CPL focus problem

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hepburn

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Wonder if any folks could give me some help?

I have a sigma 70-200 f/2.8 and it's taking some cracking shots. No complaints there. But I was given a 77mm CPL and whenever I attach it the camera focuses but out of focus - if that makes sense? Take the shot and you get a blurry photo...

The CPL looks like a cheap and cheerful one and as it cost me nothing i'm not fussed if it's a throwaway job. But I would like to know why it reacts the way it does. And before some wise alek says it, yes i've cleaned it ;)

Any ideas? and any suggestions for a replacement?
 
I know this'll seem insulting but honestly it's not... are you 100% that it is a CPL? Only I was convinced mine was a CPL until I discovered otherwise (same issue) and couldn't put my finger on the problem.
 
A reasonable quality CPL will swallow 1.5 to 2 stops of light. A poor quality one may take even more. I suspect that your cheapy CPL is putting your body beyond it's reliable AF limit (f/5.6?)

Bob
 
i have a Nikon 70-300vr like that takes good sharp images without a filter - stick a filter of any type on it and its crap... I just don't bother trying now.
 
Canon Bob said:
A reasonable quality CPL will swallow 1.5 to 2 stops of light. A poor quality one may take even more. I suspect that your cheapy CPL is putting your body beyond it's reliable AF limit (f/5.6?)

Bob
Click to expand...

I think you hit the nail on the head Bob. I've been playing with various settings and your theory appears to be the case. I suspect it was an el-cheapo ebay purchase but don't know for certain...

ps: Adsayer, yep, it says CPL on the ring. :)
 
i had the same problem whislt on holiday in Kenya - luckily i checked the images! Found the shots i took with my polariser were soft @ shutter speeds in excess of 1/2000. i tried it without the filter and it was so much clearer.

it was a cheap filter as well
 
Are you sure you have a Circular Polarisor and not a Linear one?

As that could explain the focus isses?
 
puddleduck said:
Are you sure you have a Circular Polarisor and not a Linear one?

As that could explain the focus isses?
Click to expand...

All it says on the ring is CPL and 77mm. It looks like a CPL, rotates like a CPL, and darken/lightens like a CPL, so I'm fairly sure it's a CPL...
 
mine say so as well 'cpl' its on my desk at work as a coaster!
 
hepburn said:
All it says on the ring is CPL and 77mm. It looks like a CPL, rotates like a CPL, and darken/lightens like a CPL, so I'm fairly sure it's a CPL...
Click to expand...

With all due respects, just by looking at it (ignoring the label), how does a CPL look any different from a Linear Pol? My old linear pols (from way before AF existed!) rotate just like CPLs, darken/lighten skies just like CPLs and reduce reflections off non metallic surfaces just like CPLs.

It's possible that at some point the actual filter has dropped out of the ring and been put in backwards. This would leave the filter behaving like a linear pol, since the "randomising" layer would affect the incomming light rather than the polarised light. Try rotating the questionnable filter in front of a known-to-be-good CPL - if the questionnable one is a CPL, the darkness won't be altered - if it's a linear pol, the darkness will increase to almost blackout. If the darkness does increase, try flipping the filter and see if that makes a difference - if so, turn it over in the mount - but beware sharp edges...
 
Nod said:
With all due respects, just by looking at it (ignoring the label), how does a CPL look any different from a Linear Pol? My old linear pols (from way before AF existed!) rotate just like CPLs, darken/lighten skies just like CPLs and reduce reflections off non metallic surfaces just like CPLs.

It's possible that at some point the actual filter has dropped out of the ring and been put in backwards. This would leave the filter behaving like a linear pol, since the "randomising" layer would affect the incomming light rather than the polarised light. Try rotating the questionnable filter in front of a known-to-be-good CPL - if the questionnable one is a CPL, the darkness won't be altered - if it's a linear pol, the darkness will increase to almost blackout. If the darkness does increase, try flipping the filter and see if that makes a difference - if so, turn it over in the mount - but beware sharp edges...
Click to expand...

Thanks for your input, some good advice there! (y)

I tried what you suggested with a smaller known good cpl and with the 77mm cpl facing the correct way, as it would on the lens, rotating it seems to present either green/yellow or blue hue. Reverse the 77mm cpl and it's a constant dark, although not entirely blackout. That would suggest the filter has been correctly assembled and not reversed?
 
There's something a bit odd there. At a particular point (well, 2 points) of rotation of the reversed CPL, the image should black out completely since the polarisation should be effective in all directions. You'll have to turn the outside (or inside but not both) filter to see the darkening effect.

It does sound like the 77mm CPL is correctly assembled though. Have you tried using the 77mm filter in front of another lens to see how that copes? How does the viewfinder look with the filter fitted - ie does the image look sharp? Could it be that the slower shutter speed that the drop in light is causing is giving you a shutter speed that's too slow to handhold?
 
Nod said:
There's something a bit odd there. At a particular point (well, 2 points) of rotation of the reversed CPL, the image should black out completely since the polarisation should be effective in all directions. You'll have to turn the outside (or inside but not both) filter to see the darkening effect.

It does sound like the 77mm CPL is correctly assembled though. Have you tried using the 77mm filter in front of another lens to see how that copes? How does the viewfinder look with the filter fitted - ie does the image look sharp? Could it be that the slower shutter speed that the drop in light is causing is giving you a shutter speed that's too slow to handhold?
Click to expand...

Funnily enough i've just finished doing that test and it seems better on my 24-85 f/3.5 lens. So i'm wondering if it's just at odd with the Sigma?

Yesterday I was using a monopod and the viewfinder image was as out of focus as the shot, yet the camera beeped to indicate it had focused. I've now tried it in m/av/tv/p modes at different speeds and apertures, with and without flash. Still the same result on the sigma. Weird...

I have a friend with some canon 77mm lenses and i'm hoping he will test it for me. Not sure if he has a cpl himself, but if he does I'll ask to try his on my Sigma. That may help shed some further light. Unfortunately he lives some distance away so it might be a while before we can get together to do this.

In the meantime if there's anyone in the Wycombe area with the right gear who fancies helping out...?
 
If it is a cheap polariser you will be getting refraction and internal reflection as well - so the image is being distorted on its path to the sensor. Your lens is being fed poorer information than when the filter is off - a bit like sticking cling film over your lens (only that would be MUCH more exaggerated, but you get the drift).
 
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