Megapixel mania mashed

Here's an interesting development. The Canon Powershot G10, a top-of-the-range compact camera, has recently been superseded by the Canon Powershot G11.

"And what's so special about that?" do I hear you ask? "Manufacturers are upgrading their models all the time."

Yes, but get this. The G10 had a 14 megapixel sensor. The G11 has only 10.  In other words, Canon's 'development' is to reduce the number of megapixels in their upgrade.

It just goes to show that the widespread belief - much touted by salespeople - that the more megapixels the better, may not necessarily be the whole truth.

The megapixel rating of a camera does one thing, and one thing only. It tells you how big a print you can make at a certain resolution. That's all.

It tells you nothing about the quality of the image - the sharpness, the amount of 'noise' (random speckles), the colour differentiation - nothing.

Why? What is a megapixel? how does it work?

The rectangular sensor in your digital camera is made of rows and rows and rows of tiny light-sensitive dots called pixels (short for 'picture elements'). A megapixel is simply a million of these. The number is calculated in exactly the same way as you calculate the area of a rectangle, length x width.

So, if the sensor of your camera is 2000 pixels by 3000 pixels, that is 2000x3000 = 6'000'000 pixels = 6 million = 6 megapixels. Simple.

Camera sensors are different sizes. And it's pretty obvious that an ultra-compact pocketable camera is going to have a much smaller sensor that a digital single lens reflex. Here are a few sensor sizes, drawn to scale ...

It's not exactly rocket science to work out that 10 million pixels crammed into the ultra compact camera's sensor (green) are going to be squished together a whole lot more tightly than 8 million in the DSLR sensor (red).

And when pixels are squished together so tightly they begin to interfere with each other. Electronic 'noise' (not the same as sound noise), static charges, all sorts of interference, can spill over from one pixel to the next. We're talking ultra-microscopic dimensions here. As with you and me ... well, me, anyway ... given a bit more breathing space, pixels perform much better.

In other words, more pixels don't mean better. They can simply mean a worse photograph made of more dots.

And what about that bit "... megapixels tell you how big a print you can make ...".

That's right. You have to do a little bit of maths, but it's not difficult.

When you print one of your photographs, or display it on a screen, each pixel is represented by a tiny dot of colour. Look closely at a photograph in a magazine, and you'll see what I mean. The number of these pixels/dots that are squeezed into an inch (the old imperial system of measurements is still used here) determines the quality of the final image.

High-quality printed photographs use a resolution of 300 pixels per inch (ppi). Computer monitors have a lower resolution, at 72ppi.

Lets go back to our camera sensor which was 2000 pixels by 3000 pixels - 6 megapixels. In this case each pixel translates to a dot of colour when printed. So, if you're printing at 300 pixels per inch then the biggest photograph you can get is:

2000 ÷ 300 = 6.7 inches, and 3000 ÷ 300 = 10 inches.

In other words, if you go larger than 6.7 by 10 inches the image quality will start to degrade.

Is the maths making your head hurt? Don't despair. Here's a handy chart. And, notice, underneath it the authors describe several ways of 'cheating' to get larger images.

So that's why Canon dropped the megapixel rating of their latest top-of-the-range compact camera. To get a higher quality image. They ain't daft. They know that more megapixels aren't necessarily better.