Pixels – How many do we need?

Digital Imaging

by Hans Weichselbaum

I had my 35mm film gear for 25 years, and if it wasn’t for digital it would have lasted another 25 years. Every new and improved film coming on the market gave us an automatic upgrade.

The situation is very different now in the digital age: the moment we proudly unpack our latest camera, a new and better model is announced. The megapixel race has given us shirt pocket cameras sporting 12 MPixels, with high ISO as one of the latest buzzy selling points.

How much of this is marketing hype and how many megapixels do we really need? Remember the rave reviews of the 3 MP Canon D30 in the year 2000? Another question comes to mind: looking at the rate of progress so far, is it only a matter of time before we get SLR-quality images out of our cell phones? The answer to the last question is no. As we will see just now, the laws of physics put up a natural barrier.

Pixels

Moore’s Law. For the last 30 years Moore’s law was holding up with chip density doubling and prices cut in half every 18 months. And it looks likely to remain valid into the foreseeable future. That’s why your humble desktop computer is more powerful than a multi-million dollar, room-filling Cray supercomputer used to be!

Today’s digital cameras are semiconductor devices with some optics built around them. We can expect the price of optical sensors to go down, with memory capacities and processing speeds going up. So, what prevents us from getting tiny point-and-shoot (P&S) cameras with 20 MP and excellent quality at ISO 800?

Sensor Size and Counting Photons. To compete in the pixel race camera manufacturers need to squash an ever increasing number of pixels onto a given sensor size. The following table gives you some examples for sensor and pixel sizes of various cameras. The pixel size is measured in μm (or microns, =1/1000mm)

Sensors work by converting photons into electrons at the individual pixel sites. You can think of a bucket collecting rain drops – the bigger the bucket, the more water (=light) we can collect. The less photons we collect, the less accurate the information will be. Under low light levels the signal needs to be amplified which gives us the dreaded noise, making the high ISO settings on P&S cameras with their tiny pixels virtually useless.

It turns out that the optimum pixel size is 5-9 microns.

Cameras with >9 micron pixels have problems with aliasing (low frequency artefacts showing up as moiré pattern). P&S cameras with their small pixels suffer from increased noise (especially at high ISO settings) and reduced exposure range, which leads to blow-out highlights.

In general, the noise in modern cameras, from the simplest P&S to the best SLRs is dominated by photon counting statistics. There is still room for improvement by increasing the quantum efficiency, but not by much. We have seen improved micro lenses over the pixels, gathering more light, but the fact is that we have almost reached the physical limits.

It is difficult to imagine the pixel pitch dropping to lower than 5 microns for professional cameras, which would give us a maximum resolution of 14MP for APSsized sensors. Full-frame sensors should be able to handle 33MP, however, you will need the very best lenses to utilise this kind of resolution.

How many Pixels do we need? We are back to the original question.

When I upgraded from my first digital camera, the Canon G2, to the EOS 20D about three years ago, I was quite disappointed. There was not that much of an improvement in resolution when going from 4 to 8 MP – certainly not 100%, perhaps 25% more detail. Don’t expect a noticeable jump in resolution when you upgrade from 8 to 10 MP, or even 12MP.

An image coming from a 4 MP camera can be printed to 18×25 cm or 8×10” (at 240 dpi, which is more than enough for inkjet printing). However, digital camera files can easily by upsized by 50-100% without much penalty. That would give you an A3- sized print from a 4MP camera, which, when viewed from the correct distance, will give you a very satisfying, detailed and sharp image. The original shot needs to be sharp to start with, and don’t forget to sharpen the image after upsizing.

So, today’s 10 MP cameras will give you more than enough pixels, even for A2-sized prints. Of course, having spare pixels will allow you to do some cropping without penalties.

Other Factors to Consider If you decide on a 10 MP camera you can choose between a small P&S and a larger SLR camera with APS-sized sensor. The smaller cameras with their tiny sensors will give you excellent results at low sensitivities, ISO 50-100, but will have noticeable noise at ISO 400. Anything higher is only useable for social snaps. The Olympus cameras with their ‘4/3’ sensors are an interesting compromise. Their latest E3 has received excellent reviews, but also suffers from higher noise at ISO 1600.

Most professionals prefer the optical viewfinder to the LCD screen on the back of the camera. The latest crop of SLRs have life preview – useful when you shoot from awkward angles and a big help for macro work from a tripod.

Dust on the sensor is one of the annoying tradeoffs you get for the convenience of changing lenses. Check the latest models which use an automatic ultrasonic sensor cleaning cycle.

Image-stabilisation has been around for quite some time now and has also made inroads into the P&S market. Sharp teleshops at 1/25th of a second are possible and can allow you to stay away from high ISO settings. I see this as one of the most important improvements of the last few years.

To upgrade or not to upgrade – I hope that this brief summary will give you some guidelines.

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