Professional photographers and serious enthusiasts capture still images as raw files whenever possible. The reason is simple: raw files provide more image data and give photographers much greater control over white balance, saturation, sharpening and contrast in their images.
The above image was recorded in JPEG and raw file formats.
While all digital cameras record images in JPEG format, only more sophisticated cameras provide the option of recording raw files. All DSLRs support raw file capture so you might as well learn how to take advantage of it.
Raw vs JPEG
When an image is captured in JPEG format, the image processor in the camera converts the raw image data into Red, Green and Blue (RGB) pixel values (a process known as demosaicing). It then applies white balance, saturation, sharpening and other adjustments according to pre-determined formulae.
These settings are effectively locked into the image file.
At the same time, the image is compressed to reduce the amount of storage space it occupies. And this is the main downside of the JPEG format: image data is lost during this process. The more the image is compressed (by adjusting the Quality setting), the more information is discarded.
Each time an image is saved in JPEG format, the compression process is repeated and more data is discarded. This ‘lost’ data can never be recovered. After several cycles of compression, your image may become unusable.
The actual size of a JPEG file depends on the complexity of the subject in the photograph. Shots containing large areas of blue sky can tolerate more compression so they can be two to three times smaller than pictures of detailed subjects – even though they might have originally been the same size as uncompressed files.
When you shoot raw files, all of the information recorded by the sensor is available to create the digital image. Nothing is discarded, even when the image processor compresses the raw file to make it smaller. (The compression is ‘lossless’ which means all of the image data is usable by the photographer for subsequent editing.) Raw files don’t store colour, which means you can re-set colour values ““ including the colour space ““ when files are converted into editable formats.
These crops from the centre of the image show the quality differences between raw (top) and JPEG (below) files resulting from the different amounts of image data each file contains.
Image recording quality options in an entry-level DSLR. The raw file settings are circled in red. (Source: Canon.)
Raw files make a huge amount of data available for photographers to adjust. Whereas JPEGs are only 8-bit in size and can record a maximum of 256 tonal levels for each pixel, with raw files you end up with 12- or 14-bit files. A 14-bit raw file would give you 16,384 possible tonal values per pixel, which gives you much more data to work with.
All raw files contain embedded JPEG images, which are used for the preview image on the camera’s LCD and for histogram calculation. But, because these JPEGs have been processed by the camera, if you use these histograms to evaluate exposures, make sure the camera’s exposure and white balance settings are appropriate for the subject. Mismatched settings will result in incorrect exposures.
The worst aspect of raw files used to be their size. However, nowadays storage is cheap and high-capacity memory cards widely available so this is no longer an issue.
The remaining disadvantage is that raw files are usually proprietary. Not only is Canon’s format different from Nikon’s, Olympus’s and Sony’s and everyone else’s, but raw formats may also vary from model to model within a manufacturer’s range.
Before you can edit raw files they must be converted into an editable format – normally JPEG or TIFF (Tagged Image File Format). This requires special conversion software, which is normally supplied with the camera.
See Raw file conversion article.
This article is an excerpt from Digital SLR Pocket Guide 3rd Edition.
