# Can't focus with CPL at 200 mm?



## docphysics (Oct 25, 2009)

I have a Nikon D90 with the 18-200mm VR lens.   With a UV filter on the lens it is super crisp in focus from 18-200 mm.   But when I put a CPL filter on the lens it is very crisp in focus from about 18-100 mm and from 100 mm - 200 mm it won't focus, even in manual.  Is this normal, anyone else ever have this problem?    Is the filter just a total POS?  Its a Crystal Optics filter.   Thanks.


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## KmH (Oct 25, 2009)

At what aperture, and using which focus point(s), mode?

Check out page 55 of your D90 User's Manual because there are situations difficult for AF to perform well. 

I've never heard of Crystal Optics.

Brand names associated with good filters are: Lee, some of Hoya's, B+W, Heliopan.

UV filters add nothing to an image made with a DSLR, but they can subtract.


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## Stosh (Oct 25, 2009)

Definitely not normal.  To see if it's changing your focus point, zoom to 100mm (or the max you can still focus), focus on something at infinity, and note the position on the lens.  Then remove (or install) the CPL and focus again.  If it's in exactly the same spot, it's not changing focus.  If it's not changing focus, I have no idea what's happening.


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## tirediron (Oct 25, 2009)

I've never heard of Crystal Optics either.  I'm guessing a rebadged no-name POS filter to be brutally honest.  Don't forget that your lens on the long end only has  a maximum aperure of f5.6; that filter is adding somewhere between 1.25 and 2 stops, so your effective aperture might be something between f8 and f11, typically beyond the limit at which Nikon's autofocus will work.  You might fair better with a higher quality CPOL, but likley your best bet will be to compose, lock focus, (or manual focus) and then put the filter on.  When you say it won't focus in manual, what exactly happens when you set the camera to manual focus?


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## docphysics (Oct 25, 2009)

tirediron said:


> I've never heard of Crystal Optics either.  I'm guessing a rebadged no-name POS filter to be brutally honest.  Don't forget that your lens on the long end only has  a maximum aperure of f5.6; that filter is adding somewhere between 1.25 and 2 stops, so your effective aperture might be something between f8 and f11, typically beyond the limit at which Nikon's autofocus will work.  You might fair better with a higher quality CPOL, but likley your best bet will be to compose, lock focus, (or manual focus) and then put the filter on.  When you say it won't focus in manual, what exactly happens when you set the camera to manual focus?



OK, thanks.    When I set it in manual, and adjust forward and backward, to the point where something SHOULD be in focus, it simply is never sharp.   I can see the point where its at its "best" focus, but it still blurry.   I can try to post up a pic of what I am talking about.   But sounds like I should just throw the thing out and get some other brand.   Strange though, the UV filter works great!


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## DennyCrane (Oct 25, 2009)

I got the same thing today. It was a Sunpak CPL... i.e. a "cheapie". It's great under 200mm, at 300... same deal. Couldn't focus.

Ya gets what ya's pays fer.


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## Derrel (Oct 25, 2009)

Is it possible that the light loss caused by the polarizer is dropping your lens to an effective light level that's too low for the camera's AF system to focus? I mean, with a lens that loses aperture as the focal length increases, once zoomed to a fairly long setting the aperture is f/5.6, and so with the loss of an f/stop to maybe 1.5 stops' worth f light, the effective aperture if f/8, or less...

Maybe that's the problem, if I am properly undersatanding Denny's situation.
As far as docphysics' problem--maybe the filter is really badly aligned. I had a cheap polarizer 25 years ago--it ruined many shots for me on its first day...that was the day I bought a quality Nikon polarizing filter that cost 4x more than the cheapie. Never a problem with the "good" polarizer.


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## DennyCrane (Oct 25, 2009)

I'm manually attempting to focus, and so is Docphysics... it's not an AF issue. I'm convinced it's a cheap filter issue.


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## docphysics (Oct 25, 2009)

Yea, that filter must be a total POS.   Here is the BEST that I or the camera could focus at 200 mm.  Weird!!!   and there was plenty of light so the ap. was fine.


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## JayCanon (Oct 25, 2009)

Hello, docphysics.
 If it makes you feel any better, you're not alone.  I have actually experienced the _exact_ same problem that you're having.  A few months ago, I bought this Dolica UV filter/CPL combo for my Canon 28-200mm lens:  Newegg.com - DOLICA CF-K72 72mm UV and CPL Filter Kit - Camera Filters
Many of the name brand polarizers in the 72mm size can cost well over $100.  This one cost $15.  The filter had a good polarizing effect and worked _somewhat_ well at shorter focal lengths, but like you, I had major problems when attempting to focus at lengths of 100mm and up - regardless of manual or auto focus.  

Shortly after I bought the filter, I posted essentially the same question as you on another forum.  I got a number of responses with suggestions for different things to try to achieve proper focus.  Nothing worked.  

The bottom line is you're exactly right when you say the filter is a "total POS".   The out of focus images are not the fault of you, your camera, or your lens.  You get what you pay for.  Buy a quality polarizer kiss the focusing problems goodbye.


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## DeadEye (Oct 25, 2009)

I would trash the CPL. 

I have seen a few things that bring the texture of the ground glass into the view finder as well.  Glass over 400mm and macro bellows. Battou has posted images of this effect somewere. Its only a finder issue though, the image capture is crisp as the mirror flips up and removes the ground glass from the image path.


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## tirediron (Oct 25, 2009)

docphysics said:


> Yea, that filter must be a total POS. Here is the BEST that I or the camera could focus at 200 mm. Weird!!! and there was plenty of light so the ap. was fine.


   BIN.  THAT.  FILTER!


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## DennyCrane (Oct 25, 2009)

To be fair, at lower focal lengths, my cheapie Sunpak is fine. Terrific, in fact, for pictures of creeks and waterfalls. But yes... better glass I'm sure will make a difference.


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## docphysics (Oct 25, 2009)

Yea, time to trash it and kick out some extra $$$ for another.


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## Derrel (Oct 25, 2009)

That looks suspiciously like up and down camera movement... I pulled the photo into Photoshop, but Picassa has stripped the shutter speed information. What was the shutter speed?

Curious about the labeling (most filters are made cheaply and marked up 600 to 2,000 percent at retail, after having been sold cheaply as OEM products to whatever retail concern that buys a truck-load of cheap filters) I Google searched on the label.

I found this Crystal Optics circular polarizer + UV filter kit on-line for...wait for it....$14.99 in 58mm size! CROCPL58KT | 58mm Circular Polarizer Filter Bonus Kit - with UV, Pouch, Lens Cap, Cap Keeper | BuyDig.com


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## docphysics (Oct 25, 2009)

Shutter speed was 1/400s


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