(With a passing reference to digital cameras)
Clive R. Haynes FRPS
and image quality is directly related.
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. There are some very clever resampling programs on the market, however there is nothing better than having the right quantity of pixels to begin with.
Where to start?
greyscale (i.e. monochrome / black & white) image uses one ‘byte’ per pixel
(a ‘byte’ being 8 ‘bits’).
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.A 24 bit RGB image has 8 bits per pixel for each of the R, G and B channels.
This is sometimes called ‘Pixel Depth’ or ‘Colour Depth’.
pixel with a ‘bit depth’ of 1 has two possible values: black or white.
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
Calculating File Size:
Multiply the total number of pixels by the number of 'bits' of colour (usually 24) and divide the result by 8 (because there are 8 'bits' in a 'byte').
An image containing 1200 x 800 pxls
1200 x 800 = 960,000
= 960,000 pxls x 24 (usual ‘24 bit depth’ for a digital camera)
= 23,040,000 ÷ 8
= 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).
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'.
How can the information be used as a guide?
To take an example: the same formula when applied to a colour slide (or neg) scanned into my Nikon Coolscan V When set to 2,700 ppi, it produces around 8,000,000 pixels (8 mega pixels) multiply by 24 and divide by 8 the answer is 24mb, which is about right for an image that will be acceptable as an A3 print.
Digital CamerasMany 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, cameras
at a realistic price with a specification of around 6 to 7mpxls can be considered
the lower limit for acceptable A3 prints. The 6 or 7mgpxl image when 'uncompressed'
expand to around 18mb.
range of 'consumer' digital cameras is now providing some excellent results.
However many 'consumer' ‘megapixel’ models will struggle to produce quality images larger than A4 (a lot depends upon content, your requirements and needs). Never the less, I have seen excellent images produced by models producing around 4mpxl and upwards.
There is also a big difference in quality (and cost!) between 'consumer chips' and 'pro chips'.
Digital cameras produce texturally smooth and grain-free images, the result of which frequently looks better than scanned-in slides or negs with their inherent grain structure and surface blemishes.
Pro cameras such as the 'full frame' mega pixel SLR's are fantastic but at a price beyond the budget of most most techno-dedicated, enthusiastic amateurs (we're talking about £3,000 to £7,000 price tags here!) They are, after all, aimed at the 'professional photographer'.
What is acceptable is of course subjective and depends upon content and the purpose for which the image is intended.
Quality digital cameras (usually of the 'SLR' type) shoot RAW file format and this allows 16 bit working to carry through into 'Photoshop'. For more information about 'RAW' click on the link below.