Rangefinder Magazine
May 2005

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The RAW File Format Becomes a Standard for High-end Imaging by Wendell Benedetti

Almost all digital camera image sensors incorporate the Bayer Matrix filter pattern, which has two green filters for every red or blue one.

There are dozens of different imaging and graphic file formats. For a long time, when digital camera resolutions were still low and the number of digital images being taken was limited, TIFF was the file format of choice for serious imaging.

As resolutions climbed and the number of digital captures increased exponentially, JPEG became the dominant file format. With the cost of hard disk storage space still relatively high and image electronic transmissions speeds relatively low, JPEG made sense. It provided user controllable levels of compression, file size and image quality.

	A camera’s imaging sensor is a flat, two-dimensional array of individual light-sensitive sensors, or pixels (picture elements), arranged in rows and columns.

More recently, as the per megabyte cost of hard disk storage space drops like a rock, and high-speed image transfer options become the norm for even casual photographers, the trend is back to going with an uncompressed file format. But rather than going back to TIFF, the format de jour is the RAW file format, which not only isn’t compressed (although it can be), it isn’t post-processed in the camera. That makes it possible for a photographer to do all the post-capture image processing on the computer, for greater control and higher quality images.

RAW format files have many of the same components found in other imaging file formats. That includes data values for every pixel (picture element) in an image as well as metadata content, which generally describes the camera, the camera’s settings and the circumstances in which the image was taken.

RAW format images differ from other digital images, however, because they store unmodified light data values. All other image formats store light data values that have been automatically processed by the camera’s microprocessor. Although in-camera processing optimizes image data for sharpness, contrast, brightness and color balance, among other settings, the processing capabilities of a state-of-the-art camera’s microprocessor are considerably lower than the processing capabilities of a state-of-the-art personal computer.

Before: Unprocessed RAW image of blues specialist Nick Shrivers. Virtually every image setting is off balance, including the white balance.	After: Colors snap back to their natural hues by simply changing the white balance in the Photoshop CS2 RAW file import section from “as shot” to “tungsten.”

In effect, rather than having the digital camera make arbitrary optimization decisions, the RAW format puts image optimization and conversion process into the hands of the photographer that took the picture. With RAW-based images, photographers no longer have to settle for the automatic, and in some cases, degrading optimization that takes place when image formats such as JPEG are used.

But there’s a trade-off. The price for the increased image quality is an increase in workload. Every RAW image file must be manually or batch converted (meaning processed and optimized) before it can be edited and output.

Digital image files, no matter what format they’re in, are made up of the image data values that have been recorded by a digital camera’s imaging sensor and certain text metadata, referred to EXIF (Exchangeable Image Format) information. EXIF data includes embedded exposure data and equipment information.

A camera’s imaging sensor is a flat, two-dimensional array of individual light-sensitive sensors, or pixels, arranged in rows and columns. With each image, the sensor records the amount of light that falls on each pixel position. Each sensor converts the light into electricity, which is measured as a very small voltage. That analog light value is converted into a corresponding digital value. The camera’s circuitry uses either 12- or 14-bit conversion. The resulting image is, in effect, a grayscale image. Twelve-bit conversion supports 4096 brightness levels while 14-bit conversion is capable of representing 16,384 grayscale values.

BreezeBrowser Pro offers RAW conversion for Canon, Nikon, Pentax, Minolta and Olympus cameras.	The new Photoshop CS2 includes an import section that lets users convert proprietary RAW files into DNG, JPEG, TIFF or PSD formats.

Covering the imaging sensor’s individual pixel positions with red, green or blue filters produces color images. With the filters in place, each pixel captures either the red, green or blue components of the light that falls on it. (The Foveon imaging sensor that is used in the Sigma digital SLR camera is an exception. It has separate layers for each primary color.) Since the human eye is especially sensitive to green light, almost all digital camera imaging sensors incorporate the Bayer Matrix filter pattern, which has two green filters for every red or blue one (see page 38).

Besides the individual pixel values and metadata, RAW files also include decoder information, which, among other things, tells the RAW image file converter how the filter matrix is arranged on the camera’s imaging sensor and the characteristics of the specific filter being used.

All RAW format image files must be converted by a RAW image converter before they can be utilized. RAW converters are software applications that use the camera’s decoder configuration (the embedded RAW file information) to transform the analog grayscale data values from each pixel into digital color values. This is done by interpolating each pixel’s light value with the light values of the surrounding pixels.

Before and after views of Blue Pirages drummer, Tony Alda, showing how a simple white balance tweak can dramatically change an image. Canon’s ZoomBrowser EX supports the company’s proprietary RAW format.

RAW converters also use an image’s embedded white balance setting during conversion, although that setting doesn’t actually affect the data values for each pixel. They also utilize the embedded color definitions for the red, green and blue filters that cover the individual pixel sensors. Additionally, they provide the option of applying sharpening, noise reduction and anti-aliasing optimization.

RAW image file conversion and optimization is not identical from converter to converter process. There can be variations in color and image quality, depending on which RAW image converter is used.

RAW images are often thought of as one file type, but that’s not the case. There are quite a few different proprietary formats. No two digital camera manufacturers encode their RAW files the same way. Canon utilizes the .CRW and .CR2 file format; Nikon the .NEF format, Minolta the .MRW format and Olympus the .ORF format. Some manufacturers actually support different RAW formats within different product lines.

That diversity in RAW file formats is one of the reasons for the expanding list of RAW file image converters. Some come incorporated in applications such as Photoshop CS2, ACDSee 7, BreezeBrowser Pro or Bibble.

There are also some Photoshop-compatible plug-ins available for earlier Photoshop versions and other imaging packages that accept plug-ins. Generic RAW converters accept most popular RAW files formats. Proprietary RAW converters support only the RAW file formats generated by the digital cameras they ship with.

RAW file conversion is getting easier all the time. With the new Photoshop CS2, for example, converting a proprietary RAW file to another format is as easy as pulling it into the program and saving it into the JPEG, TIFF or PSD formats. No change in settings is required. The conversion is automatic. Photoshop CS2 also includes the options of tweaking the RAW file settings before file conversion or pulling the RAW file into the main Photoshop editor, where it can be further optimized and then saved to a wide variety of different file formats.

To resolve the competing and incompatible RAW file format issue, Adobe Systems has introduced a generic RAW file format. The new format, which is based on the TIFF EP file format, is appropriately named the Digital Negative (DNG) format. (TIFF EP format files use the TIFF 6.0 specifications and the same EXIF metadata structure used in JPEG/EXIF.) Adobe is pushing digital camera manufacturers and imaging software developers to adopt the new DNG RAW format. Unlike the numerous proprietary RAW formats, the DNG format was designed with enough flexibility built in to incorporate all the image data and metadata that any digital camera might generate.

Proprietary RAW file format images that are pulled into CS2 can be saved to the new DNG file format, with all the RAW file format characteristics being retained. DNG save options include the ability to embed the original RAW file in the DNG file, to convert the image data to an interpolated, de-mosaiced format, and to vary the compression ratio of the accompanying JPEG preview image.

As increasingly more digital cameras support the RAW file format, and more photographers utilize it, it is becoming the one essential file format for serious photography.

Wendell Benedetti is technology editor at NewsWatch Feature Service. His email address is Newswatch@Prodigy.net.