsdown:

I extended test 2 by rotating the filter by 90/180/270 degrees but this made no difference to the poor focus.

Conclusions: The combination of this lens with this Hoya filter is not good. If I need a CPL filter to use with this lens then it looks like I'll have to use the Cokin one or take the lens with me when I buy another one.

Jessops won't replace the filter as I've had it for 18 months and, to be fair, I would probably take the same stance myself were I in their shoes.

Thanks to all that responded to my original post - much appreciated, as always

 

[Trig's]

I extended test 2 by rotating the filter by 90/180/270 degrees but this made no difference to the poor focus.

I had a Sigma 100-300 f4 DG EX APO & stuck a cheapie (ebay) CPL filter on it & done exactley the same thing - althou the focus was slightly better when I rotated it but not as sharp as it would've been with out it...

BUT - I now have a Nikon 70-200mm VRII & stuck a B&W CPL filter to the end of that and IQ is cracking nice blue skies & colours really stand out... So I would say that its the filter, prob some one swapped it with a fake ebay jobbie - You never know.....

 

[f1charlie]

Perhaps your Hoya is actually a linear polariser rather than a circular - I thought that was why you had to have a circular polariser because a linear affects autofocus.

Edit: just reread your post and see that only one lens is affected. I have a 18-105 and 70-300 and a Hoya CPL, and I haven't tried the CPL on the 70-300. Maybe I should!

 

[POAH]

since a CPL will reduce the amount of light getting to the AF sensor what are your subjects and exposures. it maybe that there is not enough light to AF properly.

is it one shot or servo focus you are using?

 

[hollis_f]

Jessops won't replace the filter as I've had it for 18 months and, to be fair, I would probably take the same stance myself were I in their shoes.

It makes no difference how old it is. Get back to Jessops and explain that the filter is not 'fit for purpose' and that you want it replaced under the sale of goods act. According to that you have 5 years after purchase to complain and get a replacement.

 

[Bristolian]

since a CPL will reduce the amount of light getting to the AF sensor what are your subjects and exposures. it maybe that there is not enough light to AF properly.

is it one shot or servo focus you are using?

This filter drops the light by about 2/3 of a stop. When I noticed the problem it was a blindingly bright day (last Thursday) and I was focussing on wildfowl (ducks, geese, etc) so had pretty reliable contrast to aid with AF.

During the tests I conducted today (see above) I was focussing on a clock face - white background/black hands - with excellent contrast.

I was using single servo focus both times

 

[seriousrikk]

Can you get it sharp using manual focus in live view?

 

[Bristolian]

It makes no difference how old it is. Get back to Jessops and explain that the filter is not 'fit for purpose' and that you want it replaced under the sale of goods act. According to that you have 5 years after purchase to complain and get a replacement.

I know all that but I believe in taking a reasonable view rather than a strictly to the word of the law one. Anyway, as the filter works perfectly well on both my 18-105 and 28-75 lenses (I've tested the latter since posting the test results) it's kinda hard to say it's not fit for purpose.

Thanks for the comments though