Megapixel Madness

When www.dpreview.com reviewed Canon's 30D Digital SLR, they commented:

"It wasn't surprising to see a certain level of disappointment among existing Canon owners in the evolution that is the EOS 30D, I'm sure some were at least expecting a nominal megapixel upgrade as well as the changes included ... To be fair an increase to ten megapixels would have gained little in resolution ..." (italics ours).

However, I was recently reading a respected Digital Camera review magazine, which criticised the 30D for only having 8 megapixels, when the cheaper and newer 400D had 10 megapixels. They claimed the 30D had too few megapixels for A3 prints, whereas the 400D had sufficient, which flies in the face of the quote from dpreview.

What's the real truth about this? Well, lets look at what the cameras offer in the way of resolution:

So, the 400D can yield an uninterpolated print that is about 1 inch larger on both axis. True, that's a little bit more, but it's nowhere near to an A3 print. Images from both cameras would need to be enlarged by interpolation to make an A3 print at 300DPI.

So, the camera magazine was talking utter nonsense. It's not the first one to do so and probably won't be the last.

Now, before I continue, what number of megapixels would be needed to make an A3 print at 300 DPI? A3 is 16.5 x 11.7 inches, therefore needing 4950 x 3510 pixels - 17.3 million pixels! No digital SLR comes near to that apart from the Canon 1Ds MKII - but that costs over £4,000!

What does this mean? And why is this item called magapixel madness?

DPreview further commented about the 30D as follows: "its encouraging to think that manufacturers are getting out of the routine megapixel upgrade 'for the sake of marketing'" (italics ours).

Are they being serious that more megapixels are a marketing hype rather than a quality improvement? The answer is this: the quality of the megapixels are more important that the number of them. As with stopping down to increase depth of field, there are only so many pixels that can be crammed onto an imaging chip before they start to degrade the image quality. Obviously, the more pixels crammed onto a chip, the smaller the pixels become and smaller pixels tend to introduce noise into the image. Noise is those horrible speckles that appear in digital images, especially at high ISO settings.

Now, I've already established that an A3 or larger print will almost certainly require the image to be enlarged through interpolation. Interpolation is a computer process that analyses the pixels in the image and intelligently creates extra pixels to increase the image's dimensions. Most software packages have some of 'resampling' routine, such as Bicubic, Mitchell or Lanczos that can produce very high quality interpolations. There are also dedicated packages, such as Genuine Fractals, that claim to provide even greater quality.

One thing is certain, however. The quality of the enlarged image will be in direct proportion to the quality of the original image. If the image is noisy, the enlarged image will also be noisy. In fact it will be noisier, as the interpolation process will enlarge the size of the noise. Conversely, some noise-free images that are sharp and well exposed can be interpolated even to poster size without noticable image degradation. There's an interesting discussion about this at the Luminous Landscape web site.

When I first published an article on this in an old newsletter, it was pointed out that more megapixels does increase the ability to crop an image and retain good detail. That's true, but looking at the difference between the 30D and the 400D, the number of extra pixels that comprise the 400D's extra 2mp isn't going to make that much of a difference where cropping is concerned.

The bottom line for me is that the megapixel count is not the only criteria that affects image quality. In fact, it is not even the main criteria. I would never upgrade a camera just to get a few more pixels.