Wanted: meaningful digital camera metrics
I just had a short discussion with @kalleboo on Twitter about digital camera sensors and the quality of their resulting photos. I took this a bit further by researching for a while on the web, and found some interesting facts.
Firstly, the resolution in megapixels (Mpx or MP) that is so widely used as a metric today, actually says very little about the quality of a digital camera. If we set aside lenses and craftsmanship (which of course affect quality) it’s still a lot more meaningful to compare image sensors than resolution. Megapixels are here to stay though, because they’re meaningful metrics. Had the image sensors had a more meaningful metric, this may have been a common way of referring to camera quality — as it should.
The camera’s lense takes in light in a circle with a certain diameter, and from the center of this circle, the image sensor crops a rectangle that becomes the photo. The image sensor is typically a CCD, though it could be a CMOS as well.
Differences between bigger, more expensive cameras and smaller, cheaper ones are plenty, but one is the sensor size. A compact camera and a DSLR that both have a 10 MP resolution do not produce the same results. Now, obviously optics weigh in, but aside from that, the DSLR is likely to have a much larger sensor, which makes it more light sensitive and thus its images get less noisy.
The metric used for image sensors today, the so-called type, hails from 50s TV camera tubes and is given in fractions of an inch. It actually corresponds to the imaging circle and doesn’t necessarily have anything to do with the size of the image sensor. Example sizes are 1/2.5″, 1/1.8″ or even 2/3″ which don’t exactly lend themselves well to comparison. Try it yourself — which is bigger: 1/1.8″ or 2/3″?
There is no fixed ratio between sensor size and imaging circle diameter, however the mean value is about 2:3. This means the metric is off by a lot, and we really need another one. How about the diagonal size of the actual sensor? And let’s do it in SI units instead of outdated imperial ones, so let’s use mm.
According to Digital Photography Review a type 1/3.6″ sensor, would be about 5.00 mm, 1/2.5″ about 7.18 mm, 2/3″ about 11.00 mm and 4/3″ about 22.50 mm. Why don’t we just designate them by that measurement? We can even round down to simplify. If an ad for a digital camera said “10 MP resolution, 5 mm CCD” and another “8 MP resolution, 11 mm CCD”, I’m pretty sure I would buy the second one, even though it had an inferior resolution.
But that’s not all. Consumer cameras ofter use a Bayer filter to split incoming light into base colors onto the CCD, whereas professional cameras can have three separate sensors with a prism diverting each base color onto its own sensor. This gives the expensive cameras a much better color resolution, and adds another metric — i.e. number of sensors.
If a camera had multiple sensors, it might be displayed as “12 MP resolution, 3 x 22.5 mm CCD”. Of course this could become a bit much for certain people, and they might ignore the sensor altogether. A simpler format might be just “10MP/5mm”, leaving out CCD/CMOS and number of sensors.
To conclude, I would love to see camera producers being bolder and challenging megapixels as the “people’s standard” metric. Regular people are easy to fool (digital zoom proves that) but couldn’t you give us the chance to choose for ourselves whether or not to disregard important information? Discussion is welcome through comments.