CPL causes unsharpness beyond 200mm per OP magazine

imtheguy · 11-21-2010, 03:43 PM

This was a new one on me. The latest Outdoor Photographer magazine states that polarizing filters can cause "unsharpness" on lenses larger than 200mm. Tech tips, p44. I always use a CPL on my da*300 to shoot surfing.

Would somebody shed some light on the origin of this "tip" for me? Its easy enough to test but I can't imagine its a legitimate concern if you need a polarizer for the shot.

bdery · 11-22-2010, 05:55 AM

It's a tip based on a lot of wind.

General statements about focal length, without regard for the actual lens being used, should be taken warily (as you do). There is no optical reason why this should be true. The effect of adding glass in the light path (which always, to some extend, degrades the image) is independant on the focal length.

Quite the contrary, ultra-wide angles are more likely to have polarizer issues, because the field of view is too large, and the polarizer"s effect will often be uneven.

imtheguy · 11-22-2010, 08:22 AM

Thanks for confirming BD. Next I get to email Outdoor Photographer to request they provide backup/explaination for such a casual comment. They did mention the UW issue too but I think we have all seen that one.

**dadipentak** · 11-22-2010, 10:52 AM

I guess the take-away for me is to think twice about using a CPL: "What's the need and what's the trade-off?" Shooting surfers sounds like a situation in which the benefit outweighs the cost.

Kahuna · 12-01-2010, 04:36 PM

???Since my last visit, over half of this thread has disappeared. Any clues?
Meanwhile, this from Pentax regarding CPL, 55-300mm lens, and serious blur.
"The autofocusing problem you are having with the K-r is probably being caused by the relative low speed of the DA 55-300mm lens combined with the filter factor introduced by a polarizing filter. ... With that lens I would suggest you try autofocusing without the polarizer and then attach the filter to the lens, or use manual focus only with your polarizing filter.

bdery · 12-02-2010, 05:43 AM

The Pentax recommendation makes sense, but if light levels are very low, you should probably not be using a polarizer in the first place.

For the record, ading a rectangular plate (the filter) after focusing will slightly affect the focus position. But as long as you're not trying to do 1:1 macro the effect should be minimal.

reeftool · 12-02-2010, 10:10 AM

Well, you do loose a stop or so exposure so that has to be taken into account with regards to shutter speed and ISO, things that will affect IQ. I don't think a properly used quality CPL will affect the image as long as it's in focus. My AF hunts some and is slower using a CPL, something I expect would be perfectly normal with my Sigma which is f 5.6 wide open at 300mm. I think this is more an issue of improper use of a CPL than saying CPL's don't work on long lenses.

Cuchulainn · 12-03-2010, 07:52 AM

To go back to the OP, I'd expect a polariser to create more problems on a wide angle lens since the range of angles that the lens needs to accept will lead to some spherical aberration from the filter that won't be as strong on a tele lens.

seldredg · 02-08-2011, 03:38 PM

I just tried a PL on a 55-300mm at full zoom, and K10d can't autofocus (sunny daylight, not super bright though). A few test shots determined it would AF from 55-150 or so and then wouldn't.

11-21-2010, 03:43 PM	#1
imtheguy Site Supporter Join Date: Mar 2009 Location: Virginia Beach Photos: Gallery \| Albums Posts: 2,950	CPL causes unsharpness beyond 200mm per OP magazine This was a new one on me. The latest Outdoor Photographer magazine states that polarizing filters can cause "unsharpness" on lenses larger than 200mm. Tech tips, p44. I always use a CPL on my da*300 to shoot surfing. Would somebody shed some light on the origin of this "tip" for me? Its easy enough to test but I can't imagine its a legitimate concern if you need a polarizer for the shot.

12-02-2010, 05:43 AM	#6
bdery Pentaxian Join Date: Mar 2008 Location: Quebec city, Canada Photos: Gallery Posts: 9,357	The Pentax recommendation makes sense, but if light levels are very low, you should probably not be using a polarizer in the first place. For the record, ading a rectangular plate (the filter) after focusing will slightly affect the focus position. But as long as you're not trying to do 1:1 macro the effect should be minimal. Last edited by bdery; 12-03-2010 at 06:09 AM.

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11-22-2010, 05:55 AM	#2
bdery Pentaxian Join Date: Mar 2008 Location: Quebec city, Canada Photos: Gallery Posts: 9,357	It's a tip based on a lot of wind. General statements about focal length, without regard for the actual lens being used, should be taken warily (as you do). There is no optical reason why this should be true. The effect of adding glass in the light path (which always, to some extend, degrades the image) is independant on the focal length. Quite the contrary, ultra-wide angles are more likely to have polarizer issues, because the field of view is too large, and the polarizer"s effect will often be uneven.

11-22-2010, 08:22 AM	#3
imtheguy Site Supporter Join Date: Mar 2009 Location: Virginia Beach Photos: Gallery \| Albums Posts: 2,950 Original Poster	Thanks for confirming BD. Next I get to email Outdoor Photographer to request they provide backup/explaination for such a casual comment. They did mention the UW issue too but I think we have all seen that one.

11-22-2010, 10:52 AM	#4
dadipentak Site Supporter Join Date: Jul 2008 Location: Baltimore, Maryland Photos: Gallery Posts: 11,590	I guess the take-away for me is to think twice about using a CPL: "What's the need and what's the trade-off?" Shooting surfers sounds like a situation in which the benefit outweighs the cost.

12-01-2010, 04:36 PM	#5
Kahuna New Member Join Date: Nov 2010 Location: California Posts: 7	???Since my last visit, over half of this thread has disappeared. Any clues? Meanwhile, this from Pentax regarding CPL, 55-300mm lens, and serious blur. "The autofocusing problem you are having with the K-r is probably being caused by the relative low speed of the DA 55-300mm lens combined with the filter factor introduced by a polarizing filter. ... With that lens I would suggest you try autofocusing without the polarizer and then attach the filter to the lens, or use manual focus only with your polarizing filter.

12-02-2010, 10:10 AM	#7
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,554	Well, you do loose a stop or so exposure so that has to be taken into account with regards to shutter speed and ISO, things that will affect IQ. I don't think a properly used quality CPL will affect the image as long as it's in focus. My AF hunts some and is slower using a CPL, something I expect would be perfectly normal with my Sigma which is f 5.6 wide open at 300mm. I think this is more an issue of improper use of a CPL than saying CPL's don't work on long lenses.

12-03-2010, 07:52 AM	#8
Cuchulainn Senior Member Join Date: Dec 2008 Photos: Gallery Posts: 153	To go back to the OP, I'd expect a polariser to create more problems on a wide angle lens since the range of angles that the lens needs to accept will lead to some spherical aberration from the filter that won't be as strong on a tele lens.

02-08-2011, 03:38 PM	#9
seldredg Inactive Account Join Date: Jan 2011 Location: California Posts: 6	I just tried a PL on a 55-300mm at full zoom, and K10d can't autofocus (sunny daylight, not super bright though). A few test shots determined it would AF from 55-150 or so and then wouldn't.