Polarized filter question

timk519

I've got a circular polarized filter which has enabled me to take some shots that would be impossible otherwise, but every now and then I'm running into some incurable casting problems - like what's illustrated in the following image:



Obviously the water shouldn't be purple like that, and I have no idea what's causing it, or how to fix it. Is this a situation where the filter shouldn't be used at all?

Philip Goh

I'm not sure if it's my eyes, but I don't see anything wrong with that scene. The water looks rather black to me, not purple.

adbsgicom

I think there is a blue/purple cast to the waterfall, but I don't know if it is the circular polarizer, or just the way light is coming off the falls.

timk519

I'm running this on a calibrated screen and I'm seeing a purple / magenta shift in this pic - as well as a number of other pics I've been processing.

What monitor are you using?

timk519

adbsgicom wrote:

I think there is a blue/purple cast to the waterfall, but I don't know if it is the circular polarizer, or just the way light is coming off the falls.

I'm seeing the blue/purple cast in a number of other shots taken at different perspectives, this was posted as an example of what I've been running into.

Philip Goh

I'm on a laptop that's been calibrated with a Spyder device. I guess I can see a slight purple hue to the water, but it's not very obvious. It could be my screen though.

timk519

Philip Goh wrote:

I'm on a laptop that's been calibrated with a Spyder device. I guess I can see a slight purple hue to the water, but it's not very obvious. It could be my screen though.

I'm running the same thing here.

Here's another sample of what I'm seeing:

adbsgicom

Not all filters are created equal. Did you get a good quality polarizer? I pulled the image into PS to look the historgram, and you definately are clipped in blue out the chute in the waterfall, but only blue. Don't suppose you have any w/o the polarizer on as reference in terms of cast.

Try an experiment and shoot a neutral colored wall or greycard or something with and without the polarizer. See if the color balance changes. I'll try the same experiment w/ my polarizer as well in a couple of minutes.

Edit: that said I shot a bunch of pictures at a pumpkin patch this weekend w/ the polarizer on and didn't have any blue cast...

Edit #2. So I shot raw and just brought the images into Camera Raw 5.5. When shot in shady light, the blacks are pure, and as you get more white, the blues start to pull out, which makes sense since it was shady. So looking at getting my grey target neutral, I had to go to about 8000K +18 magenta with the polarizer, and 8800K +8 without, which indicates that there was more blue in my non-polarized image. Interestingly, if I select auto for the WB, the system picks the same White Point for both even though I can tell the non-polarized image is definately bluer. So this didn't help much since this is totally opposite of your results....

By just doing a White Point adjust in Camera Raw I got:
waterfall_1.jpg

timk519

adbsgicom wrote:

Not all filters are created equal. Did you get a good quality polarizer? I pulled the image into PS to look the historgram, and you definately are clipped in blue out the chute in the waterfall, but only blue. Don't suppose you have any w/o the polarizer on as reference in terms of cast.

For the $ I paid for this, it better be good! It's a Hoya "Super Quality" filter.

It's done an outstanding job on some other pictures - like this bridge where it completely gets rid of the glare and you can see right to the bottom of the stream:



but I think it's altering other colors in the image while it's doing that.

My main thought right now is that there are times when this filter's not a good match for the shooting conditions which is leading to sub-optimal results, but I'm not sure what those conditions are.

adbsgicom

I edited my post above.

Also looking on the Hoya site:
http://www.hoyafilter.com/products/hoya/coatings.html

Blue gets reflected more off the front, which matches my findings (polarized version was less blue). Perhaps all that green in there made your AWB want to push your Red/Blue (i.e. purple) too far.

timk519

adbsgicom wrote:

I edited my post above.

Also looking on the Hoya site:
http://www.hoyafilter.com/products/hoya/coatings.html

Blue gets reflected more off the front, which matches my findings (polarized version was less blue). Perhaps all that green in there made your AWB want to push your Red/Blue (i.e. purple) too far.

I've seen this behavior in a number of different shooting conditions, so I'm not sure it's the green in the sample scene I posted. If the problem is that the blues are getting blown because the source light's too bright, and the WB system is incorrectly compensating as a result of an imbalance in the incoming colors, then that's what I need to know in the future. This filter is good, but it has it's limits and now I have a better idea what those limits are.

Thanks for your research and help!

adbsgicom

This was shot w/ a Hoya polarizer the other day in a corn maze:

20091018-153.jpg

This is all SOOC, not twiddling in LR/CS.

timk519

adbsgicom wrote:

This was shot w/ a Hoya polarizer the other day in a corn maze:

This is all SOOC, not twiddling in LR/CS.

Very nice!

adbsgicom

Thanks.

If you are using their standard coating, you'll have a percent or so loss in green (per the graph), so that could also factor into the purple cast. Re the pic: He's a friends little boy. Loves to have his picture taken though (much more so than my daughter).