For many people considering the purchase of a digital
camera for the first time, one question that keeps cropping
up is "how many megapixels to buy?"
One way to answer the question is to work on the basis of the finished
print size. The number of pixels required to print an image so that the
print becomes almost indistinguishable from a print made from film, varies
according to the size of the print. Indeed, past a certain number of pixels
(around 1.3 megapixels), the quality of the image is not directly
linked to the number of pixels on the sensor, the image contains a sufficient
amount of information to create a clear and sharp photo.
Digital images can be printed in different sizes, but to get the best quality
image, printers need a sufficient number of pixels every square inch, or
square centimetres, to produce a print that appears smooth. In short, the
number of dots placed on the paper must be sufficiently high that the eye
will not detect jaggies, or other artefacts.
The vast majority of digital images, in particular JPEG format images,
start life with a pixel per inch (PPI) count of 72. The reason for
this is that the image is destined for a monitor and that 72 pixels per
inch is the standard definition of the latter.
When this count of the image's Pixel Per Inch is converted to the Dot
Per Inch count of a printer, the image's size changes accordingly as
the printer requires many more dots per square inch than the monitor's
72 pixels per inch.
Probably the easiest way to visualize this is by looking through the chart
below. The chart assumes a printer DPI (Dot Per Inch) of 300, which
will usually yield a sharp image without any obvious artefacts:
Camera
Screen image
Printed image
Image Resolution
=
Image size at 72 PPI
(pixels per inch)
=
Image size at 300 DPI
(dots per inch)
A
640 x 480
=
22.58 cm x 16.93
cm
(8.889 in. x 6.667 in.)
=
5.42 cm x 4.06
cm
(2.133 in. x 1.6 in.)
B
800 x 600
=
28.22 cm x 21.17 cm
(11.111 in. x 8.333 in.)
=
6.77 cm x 5.08 cm
(2.667 in. x 2 in.)
C
1024 x 768
=
36.12 cm x 27.09 cm
(14.222 in. x 10.667 in.)
=
8.67 cm x 6.5 cm
(3.413 in. x 2.56 in.)
D
1280 x 960
(1.3 megapixel)
=
45.16 cm x 33.87 cm
(17.778 in. x 13.333 in.)
=
10.84 cm x 8.13 cm
(4.267 in. x 3.2 in.)
E
1600 x 1200
(2.1 megapixel)
=
56.44 cm x 42.33 cm
(22.22 in. x 16.665 in.)
=
13.55 cm x 10.16 cm
(5.333 in. x 4 in.)
F
1800 x 1200
(2.3 megapixel)
=
63.5 cm x 42.33 cm
(25 in. x 16.665 in.)
=
15.24 cm x 10.16 cm
(6 in. x 4 in.)
G
2048 x 1536
(3 megapixel)
=
72.25 cm x 54.19 cm
(28.444 in. x 21.333 in.)
=
17.34 cm x 13 cm
(6.827 in. x 5.12 in.)
H
2400 x 1600
(4 megapixel)
=
84.67 cm x 56.44 cm
(33.333 in. x 22.22 in.)
=
20.32 cm x 13.55 cm
(8 in. x 5.33 in.)
As can be seen by looking through
the dimensions of the printed photo produced by various sensors,
it is clear that the larger the final print, the more pixels
the sensor will need.
Most mini-labs produce printed photos from 35mm film that measure around
6 x 4 inches or 15.24 cm x 10.16 cm. To obtain a good print with a similar
dimension from a colour printer, the camera must be able to record an image
of at least 2.16 megapixel.
Although not to scale, the image below shows the relationship of size between
a 640 x 480 resolution (A) and a 4 megapixel resolution (H)
representing a print size of 8 x 5.33 in or 20.3 x 13.5 cm.
A note of caution is in order: colour printers, or the software driver
that they use are able to interpolate an image to larger dimensions.
Interpolation involves the use of algorithms to "invent" extra
pixels which are inserted between existing ones in the image to increase
the overall size of the image. This is a process that most printers use
to smooth the appearance of the image. However, if the interpolation
is too great — the image size is increased too much — then
artefacts will appear in the print.
Finally, many photo printing programs perform the changes from 72 DPI to
a PPI value automatically, adjusting the parameters to the image size requested
by the user. However, to get the best results, it is advisable to avoid
printing a low resolution image at too large a size.
