8 bit and 16 bit

Most Photoshop images are going to be in 8 bit pixel mode or 16 bit pixel mode.
In a Photoshop image, a pixel is a small square of digital information.
The pixels are laid out in a mathematical grid and so do not deteriorate when copied - the mathematical pattern is just reconstructed.
So what does this mean in terms of image size and quality.

A bit is a unit of colour information. A 1 bit pixel has two choices. In the computer binary system, it can be 1 or 0, on of off. In practice it can be black or white. In simplistic graphic terms, imagine a pixel as a rod with the colour on the end, so this 1 bit pixel can only represent black or white.

An 8 bit pixel has greater pixel depth or bit depth and can contain and display much more colour information. As each bit has two choices, 8 bits have 2x2x2x2x2x2x2x2 or 256 choices, which means 2 to the power of 8 or 256 colours. A complete image which is only 8 bit is likely to be a greyscale or an indexed colour image.

A colour image in this category will have 8 bits on each of the red, green and blue channels.
The total pixel content of the image combine to make the file size. An 8 bit pixel uses one 'byte' of computer memory and a thousand bytes amount to one 'megabyte' of memory.

The greater the bit depth or pixel depth in this 'rod' of digital information, the greater will be the number of colours and information stored. Therefore, the digital image will give a more accurate representation of the original.

Photoshop CS can handle most functions in 16 bits per channel, or a 48 bit image. Earlier versions of Photoshop can handle only a limited range of tasks in 16 bit, while Elements will work in an 8 bit per channel image.

Many scanners can scan film in 16 bit per channel mode and this will capture considerably more information and hence more colours, tones and details. In actual fact, Photoshop works in 15 bit + 1 unit, which amounts to 215. This produces 32,769 values, which is still considerably more than in standard 8 bit mode.

• Open any image.

• At the top of the image frame, it is likely to say RGB/8, indicating RGB Color Mode in 8 bit.

• Click on the different menu items in the top menu bar and you will see that some of them are greyed out, particulary with the Filters, meaning they will not function in 16 bit mode. However, all the Adjustments under Image are available which means doing colour and tonal corrections on a 48 bit image is a great advantage - more tones will be retained.

Converting from 8 bit to 16 bit mode in this way is for demonstration purposes only. It will not give more quality to the image. To obtain that, the original scan or digital capture must be in 16 bit mode.


Summary

• 8 bits per channel give 256 colours per channel, and an RGB image of 24 bit.

• 16 bits per channel give 32,769 colours per channel, and an RGB image of 48 bit.

More about 8 bit and 16 bit

The same applies for scanners and digital cameras. They will also produce their own dynamic ranges, which will be the difference between its darkest and lightest points. Digital cameras produce a range comparable with colour negative film or more with the better cameras, with the advantage of capturing more detail in the shadows.

For post production treatment, the darkest point will be assigned a value of zero and the lightest point will be assigned a value of 255.
The number of tones between these two points will depend on the bit depth of the digital file.
A camera file or scan of an 8 bit depth will have a tonal range of 256, 0-255, which is 2 to the power of 8.

A 16 bit Photoshop file will be in 15 bit + 1 unit, which is 215 + 1, or 32,769 colours.
Bit depth will determine the number of steps between the maximum and minimum values, so a 16 bit image will produce much smoother tonal gradations. A very large dynamic range will need more bits to reproduce all the tones within that range.

A raw file is what the digital device produces when it is sqeezes as much information as it possibly can out of the subject or the film it is scanning. For best quality, the raw file should always be in 16 bit. Every tonal adjustment made in post-production throws away information. A 16 bit file can cope with this loss much better than an 8 bit file.

An 8 bit file has only 256 levels as mapped on the histogram.
Moving the highlight triangle in to the 200 level and the shadow triangle in to the 10 level where the information begins (or doing a curves adjustment to change contrast), will mean 190 levels will be spread out over the 256 spaces.
This produces the comb effect in the new histogram, because of the loss of data.

In a 16 bit file, starting with 32,769 levels, doing the same adjustment will still leave some 24,250 levels. Converting this to 8 bit will give the full 256 levels.

If another adjustment is made later, more information will be permanently lost. If the changes are large enough, image quality will suffer and the image can start to posterize, creating blocks of colour rather than smooth transitions.

The 16 bit file, on the other hand, shows no noticeble loss of information. There is so much data in the 16 bit file that it can cope with the loss without degrading the image.
With all the extra detail in the file it is possible to make a better choice as to which parts of the image require emphasis - shadows, midtones or highlights, bearing in mind that the print will have a much smaller range.

A 16 bit file does not actually have a larger gamut or contain more colours than an 8 bit file. It is that there is greater detail within the tones of the file and the image will not degrade as much with adjustments.

The moral is:

• to scan in 16 bit as much as possible;

• save your photographs in 16 bit as much as possible;

• retouch in 16 bit as much as possible;

• only transfer to 8 bit if there is some activity your system cannot undertake in 16 bit;

• and only change to 8 bit mode for the final output.
  Printing has to be done in 8 bit.
  
  
  
  Back to the top of the 8 bit and 16 bit page.

  Back to Advanced Tutorials page.

  Home Page

  
  
  
  

  ---