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Capturing the image: 8 or 16 bit

Colour depth is a term used to describe the number of different tones that can be recorded by a single pixel.

A pixel is a photo sensor. It reacts to light, generating a tiny electrical current. This current increases as the brightness of the light increases. The level of the current from the pixel is measured and recorded in binary code. This code uses binary digits, or ‘bits’. Each digit can be ‘0’ or ‘1’.

The term 'colour depth' is used to describe the number of different tones available for red, green and blue. In effect, it is telling you how many different levels of brightness a sensor can record. With just one level, only one tone would be recorded. As you increase the number of levels, a wider range of tones is available. Here, we only show up to 64 levels, which is already giving quite a smooth transition from solid colour to white. Most EOS digital cameras capture images in 12-bit mode, which gives 4096 ltones. The EOS-1D Mark II captures images in 14-bit mode, which gives 16384 tones.

If data from a pixel were recorded using a single bit, there would only be two tones - black (no current) and white (some current).

However, binary code is not limited to single bits. You can record the tones from the pixel using multiple bits.

Consider the decimal number system for a moment. This is constructed around a base of ten. There are symbols representing the numbers zero to nine. After this, the symbols are repeated, but their position in the number indicates their value. For example, the number two hundred and fifty six is written 256. The digit at the right is in the unit position and represents its face value of five. The number next to it is in the ‘tens’ position, so is not five, but five tens, which is fifty. And the number at the left is in the ‘hundreds’ position, so is not two, but two hundreds. We don’t think of this as we look at the number 256. In the same way that we don’t read ‘d’, ‘o’ and ‘g’ when we see the word dog, we can read a decimal number without having to break it down into component parts.

The binary system works in the same way as the decimal system, but looks strange because we are less familiar with it. There are only two symbols, representing zero and one.

As with the decimal system, the value of a symbol is indicated by its position in the number. However, rather than the positions from the right representing units, tens, hundreds, thousands, they represent units, two, fours, eights. So 1111 in the binary system is 15 in the decimal system (one unit, one two, one four and one eight).

Digital photography mostly works with 8-bit binary numbers. The highest value you can obtain with this is 11111111. Convert this to the decimal system and you obtain 255 (1 + 2 + 4 + 8 +16 + 32 + 64 +128). However, if you include the string 00000000, an 8-bit binary number can be used to define 256 different tonal values from a single pixel.

256 tones might sound impressive, but it does not produce the sort of quality obtained with film. However, 256 is just the number of tones from one colour. In practice, a pixel not only captures data for its own base colour of red, green or blue, but also acquires data for the two missing colours from adjacent pixels. This means that the total number of colour tones that can be defined by a single pixel is over 16.7 million (256 x 256 x 256). 8-bit RGB is also called 24-bit colour.

EOS digital cameras capture images in 12-bit mode, which defines over 68 billion colour tones from each pixel. Only RAW gives you 12-bit colour files. These can be opened in the 16-bit mode of Photoshop, Digital Photo Professional and some other imaging applications. However, the file remains 12-bit - it simply opens in the 16-bit space.

If you shoot JPEGs, the camera takes the 12-bit data and automatically converts it to an 8-bit colour file.

Most ink-jet printers work in 8-bit mode, and 8-bit files give excellent prints. 12-bit mode is mostly useful when you intend to do a lot of processing work on the file after it has been shot. Intensive processing can lose some of the data, especially if you are adjusting the dynamic range, so there is an advantage is starting with the largest file available. After all the work has been done, the image can be saved as an 8-bit file for printing. Generally, the final image will show better quality than if the work had been done on an 8-bit file. In particular, areas of similar tone will be smoother, and the transitions between different tones and colours will be smoother.

The disadvantage of working with 12-bit files is that they are much larger than 8-bit files and take longer to process on a computer. It is worth experimenting with both 12-bit and 8-bit files of the same subject (you can use the same RAW file, opening one copy in 16-bit mode and the other in 8-bit mode). Make similar significant changes to both images, save both as 8-bit files, and then see if there is any noticeable difference when printed to your normal paper size. If not, you will save a lot of time by working in 8-bit mode.

Since RAW files always open as a copy, leaving the original file untouched, you can always go back and work in 16-bit mode if a particular image does not give the results you want when worked on in 8-bit mode. Unfortunately this is not possible in JPEG as you change the actual file.