January 20, 2010
I woke this morning to discover that my post about megapixel madness was up to 344 comments on the Reddit “technology” page, as well as generating a lot of talk at The Consumerist. It seems to have struck a nerve.
There’s nothing like having 20,000 people suddenly read your words to make you panic, “could I have explained that a little better?” Let me say a couple of things more clearly:
- Yes, some people DO need more megapixels
Anyone who makes unusually large prints, or who routinely crops out small areas of the frame, does benefit from higher megapixel counts. However, those pixels are only useful if they can add sharp, noise-free information.
The typical point & shoot CCD would fit on top of a pencil eraser. There are fundamental limits on how much detail you can wring out of them. So, the giant-print-making, ruthlessly-cropping photographer really needs to shop for an “enthusiast” camera model—one with a larger sensor chip.
- Diffraction sets theoretical limits on image detail
Many more people viewed the “Swindle” post than read my explanation of diffraction. The key point is that even if you have a lens that is well-designed and flawless, light waves will not focus to a perfect point. The small, blurred “Airy disks” set a theoretical limit on how much actual detail a lens can resolve.
Up to a point, “oversampling” a blurry image with denser pixel spacing can be useful. But today’s point & shoots have clearly crossed the line where the pixels are MUCH smaller than the Airy disk, and squeezing in more pixels accomplishes nothing.
Plus, making pixels tinier actually worsens image quality in other respects. Marketing compact cameras by boasting higher megapixel counts is simply dishonest.
- Higher-quality lenses can’t fix this
Better lenses are preferable to bad ones; but diffraction puts a ceiling on what even the best lens can do (yes, even one with a German brand-name on it).
To get greater true image detail, the entire camera must scale up in size. This makes the Airy disks a smaller fraction of the sensor dimensions.
- Tiny pixels are low-quality pixels
A pixel that intercepts less light is handicapped from the start. Its weaker signal is closer to the noise floor of the read-out electronics. There’s more random brightness variations between adjacent pixels. Each pixel reaches saturation more quickly—blowing out the highlights to a featureless white.
I’m aware of the theory that higher-resolving but noisier pixels are okay, because in any real-world output, several pixels get blended together. But I’ve seen enough photos with weird “grit” in what ought to be clean blue skies to be suspicious of this.
First, random pixel fluctuations interact in strange ways with the color demosaicing algorithm. Distracting color speckles and rainbowing seem apparent at scales much larger than the individual pixels.
Second, the camera’s noise-reduction algorithm can add its own unnatural artifacts—obscuring true detail with weird daubs of waxy color. (This was the problem highlighted in my example photo.) It’s better to have less noise from the start.
- Many compacts perform much better than this one
That’s true. But isn’t reading an exaggerated polemic much more fun?
Let me be clear that my complaint is about TINY CHIP point & shoots. The new micro Four Thirds cameras (which I am following closely) were created specifically to address the shortcomings of small-sensor cameras, while remaining pocketable. But they cost a lot, at least so far.
Mainly, my complaint is about honesty. Camera makers are slapping big “14 megapixel” stickers onto cameras with tiny chips.
I just want people to understand that—as The Consumerist headlined it—these are “Marketing Lies.”