Tuesday, March 25, 2008

To Say Nothing Of Megapixels

photographyThe first article in a series that gently explains digital photography and makes helpful recommendations.

It's about time I started writing about photography. I'll begin with a fundamental assertion: a camera is a tool for capturing light. Remember that and you can fend off those aggressive high-street sales clerks whose only interest is making the largest commission off whatever they need to push today. Photography equals light. End of story.

Well, ok, there is a bit more to it than that. But only a bit.

We are all familiar with digital compact cameras, the so-called "point and shoot" models. These cameras have a fixed lens (that is, it cannot be changed for another) which channels light to an electronic sensor, where it is converted to a digital data stream and saved on a memory card of some sort. The two main limiting factors here are the quality of the lens -- how much light it lets in and how little distortion is introduced -- and the size and quality of the sensor -- how much light it captures and how accurately.

Of course the lenses are rather small in order to fit the compact form factor. This is a limiting factor but worse still are the tiny sensors, generally 1/2.5" or 1/1.8". (That's one over the number in inches, meaning the second sensor size is larger than the first). These provide surface areas of 25 mm2 and 38mm2 respectively. All the light captured by the lens has to fit onto that tiny rectangle.

Digital SLR cameras commonly use the APS-C sensor. This is about 370 mm2 in area which is ten times larger than the compact camera sensors. One can well imagine that therefore an SLR has a much greater ability to accurately capture light, and hence to produce a good shot.

You might notice that I have not bothered talking like those over-anxious sales clerks I mentioned earlier. Specifically, I have said nothing of megapixels. That's because, by and large, megapixels don't matter. This might be surprising given the energy put into proclaiming the new 6 MP camera, followed by the improved 8 MP version and the even more "astounding" 10 MP model (each with a requisite increase in price and forced obsolescence of the older models). Marketing departments find it easy to sell cameras in this way. "Look! It goes up to eleven!"

At one time the pixel count of cameras was not high enough to do their images justice. But that point was passed in 2004. Given the tiny sensor and small lens, there is no reason whatsoever to get a point and shoot camera with more than 6 MP. Anything beyond that and your file sizes are larger for no good reason, filled with more noise that the camera firmware needs to work harder to remove.

And what about SLRs? Well, even with ten times the sensor area and much more light through the lens, 6 MP is a good target to aim for. This will print film-quality photos in standard sizes. From there you can use software interpolation methods or reduced resolution to get prints at larger sizes. If viewed from a reasonable distance no-one will notice.

If you don't believe me, consider billboards. These are printed with a resolution of only 20dpi or even down to a measly 2dpi. At that low end of the scale, 6 MP will give you an image of five by three metres!

To look at this from the other end, let's see what happens if we hold the image resolution constant and increase the pixel count. To print an image twice as big in a single dimension, you need twice the pixels. But to print an image with twice the area you need to quadruple the pixel count! Thus to print A3 size at 300dpi you need an 18 MP camera. The conclusion is that incrementally upgrading your camera, from say 6 to 8 MP, is a complete waste of resources. Invest the money elsewhere until you can afford a significant megapixel jump.

Of course sensors larger than the APS-C are a good thing if large prints are the goal. But cameras made with "full frame" sensors, that is, the size of a traditional 35mm film frame, are several times more expensive than the "cropped frame" SLRs I'm concerned with here. They also require a lot more in terms of image management since the file sizes are huge. They are reserved for professionals.

Conclusions: Have a 6MP compact camera on-hand for snapshots. Buy a 6MP cropped-frame DSLR for when you need something better. And if you need to print significantly larger for commercial work, you'd better start saving for a five thousand euro professional body.

Later in the photography article series I'll recommend an SLR for those of you starting out who require the biggest bang for the buck.

For Further Reading

6 Megapixel is a site that explains why that is the optimum amount for a compact camera.

The Megapixels Chart from Design 215 shows you how large you can print with a given camera.

What print resolution works for what viewing distance? is an informative article from photographer Keith Cooper.


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