tech: getting more from RAW

In earlier discussions we’ve looked at video of how to read a histogram and read why RAW files hold more information than JPEG. I’d suggest checking those entries out, if you haven’t already.

To date I’ve stressed proper camera exposure so file manipulation will be reduced in post-production (especially important when shooting JPEG). Speaking in general terms, the less an already processed file (JPG, TIFF, etc.) is manipulated, the more integrity there will be to the information contained within. We’ve seen that RAW files have greater flexibility for manipulation, allowing us to do much more with an image with fewer penalties (and sometimes gains) on output.

Now that we know how to obtain proper exposure, today I’m going to suggest adding a little extra exposure to your RAW files for greater detail. Yep, let’s push those RAW files toward overexposure…but without actually overexposing the image. This is also called giving the histogram a “push to the right”. We may even unlock some extra detail and more subtle tonalities.

Why would we want to do this? Because there is a lower signal to noise (s/n) ratio in the dark (shadow) areas of an image. In other words, shadows are noisier (we really saw that in the examples in RAW vs. JPEG, when we severely underexposed). But if we can add additional exposure to our image then we will lift the dark areas and shadows, record more detail and get less noise. The trick when adding exposure, though, is to not overexpose the highlights, blowing them out. We want to push the histogram to the right without pushing it off of the right. Then, when processing the RAW file, we will adjust the tones so they are where they look the best visually. By lowering the slightly overexposed shadows back down to where they belong we should (theoretically) see less noise and better detail in, especially, the darker areas of the photograph.

So let’s put it to the test. Here are two photographs taken with a Canon 20D (ISO 100, daylight WB). The image on the left was shot on aperture-priority auto exposure and on the other I pushed the histogram further to the right, using exposure compensation to +2/3 stop. You can click on the overall scenes to see them full-sized for comparison.

*Note: your camera’s histogram display doesn’t accurately represent the image’s 12- or 14-bit RAW capture, it represents the 8-bit detail in the image thumbnail. So there may be detail in the RAW capture that is not accurately represented by the camera histogram. In other words, the camera histogram is a conservative measure.

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Canon 20D on auto exposure, resulting in a pretty accurate, middle of the road, real world exposure. This is maybe a touch dark but pretty close to how the subject looked.	On this shot I overexposed 2/3 stop, pushing the histogram to the right a bit. This image is lighter than the actual subject appeared.

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Okay, now I’ve processed the files out in Photoshop CS2 (ACR) so that they look similar overall and have similar histograms. Let’s look at the detail.

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The normal RAW exposure corrected in CS2. It looks good.	The overexposed RAW exposure corrected in CS2. It looks similar to the shot at left, just a hair different.
Here’s where we see a difference, in the details. This is the bottom of the door, a dark area. These are 200% enlargements so we’re really blowing things up. More noise is apparent here in the corrected “normal” exposure.	The originally overexposed image is much smoother in the gradations. At this 200% enlargement the difference that 2/3 stop makes is obvious.

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We do see more detail when “pushing to the right”. Using this technique is especially important when using a smaller camera, say 6 megapixels. You can get more out of the smaller sensor by really maximizing your exposure technique and shooting RAW. I’d dare say you’re better off doing that than having a higher megapixel count and using sloppy exposure technique.

This was a pretty simple visual explanation. For further details and the mathematics behind this process there’s a great entry on Luminous Landscape I highly recommend. As always, I hope this has been a practical help.

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