Tag Archives: Photography

Digital camera buying tip from an engineer

Last week my digital camera (Canon A610) died, after only two years of light service. (It turns out that a large batch of cameras made by Canon late 2005 had a bad CCD connector which tends to die after a year or two.) While I was obviously frustrated, part of me was also secretly happy since it meant I could have the fun of shopping for another camera.

After a fair amount of research (but not enough, as it turns out) I ended up buying an 8 million pixel Olympus camera (SP-560UZ) to replace the 5 million dead ones on my old camera. Progress, right? Not really. I was surprised to find that the 8 MP camera, made over two years after my dead camera, produced images of lesser quality. Sure, they were higher resolution, but that was about it. How is that possible?

I looked a little more into the sensor elements used (called CCDs, for charge coupled devices) and it seems that the digital camera companies generally increase pixel counts without actually making the sensors any bigger. In fact, the CCD on the Olympus is actually about half the area of the Canon! [Note: as pointed out by Leonid, below, they don’t have to much leeway to do otherwise in the megazoom cameras.] So the pixels get much smaller, and since each pixel requires a certain amount of circuitry (that can’t shrink any further) the sensor actually becomes even less sensitive as a whole. Furthermore, the amount of noise experienced by each pixel doesn’t shrink as quickly as the pixel size (for reasons that are a bit complicated), so an 8 MP sensor experiences significantly more noise than a 5 MP sensor. They make up for this with somewhat better CCD technology and clever image processing, but there is only so much that can be done.

Another, perhaps even worse, result of smaller pixel size is that the maximum number of photoelectrons that can be stored in each pixel is lowered. A pixel on a CCD acts like a bucket for electrons. (It’s sometimes incorrectly stated that CCD pixels store photons.) A photon of light hitting the CCD pixel has a certain probability of causing an electron to be “freed” from the silicon and dropped in the bucket. While this is just a metaphor, electrons do actually follow a lot of the same rules as water filling a bucket do. Once the bucket is full, the electrons spill out, often into a neighboring pixel that isn’t yet full. Furthermore, if the bucket (pixel) is shrunk, it can’t hold as many electrons.

The increased noise plus the smaller capacity to hold electrons means that each pixel can’t handle a very large difference between light and dark in a scene. In other words, as you increase the exposure, the pixels “fill up” much quicker than if the pixels were larger. The ability to measure large variatation between light and dark in a scene is called dynamic range. A lack of dynamic range shows up as washed out highlights and lack of detail in shadows. An example of this is shown in the following zoom from a picture taken with the Olympus:

Washed out sky

It looks like a cloudy day, but this picture was actually taken at 3 pm on a nice, sunny day. The blue sky was not bright at all to the eye, but it was enough to cause the pixels which saw the sky to all max out and overflow. The fact that they maxed out is indicated by the pure white that resulted, and the overflow of photoelectrons into neighboring pixels is evidenced by the “bleeding” of the white into the tree branches. Admittedly, this is not strictly proof of anything, as I would have to provide a picture of the same scene taken with a better camera for you to be able to truly verify my claims, so you’re just going to have to trust me that this day wasn’t. Here is the full picture, to show that the rest of the picture was not overexposed:

The Olympus is a great camera in most every way, but it appears that they have pushed the pixel count so high that picture quality has suffered. All manufacturers of cameras in this class appear to do the same thing, and not one of them seems to have the guts to say “enough.” From an engineering standpoint, this pixel race makes absolutely no sense. At some point, adding pixels is counterproductive and actually lowers the effective resolution for most situations, due to the effects of increased noise and loss of detail due to dynamic range reduction. In my opinion, this point was reached on the small sensors used in compact point-and-shoot cameras at about 4-5 MP. Unfortunately, engineers don’t run companies, marketing types do. And the marketing lemmings invariably decide that putting a sticker that says “8 MP” on the side of the camera is more important that the quality of the images it produces. Most likely, they don’t even understand the trade-offs involved in doing so, and only hear the first three words when the engineering manager says “Yes, we can do that, but…”

People buy into this because they, understandably, assume that companies couldn’t possibly be so crass and cynical as to intentially fool people into paying more for an inferior product. There was probably a time when that was a fair assumption, but those days are long gone in the Persian bazaar that is the consumer electronics industry.

The counterintuitive upshot is that you can actually get a better quality image from a $200 low-end camera than from the higher-end $400 model from the same brand. If you’re buying a digital camera, consider intentionally buying a 5 or 6 MP model (if you can still find them) even if you can afford the 8 or 10 MP version. Check the specifications and buy the one with the largest CCD you can find. This means avoiding the cute pocket cameras, if you care at all about image quality. If you need to print poster-sized enlargments that require more than 5 MP, you just need to bite the bullet and splurge for a digital SLR; they use much larger sensors that operate on a fundamentally different read-out principle, and as a result they can produce 10 MP images with incredibly low noise. The idea of a small consumer level point-and-shoot camera with 8 MP is a bit crazy, if you ask me, and a terrible engineering choice.

The net result of all my research is the realization that I had a great thing in that little Canon A610, which makes its loss even worse. It was one of the last models where Canon used a relatively large 1/1.8 inch CCD, and to my eye it struck the right balance between resolution and image quality. After all this, I’m just going to try to find a used A610.