Faking a polarizer using RAW

Here’s a question which came in to my email just now:

“Could I process a RAW file in Photoshop to achieve a similar effect as if I had used a Polaroid lens filter?
Or would I be better just using the Polaroid filter?”

The answer is that you can never imitate the effect of polarizing light (which changes the way reflective surfaces look, and deepens or lightens the sky blue according to the zone of the sky relative to the sun’s position. But you can use Adobe Camera Raw (CS3 versions) to deepen skies you never thought could be rescued.

Here’s a shot in lousy light with deliberate over-exposure on the Alpha 200. The exposure has been given for the shadow areas, not the highlights, much as you would with colour negative film. Although digital files are often criticised for lack of latitude and dynamic range, they are far more useful than slide film in conditions like this, and almost as versatile as negative film. But you need to process them correctly, from raw, as very little of use can be recovered from the lighter areas of this in-camera JPEG without poor colour and tone.

So, working from the raw file, one ‘normal’ version is exported and opened in Photoshop. This is adjusted like the JPEG, for good shadow detail.

Next, a second version is exported from the same raw file. This uses some basic adjustments to tone and colour:

The important control here apart from exposure, brightness and blacks all of which shift the tonal range to favour the lighter areas (and make them darker) is Vibrance. A high Vibrance setting will allow you to see changes in greens and blues, especially. We are now going to make those changes in the selective colour HSL tab of Adobe Camera Raw:

First we go to the Blues slider, Saturation tab of the HSL adjustments. It is taken up about half way. You have to judge while doing this.

Next, to strengthen this, the Luminance slider of the Blues is taken down. Again, it’s a matter of looking at the whole image, and judging this. Since you can not select parts of the picture in raw development (unlike Nikon’s excellent Capture NX2 software which allows effects to be applied to sampled ‘similar’ zones) you must be aware of any unwanted shifted blues in the rest of the image, for the next step.

Having opened the adjusted raw in Photoshop, a selection path is drawn. I have highlighted this in red to make it easily seen:

This does not need to be dead accurate. My path took about two minutes to draw with a mouse, not an artpad. This was actually still on my screen when James (thanks!) sent his enquiry about simulating polarizer effects. I was doing some one-to-one training yesterday and grabbed this example to show the benefits of working with raw files.

I have selected the lighter areas – the sky, landscape beyond the bridge, and the sunlit parts of the stonework. In this conversion the shadows are just too heavy though it looks much like a print might do recovered from an over-exposed colour negative.

The next step is to Feather the Selection (found under the ‘Modify…’ menu). I used 150 pixels for a large (5120 pixel wide) image, but that is from experience. You can experiment with values between 50 and 250 for different size images. The resulting Feathered Selection is then Copied (to the Clipboard), and we go back to the original lighter exposure, and Paste. If the selection has touched three sides of the shot, it will always paste exactly in position. In this case, the Move tool was needed to snap it into position over the original picture (Photoshop Layers incorporate a very accurate auto snap-to intelligence).

My raw export was not quite as soft and light as the in-camera JPEG, but even so, you can see how the two exports from the raw .ARW Sony Alpha 200 image have been blended. The result can be controlled by adjusting the Layer opacity for the pasted-in sky tone.

This technique works best with skies, as for many scenes, the Blues slider can be adjusted without any apparent effect on the rest of the image. Sky and water may be the only parts influenced (and not this muddy February river-water!).

So, there is the answer – and it does also work with deep blue skies. Using the Vibrance control and the Blues luminance slider, without much need to touch Blues saturation, you can sometimes adjust a full blue sky to look polarized without any need to blend two exports.

Here is a default export from a Sony Alpha 350 shot on the 16-105mm lens at 16mm:

And here is the same shot with Colour Temperature 4550, Tint 15, Vibrance 50, Blues saturation 40, Blues luminance -20, Greens luminance +40:

I know which looks like the real Abbey (it’s two minutes walk from the office) but I also know which probably looks better in a tourist guide book to sunny Scotland!

– David Kilpatrick