Bits, Bytes & Banding: Which File Format Is Best? Ever wondered which is the best file format to use for images and why? Well, I promise that is an entirely unanswerable question! However, always in the pursuit of those pesky details, let’s take a journey through file types, image compression, bit-depth and why you would choose anything other than JPGs.
The Basics - Bits, Bytes And Nibbles!
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irstly, despite the fact it is the analogue world we capture, digital images are stored as a series of 0’s and 1’s - called bits. These ‘bits’ are grouped into ‘bytes’, with each byte having eight bits. So using 8 bits, you can represent any positive number between 0 and 255 (i.e. 256 levels.) Incidentally, if you thought the boffins who originally designed computers were humourless geeks, think again: a group of 4 bits is called a nibble. I kid you not. Why is this important? Well, the bit-depth (I have no idea why we use the word ‘depth’) describes how much detail you can store in your image. For example, 8, 12, 14, 16, or even 32 bits per channel are the most common bit depths, and it defines how many levels of brightness you can store for each pixel in its red, green and blue channels. Diving The Depths 8-bits per channel allows 256 levels of luminance for each of the red, green and blue channels. So if we multiply this for the three channels, we get 256x256x256 or 16.7 million possible colours at each pixel.
Image 1 - the number of bits per channel defines the tonal range you can capture 16.7 million colours may sound vast, but imagine an image with nothing but blue sky tones; suddenly, you are limited to something closer to 256 levels because only the blue channel (all 8-bits of it) is being used. Ever wondered why you get banding in a blue sky when you edit an 8-bit file? Well, there you have 256 reasons - there aren’t enough visible levels in your file to truly represent mother nature. In addition, you may notice that your histogram has gaps in it - these are tell-tale signs that you have lost image detail. 12 and 14 bits per channel are more specific to RAW file formats. Again, a balance between speed and 42 the PHOTOGRAPHER / 2021 / Issue Three
quality; the files aren’t as huge as 16-bit files but give a lot more information than 8-bit. For example, a 12-bit file allows 4096 levels per channel, while a 14-bit file provides 16,384. Just think of those silky-smooth blue skies! Only a handful of high-end medium-format cameras or backs use 16-bits per channel. With 16-bits, you have 65,536 levels per channel, and the files are, well, large. That said, you may well end up using 16-bits per channel when you convert your file ready for Photoshop (or whatever is your editor of choice). While RAW files may support 12- and 14-bit formats, standard image formats (JPG, PSD, TIFF etc.) don’t. The standard bit depths for editing are 8-, 16- and 32-bit. Opening a 12- or 14-bit RAW file in your editor means you have to make another decision: do you downscale to 8-bit (losing some information in the file) or upscale to 16-bit (making the file a lot bigger and slower to process)
