(With a passing reference to digital cameras)

Clive R. Haynes FRPS

File size and image quality is directly related. As digital cameras become steadily more attractive, one needs a ‘rule of thumb’ to indicate how good the image could be for the purpose intended.

Previous notes have established that an exhibition quality A3 size, ‘photo-realistic’, colour picture requires a file size of some 20 to 25mb. Such an image will contain around 7 to 9 million pixels. A ‘photo-realistic’ A4 picture would be around the 10mb size and need, therefore, only around half the number of pixels to render similar image quality at a normal viewing distance.

It is advantageous to have a good working idea of what a particular relationship of pixels will produce as a file size. Often we see figures quoted such as 640 x 480 pixels or 1200 x 800 pixels and so on. The number of pixels in an image is ABSOLUTE so in essence the more the better. Changing the number of pixels by ‘re-sampling’ is not desirable as it involves either ‘creating/inventing’ pixels or discarding pixels.

A greyscale (i.e. monochrome / black & white) image uses one ‘byte’ per pixel (a ‘byte’ being 8 ‘bits’). An 8 bit unit or a ‘byte’, as it is called, can store up to 256 levels of information. In this way we can store up to 256 levels of brightness per pixel – which gives us an ‘8 bit greyscale’.

A colour image is made when each element of the ccd array, in the camera or scanner, samples the level of a particular primary colour - Red, Green or Blue (RGB) the resultant sampling combines the information to create one full colour pixel. This full colour pixel contains three bytes (each one 8mb in depth). Three bytes per pixel (RGB) are needed so 8 x 3 = 24 bits. For a given area therefore, a colour image needs three times the number of bytes compared to its greyscale equivalent.

So at one byte per colour, (remember, that’s 8 bits x 3 = 24 bits) we have what is termed ‘24 bit colour’ and is the typical ‘bit depth’ for realistic images.

Bit Depth.

This is sometimes called ‘Pixel Depth’ or ‘Colour Depth’.

A pixel with a ‘bit depth’ of 1 has two possible values: black or white. A pixel with a ‘bit depth’ of 8 has 2⁸, or 256 possible values. A pixel with a bit depth of 24 has 2²⁴, or approx. 16 million possible values. A 24 bit RGB image has 8 bits per pixel for each of the R, G and B channels.

The greater the ‘bit depth’ the finer the levels of change that can be recorded so the higher fidelity the gradations of the image. Naturally the equipment to perform this task this is more expensive and the resulting file size is correspondingly larger. As a consequence more space is needed in the computer system to handle and store the image. Depending upon scanning options, bit depth can be 24, 30, 36, 48 or even 64.

e.g.

An image containing 1200 x 800 pxls

= 960,000 pxls x 24 (usual ‘24 bit depth’ for a digital camera)

= 2,880,000 or as we say 2.88mb

The above formula will provide a quick reference to estimating file size (and therefore a guide to resolution).

Note: the file format used to save the image information can change the figure calculated but not by a vast amount. Files can of course be 'compressed'.

To take an example: the same formula when applied to a slide scanned into my Nikon Coolscan II produces around 7,375,000 pixels (7.35mega pixels) multiply by 24 and divide by 8 the answer is 22.125mb, which is about right.

Many excellent digital cameras are now available but be wary and be aware of resolution/file size for prints larger than say A4 (a lot depends upon content, your requirements and needs). Some cameras use clever methods of interpolation to raise the pixel level - look at the specifications carefully.

So as a rough guide, when cameras are available at a realistic price with a specification of around 6mpxls (mega pixels) we can consider this to be fine for A3 prints. For A4 prints, 3.5mpxls would be satisfactory.

The upper range of digital cameras is now providing some excellent results. The images are smooth and as they are 'grain free', present silky definition.

What is acceptable is of course subjective and depends upon content and the purpose for which the image is intended.