Stills To Motion

LOG AND GAMMA

One of the features of the Canon EOS C500, and most digital cinema cameras, is a Log recording mode. Unlike RAW, Log recording isn’t available in most still cameras and requires some additional explanation. Digital sensors are composed of photosites, which act like buckets for gathering light. The more light that hits the photosite, the fuller the bucket and the higher the output value. These photosites react linearly to light that hits them, meaning they’re 1-to-1 with the light gathered. This is often called “scene-linear.” All sensors basically work this way, but it’s very different from how we see things. We humans aren’t linear, you see. We make dark things brighter and bright things darker. This is the basic idea of Log, as well as traditional gamma curves. They’re designed to scale the linear data from the sensor to make it more like how we see, and also to get rid of unneeded data. Log curves are designed to retain the full dynamic range of a sensor while reducing the amount of actual data stored. Gamma curves effectively do the same thing, though their origins are based on television reproduction.

We call Log and Gamma “curves,” and that’s exactly what they are. We can visualize a Log and Gamma curve in Photoshop’s Curves tool.

An example of a scene-linear image. The image is dark, but exposed correctly. ABOVE, RIGHT: The same image with a curve applied, adjusting the linear data of the sensor to obtain a usable image.

In this image (above), I have a scene-linear image open. I created this image from a Sony RAW clip, exported out of Sony’s RAW Viewer. As you can see, the image is very dark, but I assure you it’s exposed correctly. A linear response to light is very different from our own.

The same image with a curve applied, adjusting the linear data of the sensor to obtain a usable image.

Now, in the other image (above), you see the same image with a curve applied. I just drew in the curve, so it’s far from perfect, but it gives you an idea of how both Log and Gamma curves work. They adjust the linear data of the sensor to get a usable image. Yes, I’ve lost something along the way, but for the most part, that’s data in the shadows that I don’t need.

If I were to compress the dark linear data (think JPEG compression of MPEG4), it would be very hard to get anything out of it later. However, if I compress the image with the curve on it, then I have a full image to work with in post. This is the idea of both Log curves and Gamma curves; they both make the image more like how we see it and also make it much easier to compress. Log is just a special curve that’s designed to preserve the full dynamic range of the sensor. Most cameras don’t even give you access to linear data for this reason; they apply a curve to the image before compressing or outputting. Canon RAW, for instance, in both the C500 and still cameras, has a curve applied to it before being recorded.

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