Look at these pictures - an original and two enlargements:
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| Original | Photographic enlargement | Digital enlargement |
In the photographic enlargement you can see that the edges of the petals and the shading of the colours are continuous. The edges are a little blurred and the speckles you can see are the individual grains of the photographic film.
The digital enlargement shows the image is made up of lots of individual squares. Each square is a single solid colour. At this enlargement there is no shading and no curves.
The squares are known as pixels (short for picture elements). All digital images are made up of pixels, whether in a digital camera, a computer or a scanner.
The original picture here is 200 pixels high by 142 wide. (The larger image obtained by double clicking is 359 x 256). Because the pixels are so small the eye cannot distinguish them and the picture looks like an ordinary photograph.
The more pixels an image has the greater the detail that can be held.
And the more it can be enlarged before the picture appears to break up into individual pixels.
Always think of your digital image in terms of the number of pixels. It is about 1.5" x 2" on the computer monitor but it is probably 30" x 40" on the projector screen, so to think of it in terms of inches or centimeters is meaningless. We will get to print sizes later.
At school you learned that all colours could be made by mixing red, yellow and blue paints. This is not quite accurate in photographic terms, where the corresponding colours are magenta, yellow and cyan.
But on the screen we are talking about mixing different coloured lights, not paints, and they mix in different ways. The primary colours are Red, Blue and Green. (Try looking at your monitor or TV screen and you will see that it is made up of lots of red, blue and green dots, which light up in various proportions.
Here we can see the effect of shining red, blue and green lights on a screen. Where they overlap completely they form white. Where they overlap partially they form the 3 paint primary colours (magenta, yellow, cyan).
If we dim all the lights equally the centre area would become grey. The more we dim them the darker the grey.
If we dim the colours unequally then we get different shades. All colours are made by mixtures of the three primary colours in different intensities. So, e.g. bright yellow is made with bright red plus bright green and no blue. Orange is made with bright red, half intensity green and no blue.
There are 256 possible intensity levels of each colour. Therefore we can make 256x256x256 different colours, a total of about 16 million.
If all you want to do is display your pictures on a computer screen you need no more pixels than the screen can display. Most screens are set to 1024x768 pixels (it could be anywhere between 640x480 and 1280x1024), i.e. about 786,000 pixels, well under one megapixel.
If you want to send pictures by email you'll probably be satisfied with something smaller, say 500x350, or less than a quarter megapixel.
However, prints can show much finer detail. If you print an image of 500x350 pixels it will look very grainy (pixelated). For prints reckon on 200 pixels for every inch of the finished print. So, if you want a 5"x7" print you will need 1000x1400 pixels - or 1.4megapixels. For a 11"x8" print (A4) you will need 2200x1600 or 3.5megapixels. (If necessary you can get away with somewhat less then this).
Remember that you may want to print just a part of the original picture, in which case the original should have been bigger.