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Maybe not as much as you thought.
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Enough already with the megapixels
Originally published 2005 in Atomic: Maximum Power Computing
Last updated 18/07/06.


Generally speaking, the march of technological progress gives consumers what they expect. More stuff for less money, all the time. Hurrah.

Sometimes, though, some segment of the high-tech industry gets caught in an unprogressive eddy for a while. And it's usually the fault of oversimplification.

As Albert Einstein said, everything should be made as simple as possible, but not simpler. Boiling complex concepts down to simple numbers always makes it easier for people to think they understand them, but they often actually don't.

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Take IQ, for instance. The Intelligence Quotient wasn't intended by its inventors (a century ago) to be an actual measurement of "general intelligence"; it's pretty obvious to anybody who thinks about the issue for a moment that general intelligence, as a simple quantifiable thing, doesn't exist - you can't describe this or this with just one number. IQ was meant to be a technical tool to identify schoolchildren who needed extra help. But try telling the average person that today, especially if the last Web IQ test they took gave them a score of 145.

In computing, there's clock speed. Nobody judges the worth of a car based on its engine RPM redline, but megahertz provide an easy way to decide which computer's fastest. The fact that this way doesn't actually work does not weigh heavily on the minds of those who use it. Lots of people are confident that their 3GHz P4 must be much faster than their neighbour's stupid 2.4GHz Athlon 64 X2 - or that they've been ripped off, when WCPUID tells them their "4800+" X2 only runs at 2.4GHz.

And so it is with digital cameras. Lots of people assume that a camera with more megapixels must take better pictures than one with fewer.

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The truth can actually, quite often, be exactly the opposite. And it's getting worse.

Consumer digital cameras always have teeny tiny little sensor chips in them. Sony's popular DSC-F828, for instance, is an "eight megapixel" camera with what's known as a "2/3 inch" sensor. That sounds reasonably large, but the chip is actually only 8.8 by 6.6 millimetres - smaller than my little fingernail. The sensor size specification goes back to the old vacuum-tube TV camera days, and comes from measurements of the outside of the tube's glass envelope, not from the actual size of the imaging portion at the end.

Eight million pixels on something the size of a Rice Bubble means, of course, that the pixels are very small. So small, in fact that the "micron" - one millionth of a metre - is a suitable unit for measuring them. The F828's "photosites" - the actual physical sensors, covered with coloured filters, from which the image is built up - are 2.7 microns across.

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For comparison, the Canon EOS-5D, the cheapest DSLR with a full 35mm-frame-sized sensor, has a 12.7MP sensor, but 8.2 micron photosites. The eight megapixel Canon EOS-20D and its successor, has only 6.4 micron photosites, versus 7.4 microns for the older 6MP cameras with the same sensor size. The DALSA 33 megapixel chip that powers the Leaf Aptus 75 medium/large format back - yours for only $US30,000! - has 7.2 micron photosites.

So what, I hear you say. The F828 takes good pictures, doesn't it?

And yes, actually it does. Lots of professional photographers have an F828 in their photographic arsenal along with lots of bigger, heavier, more expensive gear, because small "prosumer" cameras like this genuinely can take top-flight pictures, and are far more manageable in strange and/or "candid" situations than the heavy artillery toted by those who hunt nude celebrities.

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But there are now plenty of rather cheaper consumer cameras out there that have 8MP sensors, because every manufacturer knows it's hard to sell a five or six megapixel camera when your competitor's shiny new 8MP model's on the shelf for not much more money. Consumers don't know much about lens specs or autofocus speed, but they can do basic megapixel arithmetic real good.

8MP consumer cameras don't take pictures as technically good as those from the F828, and the F828 isn't, in turn, as good in some respects as the more expensive, larger-sensored digital SLRs with the same pixel count.

That's partly because it's more difficult to get a lens to focus an image perfectly on a smaller sensor. As sensor size falls, the precision of the internal focussing becomes more and more critical for a given pixel count. But bigger photosites can also hold more charge than little ones, while having similar inherent noise - which, to date, nobody's figured out a way to avoid without the use of stacked Peltier devices at least and liquid nitrogen at most.

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What this means is that the signal-to-noise ratio of bigger photosites is better. All other things being equal, bigger photosites give a less noisy, "grainy" image.

Lower noise, in turn, means you can get more sensitivity out of the sensor for the same amount of image grain. Cheap consumer cameras today often deliver noticeably noisy, "fast film"-looking images at ISO 400 (the same sensitivity as the fastest film you'll find at the supermarket). Big-sensored DSLRs today commonly make cleaner images than this at ISO 1600 (the fastest rating for any colour film), or even 3200.

So, why the little sensors?

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Well, they're cheaper to make. Much cheaper. Image sensors are made with technology closely related to that used to make ordinary silicon chips, and a full frame 35mm sensor has almost eight times the area of a Prescott P4 die. A "2/3 inch" sensor has seven per cent of a full frame sensor's area - and virtually all consumer cameras have considerably smaller sensors than that!

Noise isn't a direct function of photosite size. As you'd expect, modern high-res sensors have far lower noise levels than do lower pixel count sensors from several years ago. But the sensor makers seldom make great strides in just one or two generations of sensor, and right now they're definitely not keeping up with the camera companies' demand for ever higher sensor resolution.

Sony are a good example to choose, here, because they clearly know this is a problem. Their 10MP DSC-R1 is an integrated-lens "point and shoot" camera, but it has an APS-sized sensor, the same as most DSLRs (more expensive DSLRs have a sensor as big as a full 35mm film frame). Sony proudly claim the R1 sensor has five times the sensitivity and 2.5 times the dynamic range of the sensor in the F828.

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The R1 also has a very good lens; despite its 5X zoom, it gives the camera overall resolution slightly better than that from Canon's 8MP EOS 350D DSLR with a sharp prime lens on it.

So, OK, the R1 pretty much uses all of its sensor resolution. It probably never manages to capture an actual ten million pixels worth of image detail, but it's better to have a few more pixels than you need than a few, um, fewer. Fair enough.

But, despite its sensor size, the R1 loses handily to the 350D for high sensitivity use, no matter what lens you put on the DSLR - and the R1 is far better for high sensitivity use than the zillions of tiny-sensor cameras out there.

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We're already seeing cameras like the Konica Minolta DiMAGE X1. It's one of those super-subcompact flat cameras with a folded lens system - lens window in the corner, no protruding barrel - and has an eight megapixel "1/1.8 inch" (7.18 by 5.32mm) sensor.

This is madness.

Folded lenses are great - they let you have a camera with considerable zoom capability with the form factor of a rather skinny cigarette packet. But they have a hard time delivering even 1600 by 1200 pixels worth of detail. This one's being used to make files with four times that number of pixels. And every small 8MP sensor, up to and including the one in the F828, has serious noise issues. They're fine for short exposures at low ISO settings, but if you want long exposures or high sensitivity, you just can't have them without a snowstorm of noise.