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Why do my photographs end up out of focus when using a CPL filter?


roscoedj 10 2 Australia
2 Nov 2010 3:07AM
Hi,

I am using a Canon EOS 1000D with the standard kit EF-S 75-300mm 1:4-5.6 III lens and whenever I use a CPL filter my photographs are out of focus when I am at the higher end of the zoom.

I have tried 2 different CPL filters but the results are the same.

A standard UV filter causes no problems.

Is this a "feature" of the Lens or is it something else causing the problems?

Cheers,

Ross
mattw 16 5.2k 10 United Kingdom
2 Nov 2010 6:09AM
The Polariser filter does cut out 2 stops of light - so the exposure will need to increase by 2 stops to compensate.

If you hold the ISO and aperture constant, this means the shutter speed will become 4 times longer

(for example, a shutter speed of 1 sec will become 2 sec if you add 1 stop, and 4 seconds if you add 2 stops to the exposure).

So m first though is that you may be seeing some camera shake softening you images at the long focal lengths.

To begin with, lets check the settings from the 'soft' shots. What were the following setting when you took the shot (You can find them in the EXIF data

Focal length of the lens
ISO
Aperture
Shutter Speed

Also, did you shoot from a tripod?
cheddar-caveman 17 1.2k England
2 Nov 2010 7:50AM
Always wondered, why do people spend big money for a decent lense and then spoil it by putting inferior bits of glass in front of it?Sad Modern lenses are already coated, and many already have a protective glass on the front, by design, so you don't need to add moreSmile
Big Bri 19 16.6k United Kingdom
2 Nov 2010 8:11AM

Quote:Always wondered, why do people spend big money for a decent lense and then spoil it by putting inferior bits of glass in front of it?Sad Modern lenses are already coated, and many already have a protective glass on the front, by design, so you don't need to add moreSmile


They don't come with built in polarisers......
roscoedj 10 2 Australia
2 Nov 2010 8:47AM
G'day Matt,

Thanks for getting back to me.

I went back through the EXIF on the blurred pictures and typically they had the settings below or very close to it.

Focal Length = 300mm
ISO = 400
Aperture = F7
Shutter Speed = 1/400

Pics taken at a shorter Focal Length (75mm for instance) worked fine, even with an Aperture of F32.

Cheers,
Ross
Big Bri 19 16.6k United Kingdom
2 Nov 2010 10:42AM
It's possible that since at the longer end, your maximum aperture is smaller (ie. 5.6 rather than 4.5), adding the CPL loses you 2 stops, so the autofocus will struggle or not work. I suggest you stick the camera on a tripod to test out the "camera shake" suggestion.
mattw 16 5.2k 10 United Kingdom
2 Nov 2010 10:46AM

Quote:It's possible that since at the longer end, your maximum aperture is smaller (ie. 5.6 rather than 4.5), adding the CPL loses you 2 stops, so the autofocus will struggle or not work. I suggest you stick the camera on a tripod to test out the "camera shake" suggestion.


Yes, and to test out Big Bri's suggestion, try obtainging focus before you put the filter on (remember to turn AF off after adding the filter)
roscoedj 10 2 Australia
2 Nov 2010 1:04PM
Hi All .. thanks for the suggestions.

I'll try the tripod first .. that's easiest!

I'll update once I've done it.

Cheers,
Ross
31 Mar 2011 3:17PM
Ah ha. These forums are great. I was looking for this very sort of answer. Excellent Grin
Cheers
Lou
31 Mar 2011 4:34PM
simple camera shake seems unlikely at shutter speeds of 1/400 sec. what were you shooting? something moving fast?
Grampy 11 507 71 England
31 Mar 2011 4:58PM
I know this sounds basic but sometimes it's difficult to see if a filter is free of oil from a fingerprint badly cleaned.
Phill
geoffash26 16 2.5k United Kingdom
31 Mar 2011 5:24PM

Quote:Hi All .. thanks for the suggestions.

I'll try the tripod first .. that's easiest!

I'll update once I've done it.

Cheers,
Ross



4 months no update.... Wonder if the problem was sorted after all the suggestions?
31 Mar 2011 6:17PM
bugger... hadn't noticed that Sad

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