RAW files and sensor information
I have been working on a simplified digital photography book and am going over the edited text. And I found a common mistake from the technical editor. He said that RAW files capture all of the information from the sensor. This is a very common idea, yet it is misleading at best and incorrect in many cases. It is also a common mistake from the RAW guys who have not actually worked with camera manufacturers. The problem is that it tells you some wrong information about RAW files.
Information from a sensor is analog and is converted into a digital file in what is called the A/D (analog/digital) converter. At this point, the RAW file is changed, both in getting less AND more than the sensor actually captured. It gets less information because analog data is infinite and digital is finite, so some information is lost as it is converted. It gets more information because proprietary processing in the A/D converter will affect noise, color and tonal information, and camera manufacturers are pretty proud of what they do here. Some manufacturers even do something called “preconditioning” where they process the analog data coming from the sensor before it is converted to a digital file, making that signal better.
A JPEG file actually comes from the RAW file. It is not, as is sometimes implied, simply a reduced RAW file with important data thrown out. It actually represents the smart, in-camera, automated processing of a RAW file. Like all automated processing, it has its advantages and disadvantages, but a big advantage is that a photographer doesn’t need to do the processing him or herself.
The point of RAW is that you get a whole lot more information to work with when processing a photo. A JPEG file ends up with 256 levels of tonal information, while a RAW file has over 4,000. Now if you don’t do much to that RAW file, those 4000+ levels are meaningless, because our visual system doesn’t need them to see a good digital photo. However, that 256 number is close to our minimum. So if you start making some big changes to your original image, you can lose those steps, which can reduce the quality of your photo (as long as those steps are needed for the image).
With RAW files, you gain a huge amount of flexibility. You can work hard to bring the most from an image in a program like Camera RAW or Lightroom and get excellent results. You can pull more detail out of dark and light areas. If you need this capability, JPEG will never suffice.
