Exposing for Raw Capture Instead of JPEG
When most film photographers, especially those shooting negative film, switched to shooting digital they quickly found out that they had to be much more careful about exposing their digital files. That’s because many of them began their digital experience shooting JPEG. When they overexposed by as little as a half-stop important highlight detail was often completely lost. Because of this, there was a knee-jerk reaction to begin underexposing digital files. Though underexposing does protect the highlights in a JPEG file, it causes another problem - it reveals noise. When a file is underexposed to protect highlights, it’s necessary to lighten the file in post-production to compensate. This lightening reveals any noise in the shadows that normally would not be seen in properly exposed shadow regions.
Raw is different than JPEG
As many of these photographers graduated to shooting raw, many of them continued to intentionally underexpose to protect highlights. In fact I’ve heard respected photography teachers tell their students to do this. However, raw files are very different than JPEG files. They not only contain a huge amount of information, but due to the linear nature of a raw file, much of it’s capture bandwidth is dedicated to the highlights regions. In fact, with a standard raw capture 50% of the file’s information is used to record the brightest stop. To better understand this, take a look at the graph below.
The linear nature of a raw file means that as each stop gets darker, 50% less information describes it.
This graph shows the six stop exposure range of the typical raw file with a total of 4096 distinct tones, ranging from black to white. Stop #1, the brightest stop, contains 2048 tones, or 50% of the total tonal bandwidth contained in the file. Stop #2 contains half as much as stop #1, which is 1024. This halving of tonal ranges continues with each successively darker stop.
As you can see by the histogram in the graph, this file was intentionally underexposed using the JPEG exposure philosophy. This results in half of the information available to describe this file not being used because it wasn’t used to describe highlights that weren’t captured. To make matters worse, only 1.6% of the file’s bandwidth is used to describe the darkest shadows.
This is a problem because when less information is used to describe shadow detail, noise becomes more of a problem. This noise is accentuated when the histogram is moved to the right in post-processing to lighten, compensating for the underexposure. If the file had been properly exposed, (moving the histogram one stop to the right), not only would all of the bandwidth have been used to capture the highlights, but the shadows would have moved one stop to the right where twice as much information would have been used to describe them.-
But there’s more!
Because so much bandwidth is used to describe the highlights, it’s possible to overexpose a raw file by quite a bit and still recover highlight detail in post-processing. The two photos below show a barn that I photographed a couple of years ago. When I shot the original photo (top), I forgot that I had a two-stop exposure bias set on my camera. When I realized this later, I didn’t bother to go back and shoot the barn because I knew that I was shooting raw. (I also knew that I could go back and shoot the barn again if necessary.) The second image shows the processed file with all of the highlight details. If this image had been captured using the JPEG file format, the highlights would have been toast. Additionally, because the photo was darkened overall, the shadows tones were compressed, helping to hide any noise.
The blown-out highlights in the original (left) were recovered during post-production (right).
Is this extreme overexposure the best scenario? No way. It’s always best to nail an exposure. However, if you’re going to fudge your exposures when shooting raw, be sure to overexpose a bit rather than underexpose. That way you’ll know you have plenty of highlight information to work with and you’ll leave noise where it belongs - in the dark.
