THE ANALYSIS OF
DETAIL
An image becomes digital when it is sampled or "quantised" into a form which can be understood by a computer. It simply is turned into a long string of "on"/"off" signals.
The smallest element of a conventional photograph is a piece of grain. The equivalent digital picture element is a "pixel". The "pix" part of this word is from "picture", the "el" from "element". Join them together and you have "pixel".
Digitising an image is like overlaying a very fine wire netting over a scene, analyzing the colour and brightness seen through each part of the mesh, then noting down the values in correct order in a huge list. How many of these pixels do you need to describe your image? Just like conventional photography, we don't want them to be large enough to be intrusive on our images, so the more the better - to a point. That point is when you simply can't see the extra detail that more pixels bring to the image. All the rest are wasted.
In the accompanying series of colour illustrations, I've chosen to display all the pixels used to define my image - be there one or 65,536 of them - in the same size square picture box. Note also that each square pixel is of uniform brightness and colour across its whole area, irrespective of its size.
An important concept to grasp is that while a monitor screen picture element - or pixel - does have a physical size, an image pixel does not. Your monitor screen's pixel size is fixed forever when the screen is manufactured at the factory - just as its counterpart, the grain in silver halide photography is fixed in size when the film is manufactured. On the other hand, a digital image pixel has no physical size until you give it one. A pixel is simply a mathematical definition inside the computer. It's up to you to decide how large to display that description.
One pixel is displayed here as one screen pixel (look closely!) This represents the maximum resolution (the smallest amount of detail) your monitor screen can display.
The same pixel
displayed as ten screen pixels by ten screen
pixels...
... and as forty screen
pixels by forty screen pixels.
The size you decide to display your pixels - and how many you will need - will depend on the size and/or viewing distance of the finished image. Read on!
Let's look at a typical scene - the kind of subject you might see in the viewfinder of a Hasselblad
Now let's average the colour and brightness of the scene.
The smallest possible number of pixels we can use to describe an image is one. Not exactly an exciting photograph, unless you consider it to be a work of art worthy of exhibition in the Tate Gallery. What it tells the photographer is the average colour and brightness of the whole image, just like an reflectance exposure meter.
Now let's look at the image through a coarse wire mesh - only 2 squares by 2 squares.
Now let's average the colour and brightness seen through each square.
In a four pixel image we can just begin to see some differences in colours in the four corners of the image but not much else.
Now let's make the mesh smaller - 4 squares by 4 squares.
Doubling the number in each direction again gives us 4 x 4 pixels - a total of sixteen, four times as many as before.
Note that the square law, which affects photography so much, applies here also.
Progressively doubling the number of pixels each time brings increasing resolution. Even as low as 8 x 8, a total of only 64 pixels, Martien's magenta T-shirt is just beginning to take shape.
Note how the resolution improves as the number of pixels increases. And notice, too, how fast the total number of pixels in each successive image increases. That's the effect of the square law.
By 32 x 32, a total of 1,024, the faces are beginning to take shape, although the subjects themselves are unrecognisable.
Click on the Image to continue.
Click on the Image to continue.
At 256 x 256 - a total of 65,536 pixels - the individual pixels have almost merged to become imperceptible at monitor screen resolution.
Reducing the reproduction size of the 128 x 128 and 64 x 64 pixel images on the screen renders their individual pixels less noticeable. All three of these images are now shown at a ratio of 1:1 - one image pixel is represented by one screen pixel.
For a given size of reproduction, increasing the number of pixels used to describe an image increases its resolution on the screen (or on the print or printed page) - until the limit of the resolution of the screen is reached. After that, any further increase is not apparent. This "unseen" resolution in the image does however mean that the image could be enlarged to a larger size. This is analogous to the print size obtainable from a film negative. It is no more mysterious than that. How big can you enlarge a digital image? As with traditional photography, it all depends on what you feel you can get away with.
Displaying the image on a computer screen, or making a digital print, is just like colouring in the squares on a piece of graph paper. When you remember that the digital "graph paper" could have around six million tiny squares for just one image digitised from a piece of 35mm film, the process is quite phenomenal.
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