Duncan Wherrett

Are Bits and Pixels in Photoshop Really Worth 32,769?

A pixel is a small square of digital information, but what does that mean in practice?

The article on this site titled What's Icing a Cake Got to Do With Pixels and Resolution in Photoshop? explains that 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. Now we'll look further into what this means 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. So 8 bits means 28 or 256 colours. A complete image which is only an 8 bit pixel image 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 'kilobyte' 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.  

1. Open an image.
2. At the top of the image frame, it is likely to say RGB/8, indicating RGB Color Mode in 8 bit.
3. Go to Image > Mode and you will see there is a tick by 8 Bits / Channel. Drop down to 16 Bits / Channel. This does not make the image true 16 bit but it's only for illustrative purposes.
4. Click on the different menu items in the top menu bar and you will see that some of them are greyed out, particularly with the Filters, meaning they will not function in 16 bit mode. However, under Image, all the Adjustments are available, which means doing colour and tonal corrections on a 48 bit image is a great advantage – more tones will be retained.

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

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