Digital Image Resolution

A confusing topic for beginners, here we look at image resolution.
Colour for digital imaging purposes is defined by a number of channels
of data, one for each colour required by the system of representation
that we are using. A monochrome image requires only one channel
for the brightness or luminance. For colour images the two main
systems are RGB and CMYK. RGB, which stands for red, green and
blue, models the way our eyes respond to colour. All computer
monitors display in RGB and digital cameras and scanners actually
capture the image in RGB, even if it is converted to something
else. CMYK, which stands for cyan, magenta, yellow and black, is
the colour system used by the printing industry and almost all printers
(except photo printers) use four inks or dyes to print colour
images. It is possible to convert between the two, although it is
better to do this only once.

Digital images, often known as bitmap images, have two main
characteristics: spatial resolution and colour resolution. All
digital images, whether they come via a scanner or digital camera,
comprise discrete picture elements, called pixels . The number of
pixels that make up an image, in both the horizontal and vertical
directions, defines its spatial resolution. The resolution can be
expressed in two ways, as an absolute resolution, such as a 1280 x 1024
pixel image or as a size and the number of pixels or dots per inch or
cm, such as a 4″ x 6″ print at 300 dpi. The colour resolution
indicates how finely divided and accurate are the colour values.
It is also sometimes called the colour depth. Typical digital
images have 8 bits of data for each colour channel, giving 2 to the
power of 8 or 256 levels. The norm for an RGB is 8 bits of red, 8
of green and 8 of blue, giving 24 bits per pixel and over 16 million
possible colour values. More bits give more room to manipulate
the image, so many scanners and digital cameras will actually capture
more bits per channel.

What resolution do I need?

This is probably the hardest thing for a beginner to figure out. Do I
need a 5 million pixel (5Mpixel) camera or will a 1.3Mpixel do? Should
I print at 300dpi or 1440dpi? The answers all depend on the purpose.
Let’s talk about input resolutions first, either from a camera or
scanner. You need to get enough pixels for the purpose. If you are
placing images on the web, say for a personal web site or to email to
someone, you will usually never need more than an 800 x 600 pixel
image. This is a half-mega pixel image, so you can see that
almost any digital camera will work. Also pretty much any scanner will
allow you to turn chemist shop photo prints into this resolution
digital files.

When printing images you need to take into account two factors, the
size of the desired print and the resolution your printer needs to give
a good result. Now it is very important not to be confused by a
printer’s rated resolution. Just because a printer has a resolution of,
say, 2880dpi doesn’t mean this is the resolution you need to send it.
Far from it. Printers put dots on the page. The printer’s resolution
reflects the size and spacing of these dots. However, each of these
dots is not the equivalent of one colour channel in your photograph.
The dots a printer puts down are usually only of a single colour
intensity or maybe the printer can place a limited number of different
dot sizes, so it can represent maybe eight shades of the colour. Thus
to get 256 levels of cyan the printer has to put a group of dots down.
Most desktop printers work quite well if you send them an image of
between 200 and 360dpi. So say you want a digital camera with which you
can produce prints on your inkjet printer of 8 x 10 inches. 8 x 10 at
300dpi means an 2400 x 3000 pixel image, or around 7Mpixels. Now
there aren’t many consumer level cameras that can do this. But
resolution can be interpolated (fancy word for increased through
guessing). Photoshop, for instance, allows you to resize an image and
you can add pixels to it. I have found that most digital camera
pictures can be interpolated up by a factor of two with no problems,
and usually by a little more. So instead of a 7Mpixel camera, one of
the 3.3Mpixel ones will do fine. In fact if you only occasionally print
8 x 10 and usually only 5 x 4 you can happily get away with a 2.1Mpixel

These three versions of the same
image have been saved with differing numbers of bits per channel.
With lowering colour resolution objectionable banding occurs in areas
that should be smoothly shaded, like the sky.

a. 8 bits per channel for 24 bits

b. 4 bits per channel for 12 bits

c. 3 bits per channel for 9 bits

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