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DPChallenge Forums >> Photography Discussion >> Polarizer filter and peripheral illumination Qu.
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01/19/2009 06:21:22 AM · #1
Hi

I wonder if someone can shed some light on this image problem.

I went out shooting yesterday and because of the subject matter, most of my shots used a polarizing filter (a 77mm Hoya Pro 1 digital). All of the shots I took with a wide(ish) angle with my Canon 24-105 f/4 L demonstrated a dark area in a semicircle in the top half of the sky (like the lower half of a circle). The lens came with my new camera, an Eos 5d MKII, which has 'peripheral illumination' to reduce vignetting at wide angles. My question is this - might this feature be responsible for over-lightening the corners of the image due to not taking into account the effects of the polarizer? Perhaps the upper part of the image was most affected because it was light compared to the darker ground? Or is this phenomenon something you would expect to see with a polarizer at 24mm?

Many thanks

Paul
01/19/2009 06:57:17 AM · #2
can you post a picture ??
01/19/2009 07:34:39 AM · #3
The set from yesterday was for the pending Ansel Adams challenge but I can take a new image with different subject matter in an hour or so and I'll post it up.

Good idea.

Thanks

Paul
01/19/2009 08:18:08 AM · #4
On a full-frame camera at 24 mm, what you're probably seeing is the "sweet spot" of the polarizing filter's effect on the sky. The sky is darkest at 90 degrees from the sun then becomes brighter towards and further than 90 degrees (looks like a gradiant effect in Photoshop), and at a true 24mm you're seeing more of the sky and will notice the effect more. That's one of the minor drawbacks of a polarizer at wide angles. Luminous Landscape probably explains it better than I can. Congrats on the new camera btw!
01/19/2009 08:59:40 AM · #5
Many thanks - I followed the link you gave me and the effect I was trying to describe is exactly like the one in the image and though it looks OK in colour, it looks less good in B&W, especially when I get a bit funky with curves in Photoshop.

I'll put this one down to experience!

Many thanks for your support.

Paul
01/19/2009 09:27:35 AM · #6
I run into this all the time when I am polarizing with my 10-22mm on the 20D. You can work around it in Photoshop if you want to:

1. Make new empty layer. Set layer mode to multiply and check "add multiply-neutral color (white)" option.

2. In the tool palette make sure default (black and white) colors are selected in color picker, and that black is set as foreground color.

3. In tool palette, choose gradient tool. In tool presets bar, click on the gradient example and choose "foreground to transparent" as your gradient.

4. On the empty, multiply layer draw a gradient straight down from the top in the sky. If necessary, select the sky and mask out everything else first, depends on how level the horizon is.

5. Go to edit menu and choose "fade gradient" and fade it until the portion of sky you want to darken looks about right to you; better leave it a little too dark than a little too light, because you can also fine-tune with layer opacity later.

6. Make another new layer, this time set to "screen" mode and filled with "screen neutral (black)".

7. Now use the tool palette color picker to make white the foreground color, select the brush tool, and select a largish feathered brush.

8. In the tool presets bar adjust opacity on the brush to maybe 25% and start brushing in the areas where your multiply gradient has darkened the already-dark sky too much. Keep brushing around at it until everything looks the way you want it. This step may not be necessary, depends how extreme the polarizing was.

My AA entry uses this technique; it was shot at 10mm witht he polarizer and I had to deal with this effect.

R.
01/19/2009 02:02:34 PM · #7
Robert

Thank you so much for taking the time to share this technique with me. I am a photoshop newbie and I am only on chapter 6 of Deke's book, layers are chapter 7! (though I am a dab- hand with Apple's Aperture!)

I am sure I will learn much about layers by following your advice. I had thought I had finished with my cropped AA but now I might just have a go at putting that full frame to use!

Many thanks again. I'm always so impressed by the kindness of people on this site.

Paul

Message edited by author 2009-01-19 14:04:02.
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