Mottled appearance using a circular polarising filter

[martinp2502]

The other day I took this shot while out and about and noticed a mottled appearance for the sky when viewing in Photoshop. This has happened before and I've been able to correct in photoshop but I'm puzzled why this happens. I've reduced the contrast on this image drastically to emphasise the problem. I have a UV filter permanently on the lens and the CP is a Hoya all of which I've checked and are clean.

Any ideas?

[Shadowman]

What steps did you take to remove it? Were you using a lens hood?

[Mike Buckley]

I have also noticed this occasionally. The issue becomes even more noticeable when converting to black-and-white. Years ago I used to keep a UV filter on my lens but I haven't done that for a long time. Though I usually keep a polarizer on when I'm outside, I'm not convinced that's the cause.

I have always wondered if it's some sort of refraction being caused by moisture, haze, pollution or something similar.

[martinp2502]

No John I didn't use a lens hood that day, would this help? I haven't bothered removing this in photoshop I was just curious why these marks appeared.

[Mike Buckley]

I always use a lens hood and this issue still occasionally occurs.

[Dave Humphries]

Hi Martin,

To be honest, I can't actually see the problem you refer to on this shot, but I'm perhaps looking for the wrong thing
Then again, I'd have thought increasing the contrast would show it more, not decreasing it.

UPDATE:
So then I opened it in CS5, cropping to show just the sky, then set the brightness to -100 and the Contrast to -50 and I see this;



Is that (in the blue) what you are referring to?

If so, I believe this to be an interaction between noise and jpg artefacts, at 8 x 8 blocks of pixel resolution.

Are you shooting RAW?
If so (and if I'm correct), I bet you don't see it there (you will see noise in the blue channel of course).
If shooting jpg, that means you're doubling the chances of this happening, because there's the camera's jpg generation, followed by your image editor's jpg generation when you save again. Often this will cause a resampling to happen in a different 8x 8 grid offset and that'll make things worse.


Initially I did wonder whether you had a reflex camera that shot through a semi-silvered mirror (and some optical interaction there caused it), but the EXIF shows Canon 600D, a traditional "mirror flips out the way" model, so that cannot be it.

Cheers,

[Mike Buckley]

I intentionally post-processed it to emphasize the mottling in the sky.

[Dave Humphries]

I intentionally post-processed it to emphasize the mottling in the sky.

Yeah, I think that's the same thing we're seeing there Mike.

[Mike Buckley]

Are you shooting RAW?
If so (and if I'm correct), I bet you don't see it there

I have been shooting RAW for years and I occasionally see it in that format. I have never been concerned about it because the issue is easy to eliminate by applying a little noise reduction to the sky.

[martinp2502]

Thanks Dave. I shot this in jpg as I was just snapping away without any intention of post processing. Think I'll go out when the weather is similar and shoot all in Raw and see what happens.

[Manfred M]

That is "banding" caused by using a polarizing filter on a wide angle shot. The amount of polarization is not constant. When you are shooting directly into the sun or it is right behind you, the polarizer will have no effect, but when you are shooting at a right angle to where the sun is, this effect is maximized. You were shooting at 24mm, so fairly wide angle, the direction you were shooting in made the effect fairly obvious.




It is really obvious in this shot as the sky on the right side of this image is dark while things lighten up as you look to the left side of the image. This shot was taken at an 18mm focal length. The solution is to not shoot using a wide angle setting, backing off on how agressively you have set your polarizer or not using one at all when shooting at wide angles.

[Mike Buckley]

Manfred,

Would you expect the polarizer also to cause this when using a 35mm lens? I ask because it occasionally happens when I use one.

OFF TOPIC: Is your cast finally removed?

[Shadowman]

No John I didn't use a lens hood that day, would this help? I haven't bothered removing this in photoshop I was just curious why these marks appeared.

Martin,

It wouldn't hurt, especially when you are shooting at the time of day as this image. I was considering all possibiiities for the cause of your problem. Do you see the mottling when you first convert the image in camera raw or after it is loaded into the editor?

[Manfred M]

I've generally been okay when shooting with focal lengths of 28mm or longer and in fact the effect is even reasonably subdued with Martin's 24mm shot.

Before:





After:




I didn't like the shot enough to try to remove the issue, but I played with it for a few minutes and reduced the effect a bit. I made a selection of the sky and applied a gradient to counter the banding and used blending modes to integrate the two. I adjusted the exposure and colours a bit as well. Not perfect by any means, but then I only played around with it for 3 or 4 minutes. A bit more tinkering would improve it even more.

[martinp2502]

Thanks for all your comments guys. I think I'll stop being so lazy and use the hood more but also shoot in JPG+RAW and just delete the RAW files I don't want to process.

[Dave Humphries]

I'm still a little unclear which effect was concerning you Martin;
the 'fine detail' mottling or
the 'whole picture' banding (i.e. in your shot, the darkening of sky towards left compared to right)

You used a 24mm lens on a Canon 600D, which gives a horizontal field of 50 degrees (give or take), so as Manfred says, this latter effect isn't particularly significant on your shot - it'll depend which way you're shooting relative to the sun anyway.

It will be most troublesome when the horizontal angle of view approaches or exceeds 90 degrees, because then, whichever way you point, you'll likely have both the maximum and minimum effects in shot
This renders a CPL on an UWA lens almost unusable on a 'blue sky' day, unless you process as Manfred did.

When I shoot using a polariser, especially semi-wide like your shot, I now have the experience to pay attention to the sky and rotate the filter to balance any unwanted effect there against the desired effect on what ever surface I am trying to reduce reflection/increase contrast on (or under).

Cheers,