What is a RAW File?

Like in traditional film photography, digital photography starts with a latent image, representing the conversion from light to electronic data to be processed by a computer.  This is known as a RAW file.  A raw file allows for maximum control over an image's parameters, normally lost by in-camera jpeg processing.  A raw file contains typically 12 to 14 bits of information, versus the traditional 8 bits of a jpeg.  This results in a minimum of 4,096 levels of brightness in a 12 bit file, as opposed to the 256 levels found in a jpeg. 

Henceforth, image processing with a RAW file assures the retention of the highest amount of digital information, preventing highlights from going white and shadows from going black.   RAW data straight from the sensor is useless for image editing, all raw data requires a variety of processing from the camera's built in computer.  This creates the base image which is then edited in a raw-converter, such as Adobe Camera Raw or Phase One Capture One.  

One of the driving principals behind raw capture is the amount of control one has over the file in post-processing.  White balance, exposure compensation, contrast, and many other image parameters normally permanently applied to a jpeg can all be changed using these various raw converters mentioned above.   All raw files have three key components that every raw converter must address when preparing a file to be viewed by the human eye:  CFA (color filter array) interpolation, white (neutral) balance, and gamma correction.

As a digital sensor is made up of millions of individual photo sites, or pixels, there is an inherent process involved converting this observed light into a file format for viewing, storage, and output.  The most popular method of arranging and configuring these pixels is known as the Bayer filter, developed by Dr. Bruce Bayer of Eastman Kodak.  A pixel itself is incapable of capturing all colors.  Within this filter, 50% of recorded light is green, 25% is red, and 25% blue.  Colored dyes placed over the photo sites are used to manage this effect.  This means that when a camera is advertised to have 12 megapixels, not every single pixel is used to capture the full scene.  The human eye has its greatest resolving power whilst observing green, thus the reason for the excess of green image data in this system.  

All of this data is fed to the camera's computer.  This color filter array produces an image that would be unrecognizable to the human eye; software is needed to add in the remaining color information to each pixel.  This is known as demosaicing, or CFA interpolation.   This process creates three channels in an image, a red, green, and blue channel.  Every piece of raw conversion software handles this process differently, resulting in each having a "look" that is either valued or despised by professional photographers.  

Another problem with raw image capture is that the dyes used to give each pixel its color-capturing ability are not perfect.  This results in an image in which the colors do not accurately match those in the real-life scene captured.  Under various light sources, different wavelengths penetrate the filters at different depths, resulting in color casts.  For example, if the color white were being photographed, its luminosity values would be 255, 255, 255 for red, green, and blue.  However, these imperfect dyes might result in data that says this white is 230, 244, 250.  Thus, a multiplying factor is needed to turn those numbers back to 255, 255, 255.  This is known as white balance, or, more appropriately, neutral balance.  

A third, and less discussed issue with raw capture is known as gamma correction.  When a scene is captured, the sensor records this data on a linear scale, that is, a one to one ratio of luminance to what was in the scene versus the data recorded digitally.  However, a monitor does not respond to tones in a linear fashion, due to the variation of voltage native to this display technology.  

If a non-gamma corrected RAW file were shown on a monitor, it would appear extremely dark.  Thus, a multiplier is used on the luminance values of the pixels in the raw file, brightening it to the level at which it was when the scene was captured.  

Shooting RAW is the absolute best way to get the most out of your camera's sensor.  It can be intimidating, and working with these files is material for another article.  By having an understanding of what a RAW file actually is, the process behind working with them can be demystified.  The best analogy I have heard relates to cooking.  If you want to make an excellent meal with vegetables, you would not want someone else cooking them for you.  This is what a jpeg file is.  Getting the uncooked vegetables is exactly like the RAW file.  However, you would not want the seeds!  The "latent images" that are created by the camera before the 3 key parameters of processing discussed in this article are the "seeds."  It is a new and exciting challenge to shoot RAW for the first time, and it is important to understand that while not as "simple" out of camera as shooting jpeg, the final results will amaze you.

jf.gifJoshua Lehrer | Website
Josh is a recent graduate of the Advertising Photography program at the Rochester Institute of Technology.  His career started in the NJ/NYC area where he worked as a freelance photographer, writer, and consultant.  He also worked as marketing coordinator for a large photography retailer.  He currently resides in South Florida, where he continues to be heavily involved in the photography industry.