Pixels aren't created equal, but how many is enough?

Pixels: It's not just the quantity, but the quality that counts.
One of the most commonly repeated advices in the past and--to a certain extent--the present has been that same axiom. We don't deny its merits, but it has lost much of its purpose, and if it was part of the machine world of The Matrix, it would have been deleted. Let me quantify this statement. Consumer cameras have approached or surpassed the levels of digital SLR (Single-lens reflex) cameras used by professionals in terms of megapixel ratings. For example, Sony's recently launched Cyber-shot DSC-T1 has a 5-megapixel sensor, outstripping the 4-megapixel sensor of the newly announced Nikon D2H, a dSLR. Therefore, if someone had followed that advice, the DSC-T1 would seem the better digicam.

Hold up you say, then let price be the judge. While this would distinguish the better shooter for this scenario, we hazard that this will cause other inconsistencies related to branding. It may sound silly now, but pixels just ain't pixels.

If the basic building block is different, it stands to reason that the resulting constructed digital image will have unequal image quality.

Constructing color
Contrary to belief, image sensors are color blind. To collect color data, filters are applied to the photodiodes. The problem with traditional camera sensors is that each pixel is only capable of capturing red, green or blue. These are then arranged in alternating order (as shown in the diagram) to cover the whole sensor, hence you'll sometimes hear of the term mosaic sensor.

As each pixel does not contain all the colors necessary to reconstruct the photographed scene, most cameras perform a process known as interpolation. Complex calculations are undertaken to recreate the lost color information.

We should add that there's currently one sensor without the interpolation deficiencies; the Foveon X3 sensor. It gets round the problem by incorporating three separate layers of photodiodes per pixel, allowing each pixel to capture all three colors (red, green and blue).

Who needs those extra pixels, anyway?
One of the major complaints of bad prints is pixelation. Although the term was first coined for digitized images, it is now used very widely for anything that relates to digital images. Basically, pixelation means the individual pixels are apparent to the viewer, thereby resulting in jagged outlines or bad color gradations. The common cause of this is an image being printed too large.

With digital cameras coming out with at least 2 megapixels, 4R (6 by 4 inch) photos should not be a problem. Issues only arise when you want larger pictures of at least 8R (10 by 8 inch) in size. Consumers have to realize digital films will not rival traditional silver-halide films--which typically have resolution of more than 20-megapixels--anytime soon. Consequently, they cannot expect to pick a particular digital photo they like and have it enlarged like in the old days. The following chart will give you a realistic expectation of good quality prints (at least 100pixels per inch) versus megapixel ratings. Depending on how you perceive quality, you may find yourself having to downsize, or conversely be able to upsize the photos by a factor of one or two.

Looking at the table above, you should realize that for most people who go straight from shoot to print, a 3-megapixel camera will be more than enough. If you're not the type to spend endless hours in front of a computer digitally editing your pictures, don't waste your time and money for anything more.

Conclusion
There are other factors involved in taking and producing a good picture. Resolution should not be the sole basis of your purchase. You should take into account other constituents such as lens aberrations, dynamic range and sensitivity just to name a few. Unless you don't mind purple fringing 10 to 20 pixels thick or odd artifacts littering all over your photos, you should also check out other features and specifications.