Make That Digital Camera

Digital Pixels - How many do I need?

People assume that a camera with 6mp will produce better images than one with 2mp and that 8 or 10 are even better. This unfortunately is not necessarily true. In fact it is not even half the story.

This article began as an attempt to clear up many of the misconceptions over digital camera resolution, but the more I looked into it the more I discovered and the more complex things got! I am going to keep things as straightforward as possible but I have a feeling one article might not be enough ...

Two Types of Sensor

First of all a big difference between many less expensive compact cameras and their more expensive DSLR relatives is that they use CCD sensors. DSLR's use CMOS devices. As far as we are concerned here, the big differences between these two manufacturing systems are cost and physical size.

Sensors manufactured in CMOS technology will be larger and more expensive than CCD equivalents.

A compact camera will typically be fitted with a CCD device measuring as little as 5mm square whereas DSLR's are creeping towards full frame 35mm size which is 24x16mm. Clearly the CMOS sensors at these dimensions can use larger pixels and more of them thus producing much better images.

Pixel Size

In order to fit a couple of million pixels onto a device measuring as little as 4 or 5mm square they obviously need to be tiny. Around 3 to 4 micro metres being typical. DSLR sensors are usually in the region of 6 to 9 micro metres.

The smaller sizes suffer from problems with noise, which manifests itself as poor resolution in the finished image, and reduced sensitivity. This limits the range of f/stops that can be used as well as the ISO range.

Large pixels reduce all of these problems but bring a new one all of their own. This is aliasing which produces what are known as moire patterns on the image. We've all seen these in photographs, or sometimes on the television, where someone is wearing a shirt or jacket with narrow stripes or a chequered pattern. The stripes or chequers tend to look blurred and indistinct with maybe a wavy pattern forming.

This can be reduced or sometimes fixed completely by using a low pass filter on the image data, but this reduces resolution which negates the advantage of the high resolution sensor.

Pixels in the 6 to 9 micro metre range seem to be the optimum size to reduce these problems.

With the smaller CCD sensors the maximum resolution is affected by the fact that pixels smaller than 2 micro metres become unusable due to their lack of sensitivty.

All (enter number here)mp Cameras are the Same

Oh no they aren't!

There are many reasons why this is not true, but the guy in the shop won't tell you. In fact, he probably doesn't even know.

If a camera is described as having 6mp it will have 6mp, but whether they are all used in recording the image is another

thing. A bit of pushing and shoving on behalf of us consumers has recently seen the word 'effective' creep into some camera spec sheets to give a more accurate description. This is why.

The camera lens is circular but the sensor containing all of those pixels is rectangular. Clearly, trying to project one onto the other produces a less than perfect fit. You will end up with either unused pixels at the corners of the sensor or wasted areas of the image that spill off the edges of it.

In order to compare like with like you need to know the number of effective pixels, that is those pixels that are actually used to form the image excluding any wasted at the corners.

We also need to think of the actual size of the pixels here. If you are comparing a compact camera against a DSLR the tiny CCD sensor most likely fitted to the compact will seriously hamper its performance.

Remember also that the DSLR will probably be much more expensive than the compact. This difference is accounted for by more than just the more expensive CMOS sensor. There will almost always be sophisticated software in the DSLR that won't be in the compact. Manufacturers take advantage of the fact that very few photographs coming out of a compact camera will ever appear as magazine cover pictures or as huge enlargements on billboards.

Colour Filter

Until the new Foveon technology began to make its presence felt all sensors were fitted with color filters so that each pixel in the array would record data for one color. Each pixel would record either red, blue or green which the software then converts back to a recognisable image.

The filter is a grid of red, blue and green cells which can be arranged in a number of different ways. Just to complicate things further, our eyes are more sensitve to green than the other colours. To compensate for this more green cells are included in the filter array. Up to 50% may be green, but again the manufacturer can do as he wishes here so not all cameras will produce an identical image.

Sony actually use two shades of green in their filter arrays.

All of the above will have some effect on the finished image, as will the quality of the lens fitted and the built-in software so there is no guarantee that cameras having the same number of pixels will produce similar results, or that more is better.

The truth is that, rather like buying a new hi-fi system, we all have different ideas. You need to take note of the number of pixels as a guide, but try to see the finished results before parting with lots of money.

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