Have you ever wondered which file format to use for images and why? Well, I promise that is an entirely unanswerable question! However, in pursuing 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!
Firstly, 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’, each having eight bits. So, using 8 bits, you can represent any positive number between 0 and 255 (i.e. 256 levels.)
By the way, 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 32 bits per channel are the most common bit depths. Each defines how many brightness levels you can store for each pixel in its red, green and blue “channels” (every humanly visible colour can be created by mixing just Red, Green and Blue light).
Diving The Depths
8 bits per channel allows 256 levels of luminance for each red, green and blue channel. So, if we multiply this for the three channels, we get 256x256x256 or 16.7 million possible colours at each pixel.
16.7 million colours may sound vast, but imagine an image with nothing but blue sky tones or a red studio backdrop; you are suddenly limited to something closer to 256 levels because only the blue or red channel (just 8 bits) is being used.
Have you 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 – 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, there is a balance between speed and quality; the files aren’t as huge as 16-bit files but give much more information than 8-bit files. 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 or flawless red backdrops!
Only a handful of high-end medium-format cameras or backs use 16 bits per channel on the sensor. With 16 bits, you have 65,536 levels per channel, and the files are huge. That said, you may end up using 16 bits per channel when you convert your…