Number of pixels is broken as a performance measure

29 October, 2007 at 4:57 pm Leave a comment

I’m a few years too late to this one – the boom days of digital cameras started years ago. Still, manufacturers are still promoting the number of pixels in their sensors as a measure of performance for cameras. There are multiple reasons why pixel counts are misleading both for still images (megapixels) and for video (SD vs HD).

The first is a mathematical reason. The number of pixels increases with the square of the resolution. That is, if you double the resolution of a digital image, you are increasing the number of pixels by four times.

So, while a 10 megapixel camera seems much better value than a 5 megapixel camera, it may not be the increase in resolution you expect. A 10 megapixel camera is not 2 times the resolution, it’s only 1.4 times the resolution in the horizontal or vertical direction. That means that comparing 8 megapixel with 10 megapixel cameras, you’re only considering 11 percent improvement in resolution. While this difference isn’t completely insignificant, it is usually easily overshadowed by other factors such as how much you’re going to resize, process and sharpen the hell out of it once you get it into Photoshop.

This brings me to the second reason that number of pixels doesn’t work as a performance measure.

Simply, the perceived quality of an image at a given viewing distance in a given medium has more to do with the characteristics of the original image than at which resolution it is presented. In other words, a blurry image at a high resolution is still a blurry image. The quality of the image can be affected by the quality of the original lens, any image processing that was applied including sharpening, softening, interpolation, or scaling, which often all happens inside a camera as a crucial part of its operation, and the medium in which it is presented – LCD or CRT, paper or film.

If you bought a high definition television and watch a true HD picture in it, say Becker on channel 10 (which isn’t on anymore, this was a while back), you may just be slightly disappointed at the slightly soft, grainy appearance, because this was originally shot on film. This is more noticeable to the casual viewer than the approx 2x increase in resolution.

Some people who buy expensive television sets and cameras spend lots of time searching for material to play, or things to photograph, that shows off the resolution of their equipment. People even give reviews of DVDs online recommending a movie or not based on whether the DVD encoded quality is high enough to justify their expensive TV purchases. What if it’s a good movie but blurry, grainy and slightly oversharpened?

One the other hand, I could take the argument too far and become one of those trendy people who purposefully buy cameras that will take grainy, blurry photos in order to show off the fact that I don’t care about visual quality and, supposedly, I therefore care more about taking an interesting photo. But I think once you get to the point of paying more for something of poorer quality, you are a bit of a fashion victim.

It you are considering buying a digital camera, and have a megapixel value in mind, perhaps I can convince you to rethink – dropping those pixels by 50% is only a 30% decrease in resolution, and you may find that you never notice the difference if the camera you’re using has an autofocus that’s too hard to get right, resulting in slightly blurred images, or can’t take decent pictures indoors.

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Entry filed under: Photography. Tags: , , , , .

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