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Just What is a RAW File Anyway?

Based on last week's article, you may have decided to try shooting in raw format in order to maximize the potential of your images. A raw file (Nikon calls theirs NEF for "Nikon Electronic Format") is just a dump of data straight from the camera's imaging sensor. In order to do much with your raw images, you have to have a raw converter program such as Adobe Camera Raw or Nikon Capture. Since you can never really see a raw file in its natural state, you may be wondering what they actually are.

A digital camera records information using an array of extremely small light sensitive spots known as photosites. Each photosite knows how to do one thing: record how much light falls on it during a given amount of time. It does not understand color at all. The entire world is black and white as far as it is concerned. Each photosite has a very simple job to do, and it does it quite well, assuming the camera manufacturer has done their job well. In extremely long exposures and other exceptional situations they don't always, but for now let's just assume that each one accurately counts how many photons of light falls on it during your exposure.

The arrangements of colors in the Bayer MosaicIn order to generate a color image from this array of data lacking color, the standard solution used by most cameras is to overlay the photosites with what is known as a Bayer matrix of colored filters (named after Dr. Bryce Bayer, the guy at Kodak who came up with this way of doing things). Each photosite is thereby made to register only light with the color of the filter overtop of it. Colored filters in a Bayer mosaic come in only red, green and blue. They are arranged in rows such that every other row contains alternating red and green filters, with the remaining rows containing alternating green and blue filters. To put it another way, if the first row starts out with red, green, red, green, red, green, the second row will have green, blue, green, blue, green, blue and so on. The third row and every odd-numbered row will be just like the first, with the fourth and every even-numbered row being like the second.

It may sound odd that red and blue get the short end of the stick in terms of number of photosites, but there is a method to the madness more than just that dividing them equally in thirds would be difficult. Reg, green and blue channels in the Bayer MosaicHuman vision is more sensitive to the green portion of the spectrum, and green wavelengths lie between red and blue so green is a good information-bearing color for allowing us to perceive the natural world.

A photosite with a red filter sees only red light and must rely on its neighbors to detect blue and green. Likewise blue and green photosites see only blue and green respectively and can't detect red at all. The colors that each photosite can't see have to be filled in later either by the camera (in JPEG or TIFF mode) or by conversion software running on your computer (if you are shooting in RAW). This process of making educated guesses about what each photosite couldn't see is called de-mosaicing. If you think about, the process works amazingly well. Since the value for two out of three colors for each pixel have to be interpolated, only one third of the eventual data was actually seen by the camera. The rest is made up by software. That's mighty sophisticated software and how it works its magic is a secret closely guarded by camera makers. It may sound like you could avoid all this color guessing by shooting JPEG or TIFF, but such is not the case. Interpolation from de-mosaicing happens regardless of what mode you are shooting in, but the process happens in-camera for formats other than RAW.

In addition to filling in all the missing data, the conversion program has one more job to do. Photosites simply count brightness in a linear fashion, but human vision doesn't work that way. Our eyes respond in a more logarithmic manner — to doubling and halving of brightness. Film has been designed this way all along, but in the case of digital, this has to be done in software or else we will have an incredibly flat looking image.

Accompanying the actual raw data, a raw file also contains a list of what settings were used in the camera when the image was taken. This includes not only basic EXIF data such as the lens used, aperture and shutter speed settings and such, but also settings like white balance, color space, saturation and sharpening that are used in the conversion process to customize the result. These settings become a kind of "instruction set" for the raw converter. Since they are not applied until the image is converted to a usable image on your computer, you can change them without loss in the conversion software. The "as shot" values become simply defaults for the conversion process (Adobe Camera Raw uses its own defaults for most settings though).

There are a growing number of raw converter programs on the market, and which one you use generally comes down to a matter of personal preference. For Nikon shooters, Nikon Capture is the software Nikon makes for the job, but there are others. Eric Hyman created Bibble as a third-party alternative back in 2000. The program is now up to version 4 and remains a strong competitor. Phase One, a longtime maker of digital backs for medium format cameras and the software that drives them came out with Capture One in 2003 as a high-end solution for raw conversion and augmented it with their "LE" Edition in 2004. Starting as an added cost option for Photoshop 7, Adobe now bundles Adobe Camera Raw for free with Photoshop CS. Because of its excellent integration with Photoshop itself, Adobe Camera Raw (ACR) is now probably the most widely used raw converter. Photoshop Elements 3.0 now also includes a version of ACR. And these aren't all of them. The number of raw converters continues to increase, evidence of the popularity of digital imaging and raw file formats in particular.

Your camera always shoots in raw mode. It can't do any different owing to how the photosites themselves work. The decision you have to make is whether you want to convert them to something else in-camera, or defer that process until you get the raw images loaded onto your computer.

Date posted: March 6, 2005


Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Related articles:
File Formats for Digital Imaging
Making Stuff Up and Filling in the Blanks

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