Most digital cameras use either CCD (charge-coupled device) or CMOS (complimentary metal oxide semiconductor) sensor technology to capture image data. Raw image files record the precise data captured by the photosensitive sensors of the camera without applying any of the processing that would be required to convert it into JPEG or TIFF file format.
Early digital cameras and most entry level digital cameras convert raw data into a format such as JPEG. This is done by the camera at time of capture by an in-built converter. Typically, this on-board camera processing calculates settings for white balance, gamma correction, noise reduction, anti aliasing and sharpening together with colorimetric rendering. The image is also compressed using the JPEG algorithm.
JPEG compression has a significant disadvantage particularly where high quality, high resolution images are required. To compress image data, the JPEG compression algorithm averages out some of the color information with the potential loss of color values and detail. High JPEG compression settings can also introduce noticeable compression artifacts. These can be particularly visible along hard edges and in high contrast areas. Skin tones and gradients can also prove difficult for JPEG compression.
What is Raw ?
A raw file records the precise data captured by the sensors of the camera without any processing. Raw files consist of data recording color and luminosity values captured by the sensors and image metadata. Metadata is data about data. For example, raw files, as well as JPEG files, contain ExIF (Exchangeable Image Format) metadata which includes information such as camera model, shutter speed, aperture and focal length.
When you capture an image using your camera’s raw format it is only ISO speed, shutter speed and aperture that have an effect on the captured pixels. When you open the raw file using the Adobe Photoshop CS2 Camera Raw dialog box you can control settings for white point, colorimetric rendering, noise reduction sharpening and so on.
In Photoshop CS2, use Adobe Bridge to locate raw image files downloaded to your computer. (See Chapter 3, Opening and Saving Files in Photoshop CS2 in easy steps, for further information on using Adobe Bridge.) Double-click a raw file thumbnail to open the image in the Camera Raw dialog box. Create the settings you require, then click the Open button to convert the image to an RGB image for use in Photoshop.
For professional photographers a significant advantage of raw files is that the sensors on cameras capable of saving raw files can typically record data at at least 12 bits per channel (providing a possible 4096 shades per pixel). JPEG compression reduces color data it records to 8 bits per color channel.
This can become a significant disadvantage for many amateur users in that raw image files are considerably larger than files saved in other file formats – up to 5 or 6 six times greater than the equivalent JPEG. Capturing raw image data can dramatically reduce the number of images you can record to the camera’s storage card.
Camera Raw is a general term which encompasses a range of proprietary files formats from different camera manufacturers, such as Nikon’s .NEF, Olympus’ .ORF or Canon’s .CRW. The Photoshop Camera Raw plug-in for Photoshop CS, which allows you to open and process raw image files, was released in February 2003. An upgraded version supporting more cameras and with added functionality became available in Photoshop CS2.
Visit: www.adobe.com/products/photoshop/cameraraw.html for a complete list of supported cameras.
Digital Negative (DNG) Format
One of the issues surrounding raw file formats is that they are typically proprietary formats; belonging to the camera manufacturers and not publicly available. Understandably, camera manufacturers that spend large amounts of money on research and development are reluctant to divulge the exact technical details of their own raw file formats as this could prove costly and result in loss of competitive advantage.
The intention of Digital Negative (DNG), developed by Adobe as a file format for raw files, is to establish an open industry standard so that all camera manufactures can work to a common set of specifications which will ultimately benefit the entire industry and especially the end user.
Crucially, DNG has the potential to provide photographers with a format they can use to archive images, safe in the knowledge that they will be able to access these images in the future. This removes the reliance on a proprietary format that may, or may not, be supported in the future.
The success of DNG as an open standard would also make it much easier to transfer and share raw images in complex workflows and between photographers and agencies.
The core consideration, when deciding whether or not to capture images using raw data, is whether or not you want ultimate control over settings such as white balance, brightness and contrast, noise reduction, sharpening and color rendition and are prepared to take the time to make these decisions. The Camera Raw functionality in Adobe Photoshop CS2 gives you access to the set of tools you need if you decide to follow this route.
About the author
Robert Shufflebotham is a software training consultant with over 21 years experience of delivering software training. He trains on a regular basis for some of the largest and most prestigious magazine, book and newspaper publishers in the world. He also has a MA in Electronic Media.