The Camera Raw File Format

Last updated: 12 February 2008
Published in: Digitising analogue media | Creating new digital media
Tags: cameras | file formats | photography

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Summary

The Raw file format is the native format offered by most digital SLR cameras. The format can offer greater levels of control over the conversion of capture data to more widely accepted formats such as TIFF or JPEG formats. This document is an introduction to the format and how to make use of it.

Introduction

The process involved in creating a colour image file from the data captured by the camera's sensor is a highly complex series of calculations carried out over a very short period of time. Because the photo sites on a digital camera's sensor can only measure luminance (greyscale) values, camera manufacturers have had to develop systems which convert this data into a colour image.

The most common solution is to employ a Bayer mosaic of tiny red, green and blue filters above the photo sites, whereby each pixel captures the light values at a colour depth of 12 to 14 bits/pixel. This data must then be averaged (or de-mosaiced) processed and down-sampled to an 8 bit/pixel JPEG or TIFF file.

During the file format conversion process, user defined settings such as white balance, saturation, sharpening and colour space all contribute to the final appearance of the image.

The vast majority of images taken with digital cameras are stored in the JPEG format. The JPEG format satisfies most amateur users whose priority is for good quality images that are widely supported and require little if any post processing. They also do not occupy too much space on the camera's memory cards.

However, for applications where quality is the priority, users must choose a format that provides greater control over the image conversion process. A handful of compact digital cameras and all digital SLR cameras offer a native or Raw format, which contains the unprocessed data as captured by the digital sensor.

This Raw format contains the luminance values for every sensor pixel along with a record of the user-defined settings. As a 16 bit greyscale image a Raw file is smaller than the equivalent uncompressed TIFF file but significantly larger than the equivalent JPEG.

Unlike JPEG, the Raw format is not a single file type but the name given to any image file containing unprocessed data. Most of the main camera manufacturers have created their own unique Raw formats. Native Raw files are encrypted and the formats are largely undocumented which would make it risky at best to save changes back to the native format.

For this reason while capture (EXIF) metadata is written into the native Raw file by the camera, additional metadata must be stored in a separate XMP (Extensible Metadata Platform) 'sidecar' file and stored alongside the image file. The abundance of native Raw files without a non-proprietary standard format has lead to some concern over future support and longevity of these images.

Adobe's Digital Negative format (DNG)

In response to widespread concern over the future support for Raw files Adobe have produced the DNG (Digital Negative) format, which while not being an open standard it is a well documented format and is a possible alternative to native Raw.

When you convert a proprietary Raw into a DNG you are offered two options. The Raw data can either be preserved but converted into DNG, this creates a new file similar in size to the original. Alternatively you can embed the original Raw data within the DNG file, this allows you to extract the original native Raw or open it as a normal DNG. A DNG with the embedded Raw will be larger than the native Raw.

The DNG format is a fully documented file format. This means that metadata can be written to the file without the risk of overwriting valuable data. This avoids the need for separate XMP 'sidecar' files and the risk of them being separated from the image files.

Raw conversion

The standard TIFF and JPEG camera formats can be viewed and edited relatively easily, however the Raw file requires a special application which can convert the captured data into a more widely supported format.

Camera manufacturers have produced their own Raw converters but there are also a variety of third party applications available. These applications allow you to apply the user-defined settings as recorded in the Raw file or to override these settings.

Raw converters offer a wide variety of controls some of which are very specific to images captured with particular camera or lens combinations while others are common to all Raw images. All Raw converters will allow you to assign new white balance settings, adjust brightness, contrast and colour saturation. Most converters will allow you to sharpen the image or reduce noise and a few will allow you to remove image problems such as chromatic aberration or vignetting or even sensor dust.

While 'in camera' conversion reduces the 12 or 14 bit captured data down to 8 bit, a Raw image retains the captured 12 or 14 bit data which can be interpolated up to 16 bit/pixel during conversion. A 16 bit image can offer a much greater tonal range and can stand up to greater amounts of tone or colour adjustment than the equivalent 8 bit image.

As the technology behind digital photography improves we are finding that today's Raw conversion programs are often able to reveal more information from older Raw files than could be retrieved with the conversion programs available at the time the original images were created.

At first glance the Raw format would appear to offer some real benefits over JPEG or TIFF, however careful thought should be given to your choice of capture format. These are just a few of the pros and cons of the Raw format:

Advantages

• 16 bit/channel image
• Full control over conversion particularly white balance
• Smaller than equivalent TIFF files (more images/memory card)
• Master Raw images can be reprocessed according to final application

Disadvantages

• Proprietary format (DNG may offer solution)
• Raw conversion adds another stage to image workflow
• Raw formats are not widely supported

Raw conversion software

Most of the more powerful image editing applications support Raw conversion. The main camera manufacturers and a few third party organisations have also produced stand-alone or 'plug-in' Raw converters.

These converters range in price from free, open source applications to a few hundred pounds for the high spec' commercial products. The features will also vary between converters but the most important factor to consider is that the software supports your camera.

Most of the converters are available as trial versions so you can compare the features before you make your decision. If you already have an image optimisation program then Raw conversion might be 'built in' already.

A selection of Raw conversion programs

For all cameras that offer the Raw format there will be a Raw converter produced by the manufacturer. This software may be supplied free with the camera or at an additional charge.

There are also several third party Raw converters which can open a variety of different Raw formats, many of which are available as trial downloads. Many photographers have a variety of Raw converters installed on their computers, like darkroom chemicals they will each produce a slightly different interpretation of the original Raw data. Here is a list of some converters currently available:

• Adobe Camera Raw
  Adobe Camera Raw (ACR) is bundled with Adobe Photoshop CS and Photoshop Elements making it one of the most popular Raw converters, it offers a familiar range of conversion tools as well as the option to convert images into Adobe's DNG format.
• Bibble / Bibble Pro
  Bibble is one of the oldest third party Raw converters, it is regularly updated to support recent cameras and is available in two versions, a standard version offered as a standalone program or as a Photoshop Elements plug-in, the Pro version offers additional features such as a full Photoshop plug-in and tethered operation. Bibble also integrates the powerful noise reduction tool Noise Ninja into the program.
• Capture One Pro / Capture One
  Phase One offers two stand alone Raw converters, one pitched at the high end user who requires powerful features such as tethered camera operation, and a more basic version which offers the standard tools required for Raw conversion.
• Unidentified Flying Raw (UFRaw)
  UFRaw is an open source Raw converter which is available as a standalone program or as a plug in for the open source image optimisation application GIMP. It is regularly updated to support recently released camera models.

Last updated: 12 February 2008
Published in: Digitising analogue media | Creating new digital media
Tags: cameras | file formats | photography

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