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Most medium and high-end digital cameras have a setting that allows you to record images in either JPG or RAW format on the memory card. We’ve all seen JPG images on the web and in folders on our personal computers, but it is often not very clear what the RAW format is and why we should use it. Most novices know that JPG files are compressed files, that is, they somehow take up less space on the memory card than they would otherwise, and that RAW files are not compressed and so take up more space on the memory card. This is not strictly true, RAW files are usually compressed, but the type of compression is loss-less. This short paper discusses the reasons why you should use the RAW format if you’re serious about photography and the quality of your images, but first we’ll start with a little background into what the RAW format is.
RAW is a general term for a number of different camera manufacturers proprietary formats; Canon’s .CRW and .CR2, Nikon’s .NEF, Minolta’s .MRW and Olympus’ .ORF formats. All share common features, but essentially record the unprocessed data captured by the camera’s sensor. This means that before the RAW data can be used, there needs to be a piece of software, the RAW converter, that converts this data into a recognisable image; more on this later. As the RAW format is not a Standard, this software continually needs to be updated when the manufacturer brings out a new camera.
The camera’s sensor is an array of individual sensors, when photons land on these sensors, an electrical signal is produced, more photons produce a larger signal (brighter light). However, these represent a monochrome image, colour information is obtained by using colour filters in the form of a Bayer matrix over the sensors.
The RAW file is comparable to the latent image on an exposed, but undeveloped film, it contains exactly what the image sensor recorded. It’s important to note that the camera might support many features such as auto white balance, the ability to ‘take’ sepia toned images or vivid colours for landscapes, all these features are essentially disabled when we use RAW as the data recorded is only
The White Balance setting has no effect on the RAW image data whatsoever, but a meta-data tag is embedded into the RAW data file so that the RAW converter can know what the photographer had intended and so render the default image accordingly.
RAW files use the full dynamic range of the camera’s sensor, on high-end digital cameras this is currently 14-bits which can record 2^14 or 16,384 different shades. On the other hand, JPGs are 8-bit hence can only record 2^8 or 256 different shades which is a serious problem when you start to post-process your image; there are very few bits representing the shadows and so when these are manipulated, banding (posterisation) often becomes apparent.
The RAW image is usually compressed using a loss-less algorithm, the file size thus depends upon the actual image and accounts for why each RAW image you take has a different file size (Nikon with their .NEF format offers uncompressed, lossless compressed and lossy compressed). A JPG image is always compressed and uses a lossy algorithm, that is, it throws information away that can never be recovered by software later. The RAW image will take more space on the memory card than a JPG.
Some cameras are now giving the additional option for a reduced resolution RAW image so you can have the advantages of a RAW if you are really short of space on your memory card. Cameras also allow different resolutions of JPG images so you can opt for fewer high resolution images, or a very large number of lower resolution images.
The JPG image has been processed by the camera at the time of shooting. It will have been sharpened (some cameras allow you to set the amount of sharpening) using an ‘unsharp mask’. Therefor all your photographs will have the same amount of sharpening applied. The amount of sharpening that you want to apply should be dependant upon your art (is your photo a soft portrait or a very detailed architectural shot?), the intended output device (screen or print) and the intended viewing distance.
Embedded within the RAW file is (usually a lower resolution) JPG image (thumbnail), this is used by your upload software so that you can see what the image is. After all, the RAW file, is, well … RAW, it can only be recognisable after it has been processed by the RAW Converter.
But be warned; it is this JPG thumbnail that is also displayed on your camera’s LCD and it is also this JPG thumbnail that provides the data for your exposure histogram. Thus if you shoot in RAW, it is recommended that you turn all of your camera’s ‘artistic’ settings to neutral, no sharpening, no saturation or contrast enhancements etc, so that you can make resonably accurate assumptions from the JPG displayed on your camera’s LCD.
The most important reason why you should shoot in RAW format is that programs like Adobe’s Lightroom and Photoshop RAW Converter are NON-DESCTRUCTIVE. This means that you can edit the RAW image at any time in the future again without affecting the original file. As your photographic skills develop and software gets better (for example noise reduction algorithms), we might have a different visualisation of our image in the future to what we have today.
Here are some bullet points giving the advantages of each format: