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All About RAW

 RAW is a file format and unlike JPEG or TIFF, RAW is exactly that - RAW. In other words, it's the raw (unprocessed) image data from a camera CCD, the original image information generated by the sensor BEFORE any in-camera processing.

The technical stuff.
In order to understand how your image is recorded, it can help to understand how a sensor works and stores information.

So how does it work?
A sensor array is made up of millions of little pixels. Each pixel generates a voltage when light falls upon it. The more light, the more voltage.

So what about colour?
On it's own, no sensor can record colour. A typical sensor is called a Colour Filter Array, or Bayer Matrix. Half the pixels are covered with a green filter and the remainder are either red or blue. The values generated by each pixel are compared with neighbouring pixels and hence the full colour information is derived.

There is a different type of sensor called Foveon, which uses three separate layers of silicon. One for each primary colour. (Sony produce a 4 colour array which includes an Emerald Green as well!)

And the end result?
Each pixel in a typical sensor captures only one colour. The data produced from each pixel is typically about 12 bits per pixel (depending on the manufacturer). This data is stored as RAW information. This means that each pixel can handle 4,096 different levels of brightness (2^12).

So why is RAW better?
Working with a RAW file in your image software means you can save your files as a TIFF or PSD in 16 bit mode. This means that the 12 bits per pixel are spread out over the full 16 bit workspace. However, if you allowed your camera's software to save as TIFF or JPEG, the files are saved in 8 bit mode. This means you only have 256 levels of brightness to work with.
The overall TIFF and JPEG files are however 24 bits - 8 bits per channel - Red, Green and Blue = 8 + 8 + 8 = 24.
This is why a TIFF file is bigger than a RAW file. Even though it retains only 8 bits per channel, it takes up twice the storage because you have three 8 bit colour channels compared to one 12 bit RAW channel. JPEG files are compressed, at the cost of image quality. The higher the compression, the greater the loss of quality.

That said, RAW files can be compressed too, depending on the camera manufacturer but not to the extent of JPEG. RAW compression is still a lossy compression but termed Nearly Lossless.

A typical example of file sizes from a 5 megapixel camera would look like this...

 

Format	Typical size (Mb)
Uncompressed TIFF	14.1	3 Channels of 8 bits
Uncompressed 12 bit RAW	7.7	1 Channel of 12 bits
Compressed TIFF	6.0	Lossless Compression
Compressed 12 bit RAW	4.3	Nearly Lossless Compression
100% Quality JPEG	2.3	Indistinguishable from Uncompressed
80% Quality JPEG	1.3	Suitable for 4X6 prints
60% Quality JPEG	0.7	Suitable for Web Images
20% Quality JPEG	0.2	Very low quality image

So why RAW? Isn't TIFF the better format?
Not from in-camera processing, no. Remember, you only have 256 levels of brightness to work with as opposed to 4,096 levels of brightness with RAW files. As mentioned above, the RAW file can be converted to TIFF as a 16 bit image. This means you now have 65,536 levels to work with.

OK, so what are the advantages of RAW?
Basically, you have far more control over the final image. A RAW file is comparable to the latent image contained in an exposed but undeveloped piece of film. It holds exactly what the imaging chip recorded. How the final image comes out depends on the developing process used. In film, this can include different types of chemical, temperature and development time (push-processing etc). With digital (RAW files), this means we have more control with imaging software such as Photoshop.

What sort of control?
White balance for example. The EXIF information will show what the camera's setting was, but the RAW file is not set with any white balance. This means you can change the white balance and colour temperature AFTER you have taken the photo, without any degradation in the image.

Contrast and Saturation. As with the white balance, the EXIF will show the camera's settings, but the settings are not applied to the actual image data. This means you are able to apply your own settings on a per-image basis rather than use the camera's general settings for all images taken.

File linearision and colour filter array (Bayer) conversion is done on a computer with a fast and powerful microprocessor. This allows much more sophisticated algorithms to be used than those done in a camera with its slower and less powerful processor and with less space for complex conversion programs.

So, should I shoot RAW all the time?
That depends how enthusiastic you are and if you have time on your hands. JPEG files are smaller, so you will fit more on your memory card. Because they are smaller, less time is spent transferring them from the card to your computer. They are also quicker to open in your imaging software should you need to do any corrections and to print them. For most people, image quality is more than sufficient for family snapshots, web display and News & Magazine images. Processing and converting a RAW file takes time. Because RAW files are bigger, many cameras (especially compacts) cannot shoot quickly because time is needed to write the data to the memory card.

In Conclusion.
You simply decide which would work best for you. There's no reason not to work with both. You can quite happily switch between RAW shooting and convert those files later on your computer, and JPEG shooting and have the processing done in-camera. Your memory card can store both types of file together.

With a JPEG file you are largely committing yourself at the time of exposure to several of the most important aspects of image quality, namely white balance, overall contrast, colour saturation and the like. With a raw file you are free to make decisions about these settings at your leisure.

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