24 Feb 2006
I’ve just bought a Pentax Optio WPi to use in situations I’d rather not take my expensive DSLR into. It is waterproof to 1.5m (and so also dust-proof), but apart from that it is a pretty run-of-the-mill compact digicam. Reviews have been fairly mixed, the main complaint being poor focusing in low light and slightly higher CCD noise than one might expect. I haven’t had the camera long enough yet to form my own opinion either of these.
It is a 6 megapixel camera (replacing its 5MP predecessor, the WP). I had some doubts over whether the extra pixels are really worthwhile, so I decided to test it for myself.
I took a series of shots of an ISO-12233 (look-alike) photographic resolution chart, with the camera’s recorded pixels set to 6MP, 5MP, 4MP, 3MP, 2MP, 1024 and 640. The chart I used is not the best reproduction in the world and the results should be taken as comparative, not absolute.
The two images below are crops from the 6MP image and the 3MP image. The 6MP image is not edited in any way. The 3MP image has been upsampled using a standard bicubic resampling by a factor of 141% to bring it to the same number of pixels as the 6MP image. The important thing to note here is that this upsampling cannot add any information that wasn’t there in the original 3MP image.
I can’t see any significant difference between them.
What that means is that the actual detail captured in a 6MP image is no more than in a 3MP image. The extra pixels are wasted. The factor that is limiting the amount of detail captured could be the quality of the lens, or perhaps the noise-removal algorithms used in the camera are destroying detail. (Compact cameras with such a small image sensor have to do a lot of noise reduction because small sensors are inherently noisy.)
The image at 2MP was only marginally worse. In fact images at 5MP and 4MP were much worse than the 2MP one – they looked fuzzy. I think this is because the camera is having a hard time resampling down from the 6MP of the sensor to 5MP or 4MP. The images recorded at 1024×768 and 640×480 showed strong aliasing – an artifact of poor resampling, so I won’t ever be using those settings.
Images recorded at 6MP on the Pentax Optio WPi take up about twice as much space as those recorded at 3MP but are no sharper. Images recorded at 5MP and 4MP are rather fuzzy, and images recorded at 1024×768 or 640×480 have strong aliasing artifacts. My conclusion is to use the 3MP setting for everything.