When you take a picture in JPEG format with a digital camera, there are things that your camera does to the picture before it is saved to the memory card.
-The image sensor gathers information, converts it from analog to digital, and holds it for more processing. At this point, the image data is unprocessed…it is RAW data.
-If you specified a white balance, sharpening, saturation, image effect, digital zoom, etc, these changes are applied to the RAW data.
-Any changes in image quality and size, if specified, are applied to the RAW data.
-The result of all of these changes being applied is a JPEG image, and it is compressed, then written to your memory card.
Once you have transferred the images on your memory card to your computer, you can decide to add “post processing” in image editing software, like Photoshop. Most will usually adjust levels and sharpen the image a bit.
RAW data is uncompressed, and unprocessed data captured by the image sensor, without any in camera processing has been applied. RAW data is the equivalent of a film negative, exposed, but not yet processed. If you choose a RAW data format, the camera will ignore your settings for white balance, contrast, saturation, and contrast. It will save those settings in a separate heading associated with the RAW data file.
Note that you will need to have an appropriate RAW plug-in installed to read RAW data with image editing software, because each manufacturer encodes RAW a little differently.
The reason people are fascinated with RAW data is that it seems to give you the ability to magically correct any mistakes on that photo. Why? When you save an image in RAW, your settings are ignored. When you open the RAW data in your image software, that header where your settings were saved is read and used to display the image, but the actual RAW data is not affected. Now you can adjust those settings and see the effect on the data, and when you are happy with the changes, you can save a copy as a JPEG. This makes RAW a powerful option because it leaves the users with more adjustability in post processing. To a limited extent, RAW will even allow you to correct exposure errors…but ONLY to a limited extent! You cannot magically fix a picture that is VERY over or under exposed.
RAW takes longer to write to a memory card, and you will need to post-process every single picture. These can be a major nuisance if you need to take pictures in rapid succession, and your camera doesn’t have a large RAW buffer. Post processing is also a chose that most do not enjoy doing. For most practical uses, RAW is not a convenient option, and shooting in JPEG is more than sufficient. The exceptions to that rule being as follows:
-You are unsure of what white balance to use, or you need absolutely accurate color reproductions (subtle colors, like off white, instead of white, etc)
-You want optimum control over contrast, saturation, and sharpness
-You will want to print the picture extra large
Some people want to be able to make all the adjustments that RAW makes available, but they don’t want to HAVE to post process every image. In cases like that, some more advanced cameras offer a RAW+JPEG option, where the camera will save the RAW data AND a processed JPEG image to the memory card. It will take slightly longer to write these files to the memory card.