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-   -   focus issues with polarising filter (https://e-group.uk.net/forum/showthread.php?t=17701)

ianbirkett 10th November 2011 10:15 AM

focus issues with polarising filter
 
Hi

I've been taking some landscape shots with a Hoya circular polarising filter attached. Usually in really good clear light. When I've looked at them on my computer sometimes they aren't as pin sharp as ones I've taken without the filter. The filter is brand new, very clean etc.

Is it just me or has anyone else had a similar experience. I wondered whether the filter might make the AF system not quite so reliable.

Cheers

Ian :confused:

Tordan58 15th November 2011 11:15 AM

Re: focus issues with polarising filter
 
Could you share some samples for us to look at?


/Tord

ianbirkett 19th November 2011 02:12 PM

Re: focus issues with polarising filter
 
Hi, Thanks for your reply. In my gallery I've posted 2 images, the one titled Leighton Moss is out of focus, I can't see any part of the image that is crisp. The other titled Constantine Bay, also shot using the polarising filter is crisp. I thought that maybe when you twist the outer ring of the filter it might disrupt the Auto Focus system in the camera. Any ideas welcome, Thanks

Nick Temple-Fry 19th November 2011 02:50 PM

Re: focus issues with polarising filter
 
It might be easier if you included the exif data from the shots, and details of the lenses used.

Whilst I'm not particularly keen on polarising filters I have used them on lenses attached to the E-PL1 without issue (effectively the same technology as the E-P1). I wouldn't expect that rotating the ring would be likely to have any effect (other than altering the light levels), though I wouldn't recommend doing it whilst the camera is actually focussing.

Without the exif I do wonder if this is acually an issue of dof/aperture/focus point, rather than related to the filter.

Nick

David M 19th November 2011 03:36 PM

Re: focus issues with polarising filter
 
Quote:

Originally Posted by Nick Temple-Fry (Post 141692)
Without the exif I do wonder if this is acually an issue of dof/aperture/focus point, rather than related to the filter.

Nick

Or camera shake.

ianbirkett 19th November 2011 05:00 PM

Re: focus issues with polarising filter
 
The image data for the blurred shot of Leighton Moss is:
1/100 sec at f6.3
42mm
ISO 200
I was resting the camera against a wooden platform so I doubt very much whether camera shake will be the issue.

For the one the other shot of Constantine Bay the image data is:
1/200 sec at f5.6
14mm
ISO 100

Cheers!!

Daveart 19th November 2011 09:07 PM

Re: focus issues with polarising filter
 
Hi I think I know the problem, you say you were resting the camera against a post, did you have IS on or off, if it was left on, then the IS would create some motion giving a blurr effect, the same as if mounted on a tripod with IS on would give this motion blurr, as their would not be enough motion for the IS to reduce so would create some.

If that makes sense.

Dave

Nick Temple-Fry 19th November 2011 10:08 PM

Re: focus issues with polarising filter
 
Well, it's difficult to say on the small images we post in the group, but the one titled Leighton Moss seems sharpest in the areas closer to the camera, I'd hazard a guess that the focus point was nearer than 60 ft away, so your depth of field wouldn't have included objects either close to the camera or in the far distance.

The one titled Constantine Bay well the hyperfocal distance would have been about 10ft (which if you focussed on it) would give you anything from about 4ft to infinity as sharp.

But I may be totally wrong (a good exif viewer will tell you - probably - how far away the camera thought the focus point was)

Nick

ianbirkett 20th November 2011 06:16 PM

Re: focus issues with polarising filter
 
Thanks for all the ideas. I have IS switched on by default, I'll try switching it off and see if that makes a difference.

Regarding the DOF comments, I remember the viewfinder telling me it was focusing on the mid-ground rather than foreground. Also, looking at the high-res images the foreground isn't sharp either.

Nick Temple-Fry 21st November 2011 12:33 AM

Re: focus issues with polarising filter
 
Quote:

Originally Posted by ianbirkett (Post 141896)
Thanks for all the ideas. I have IS switched on by default, I'll try switching it off and see if that makes a difference.

Regarding the DOF comments, I remember the viewfinder telling me it was focusing on the mid-ground rather than foreground. Also, looking at the high-res images the foreground isn't sharp either.

I suspect you haven't warmed to the dof argument, fair enough, it may be wrong. But if it was confirming focus around the centre point, which was for argument 40ft away, then everything under 25ft would be oof, as would anything over 110 ft. Which would give an appearance similar to your Leighton Moss shot.

On the Constantine Bay, a centre point focus would be around the boundary between grass and the more distant beach, quite likely on the edge of the grass. At 14mm F5.6 then if that edge was 15 foot away then everything from about 5 ft to the horizon would be sharp. A 14mm lens goes 'hyperfocal' at 5.6 just below 8ft, which gives you dof of about 4ft to infinity.

Nick

ianbirkett 21st November 2011 10:25 AM

Re: focus issues with polarising filter
 
Nick, Thanks for your advice. I'll certainly keep an eye on dof.

I'm still getting used to the E-P1 and I must admit I probably haven't really been paying close enough attention to the apeture/dof, apart from when I've actively been trying to get very blurred backgrounds.

I don't want to sound like an old man but I've got to say pre-digital was probably easier when you could see focal range on outer of the lens.

Thanks again *chr

ianbirkett 1st December 2011 11:46 AM

Re: focus issues with polarising filter
 
Hi. Thanks to all for the advice.

Firstly, I've tried a number of test shots on a tripod with IS switched off, this seemed to have done the trick regarding the overall lack of sharpness issue.

Also, I've been experimenting with DOF and noticed when composing a shot the display doesn't accurately show DOF, previously I'd been relying on the preview image too, so I'm much more aware of this now.

2 problems solved with one post - thanks again.

I was thinking of buying the Magic Lantern Guide: Olympus E-P1 to find out what else I might be doing wrong.

Ian


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