In order to save space on the memory card, produce faster transfers and make it easier to display and print the photograph on a computer, the image is usually converted to a compressed JPEG file. Unfortunately JPEG compression also reduces the image quality, and can introduce artefacts into some areas of the image.
For most day-to-day snapshots or small prints, these small trade-offs between convenience and image quality donâ€™t matter at all, and the average user will never notice the difference, which is why consumer cameras only offer JPEG as a still image format. However for a professional photographer for whom maximum image quality is vitally important, even a slight degradation is unacceptable. RAW file data is unprocessed and usually uncompressed, or at least compressed using a â€˜losslessâ€™ process, so by shooting in RAW mode it is possible to obtain the maximum image quality of which the camera is capable.
The main advantage of RAW shooting, apart from the improvement in overall image quality, is that it bypasses the in-camera image processing, such as white balance, saturation, sharpness etc. This sounds illogical, but itâ€™s not really. The processing that takes place in the camera is pre-set before the shot is taken, and irreversible once the image has been converted to a JPEG file. By shooting in RAW mode, you can make the processing adjustments after the shot has been taken, and if you need to change them you can do so as long as you have the original RAW file.
The downside to RAW file recording is of course that the saved files are much larger, so they take a lot longer to save to the memory card, and you can fit fewer of them on there. For a typical 10MP DSLR, a high quality JPEG file will be around 4.8MB, while a RAW file will be around 15.8MB, over three times the size.
Many of the more recent DSLRs have the ability to record RAW and JPEG image files simultaneously. This has the advantage of the convenience of JPEG with the versatility and quality of RAW, but takes up even more space on your memory card. Fortunately the price of very large memory cards is falling almost daily, so this neednâ€™t be much of a problem.