The Noise of Things to Come

2007.10.23

Megapixels are increasing. Ultra-compacts such as the just-reviewed Fuji Finepix F50 now sport 12 megapixels sensors. Some people love it, some don't. The controversy is mostly about image noise. More megapixels generally means more image noise, right? Well, it depends how you see it.

What we have to think about is our medium. How do we look at - not analyze - our pictures? How do we share them? For digital pictures, there are 3 common answers:

• Upload them to an online gallery.
• View them full-screen in a slide-show or screen saver.
• Print them. 4"x6" is still very popular. Up to 9"x12" is common now.

If we think about these different mediums and ask ourselves how many megapixels are needed, the answer is less than what most modern cameras offer. For on-screen viewing, the most megapixels of any screen still in-production is 4 megapixels. And that would be for a 30" LCD such as the HP LP3065. The typical monitor has less than 2 megapixels and so does 1080p HDTV. Prints are more demanding, but even a 300dpi 12"x9" print only uses 9 megapixels.

So if we have a camera with too many megapixels, what happens to the megapixels we do not need? They do not simply get discarded, they get filtered. The difference? Discarding pixels is skipping pixels and making an image with the rest. Filtering is producing new pixels from a greater number of original pixels.

By filtering, we are affecting the noise-characteristics of an image. When an image is filtered to produce an image with less megapixels, noise decreases. The reasons can be explained by signal processing, but the details are not important for this discussion.

Consequently, the amount of noise a camera produces is not the same as the amount of noise appearing in our prints or on our screens. These noise levels are related, but they are different. Obviously, we want to judge a camera's image quality based on our medium since that is our ultimate use for images. To this effect, we should not measure noise itself but something like noise-per-megapixel. Lets call this noise-density.

Noise is what we see when view an part of an image on our monitors because we called the image to 100%. Noise-density is what we see when view a photograph in our medium of choice. For large prints, noise and noise-density can be the same, but, for most prints and on-screen viewing, the former higher than the latter. Between a 12 megapixels image and a 4"x6" print, the difference can be quite striking.

The conclusion from all this is that a camera should be judged on the quality of its output in the medium we use, not by 100% viewing. Viewing at 100% is informative and interesting, but alone it cannot determine wether one camera's output is better than another. To judge which of two cameras produces better output, they have to be compared using the same medium. That means the same print sizes and the same screen resolution.

For the Fuji Finepix F50's review, there was a hesitation on wether to give it a Good or an Excellent rating because its image noise is higher than that of its predecessors. However, after seeing that the quality of its prints were so good, the decision became clear: the F50 could not be punished for having that many megapixels if it ended up producing prints that were comparable in quality to other cameras rated Excellent.

• Digital Camera