In our last post, we discussed the difference between noise and noise-density. Basically, noise-density is noise relative to resolution. Output quality, on a particular medium, is more related to noise-density than to noise because most mediums have an optimal resolution. For example, a typical computer display running at 1600×1200, needs about 2 megapixels. More pixels are simply wasted. The quality of an image seen at 1600×1200 is therefore related to the amount of noise that appears at 2 megapixels.
To illustrate the difference, I shot the same scene at 3, 6 and 12 megapixels using the Fuji Finepix F50SE. The experiment was repeated for various ISO sensitivities. The difference is striking: As the resolution decreases, noise decreases in proportion. What is a very noisy ISO 1600 image at 12 megapixels, becomes much less noisy at 3 megapixels. At ISO 400, the crop is noise at 12 megapixels but rather clean at 3 megapixels.
All these images come from the same camera yet they show very different noise characteristics. The only difference is resolution. The top left image was shot at 12 megapixels, the top-right at 6 megapixels and the left image at 3 megapixels.
For an ultra-compact digital camera with a tiny sensor, the performance at 6 megapixels is excellent. At 12 megapixels, it does not seem so great, although it would take a relatively large print to take advantage of all those pixels. When using a medium that requires a resolution lower than that of the image, pixels are down-sampled to the optimal resolution anyways.
Here is the same setup except at ISO 1600 rather than 400. While the 12 megapixels crop shows strong image noise, the 6 megapixels crop is good for an ultra-compact. Indeed, other than previous F-series Finepix cameras, no ultra-compact digital camera can produce 6 megapixels images that are this clean. At 3 megapixels, which is sufficient for 8″x6″ prints, only a little noise remains visible.
Note that these tests were made with the camera set to the desired resolution but nothing prevents users from taking their images at full-resolution in the camera and reducing the resolution later. Actually, this method is generally preferable because you can effectively reduce the resolution in software but not vice-versa. There are advantages though to reducing the resolution of images in-camera: reduced memory requirement and increased speed of operation.