Re: Digital Photos: how many bits in a pixel and how many pixels in a byte?

Thanks for the question, Rowland – It’s probably one a lot of people who 
wonder about digital cameras want to ask. 

We need to start from the definitions of bits and bytes:

The basic definition of a bit is a single binary digit - 0 or 1. A byte is 
normally defined as 8 bits – which means a byte can handle values in the 
decimal system from 0 to 255 (256 levels or 2 to the power 8 levels.) The 
term bit depth is used to indicate the number of bits in a byte, so a bit 
depth of 12 would mean 12 bits per byte, or the capability of 
discriminating 2 to the 12 different levels – 4096 levels.

Often colour digital cameras are quoted as 24 bit – this is a little 
confusing. It simply means that each primary colour, red, green and blue 
is separately recorded at 8 bits per byte. So the definition of an 
individual pixel (in colour) requires 24 bits, or three bytes. You may 
often see a computer display being quoted as able to produce 16 million 
colours. This is simply a way of hyping the simple fact that it determines 
colours at 8 bits per byte, three bytes per pixel. There are therefore 256 
x 256 x 256 =  16,777,216 possible variations.

So with that, to your question – “how many bits in a pixel and how many 
pixels in a byte?”
For most (but not all colour imaging systems, including digital cameras 
there are 24 bits in a pixel, and one-third of a pixel in a byte.

So, if your camera is, say a 2.0 megapixel camera, it should produce an 
uncompressed file of 6 megabytes. Actually, a megabyte is not exactly on 
million bytes – it is 2 to the power 20 bytes which is 1,048,567 bytes. So 
six million bytes is really only 5.72 megabytes. Here again, different 
practice is rife – with some defining a million bytes as a megabyte, while 
others use the true value. You will notice this in Windows explorer when 
you look at disk sizes determined by the PC vs. those quoted by the seller 
of the drive, for instance. Another complication is the need to store 
other information along with the three colour values – like taking 
information, pixel co-ordinates and other stuff. These extra pieces of 
information add to file sizes. (As an aside, you should be aware that a 
pixel can actually only record one colour – red, green or blue. The other 
colours are made up by “interpolation”. A typical digital camera will have 
an array of filters over the pixels, which has 25% of the pixels measuring 
red, 25% measuring blue and 50% measuring green. Green predominates 
because to our eyes, green is the most important colour – it tends to be 
where most scene information is carried, at least as far as our brains 
interpret things. Film images capture all three colours at any co-ordinate 
simultaneously.)

As to storage of digitally recorded images – if a camera takes pictures at 
4 megapixels then you can see that you need roughly 12 megabytes to store 
it, so a 16mb card will not hold four such images without compression. In 
practice, most images are compressed for storage – typically in a way that 
gives a file with the extension .jpeg – so called because the standard was 
agreed by a committee called the Joint Photographic Experts Group. There 
are other, and better ways to compress images but this one is the most 
common, particularly for use on the web. It does, however, lose 
information each time the image is re-compressed. Any image modifications 
made after decompression, followed by re-saving compressed, loses more 
information. 
It is typical for digital cameras to be sold with miserly small storage 
cards compared with the capabilities of the camera to use its full 
potential. Such is business!

I hope I’ve made things clearer on this subject for you, and I suspect 
that there are attendant questions about - how many megapixels do I need 
to print a photograph at a particular size? But that’s another topic… 
(Feel free to ask again.)