friendly warning re:polarisors

PentaxAmPho · #1 (**permalink**) 13-08-2011, 08:46 PM

I would like to give people a "heads up" about using extra-wide angle lenses and circular polarisor filters. In my case, everything was fine with my Pentax K7 and my SIGMA 10-20mm lens. When used together with my Cokin X-series pro filters - no problems!

Now, I added to my the circular polarisor (X Pro series).

KA-POW!! Waht the hell is happening? I went out on a sunny day, setup all the gear, added the "polly" (circ pol.) and I couldn't believe my eyes ....the sky had uneven patches of blue colour. even went I rotated the polly the patches never went away, only minimised!

I immediately thought that the polly was faulty until I got some input from a pro. Polarised light in the sky is not always constant. I have been reliably informed that when using a polly with an ultrawide lens (on cameras with APS sensors, such as mine) you get a very distorted view of it, the result is
these patches. Be warned (nicely though) I had no choice, I had to buy teh cokin x-pro series becasue of the nature of the sigma lens and that its thread size is 77mm. The polly for that size is approx £250 !!!! so, be very careful before taking the plunge! From all this, I only use my polly when I want to accentuate colour in natural wooodland, forests, waterfalls and surrounding areas - I dont use it in sunlight now.

This doesn't happen on full frame cameras so I believe

Gaz

Alan Hewitt · #2 (**permalink**) 15-08-2011, 04:26 PM

It is a characteristic of ultra wide angle lenses. Something to do with the effect of a polariser being narrower than the ultra wide field of view afforded by the lens itself. Not sure but I think it would be the same on full frame as it is a lens focal length issue. I've tried it with a sig 10-20 on a D300 but can't get a polariser for 14-24 on D700 bit I imagine it is the same.

PentaxAmPho · #3 (**permalink**) 15-08-2011, 04:47 PM

Quote:

Originally Posted by Alan Hewitt

It is a characteristic of ultra wide angle lenses. Something to do with the effect of a polariser being narrower than the ultra wide field of view afforded by the lens itself. Not sure but I think it would be the same on full frame as it is a lens focal length issue. I've tried it with a sig 10-20 on a D300 but can't get a polariser for 14-24 on D700 bit I imagine it is the same.

Apparently, the effect isn't there on full-frame cameras. Andrew Kime (pro photographer) told me that he doesn't get this effect on his kit (top Canon etc). ....but I agree it's basically the polarised light coming through obliquely etc. It's such a shame that I wasn't aware of this BEFORE I bought the polly. ah well.

sometimes · #4 (**permalink**) 02-09-2011, 08:49 AM

I presume you're talking about something other than vignetting?

Jim Ford · #5 (**permalink**) 02-09-2011, 10:34 AM

I think Alan has got it pretty well right.

Natural light is polarised to some extent, depending on the direction it comes from (IIRC North light is polarised the strongest). With a narrow field of view lens the light is coming from a narrow range and is consequently of a uniform polarisation. With a wide field of view, the light is coming from all directions, with different degrees of polarisation. The filter will filter these differing degrees of polarisation to differing extents, resulting in the patchiness the O.P. observed.

There is no reason why a 'full frame' or any other format camera would be immune to this effect.

Jim