For many members considering the purchase of a digital camera for the first time, one question that keeps cropping up is "how many megapixels should my camera have?"
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:
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. NOTE: 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 that 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.