Megapixels

[mmcirvin]

In the comments to Reid Stott's interesting article on the ongoing film-versus-digital debate in high-end photography, he mentioned something that I've talked about before, but needs to be said again: When shopping for a digital camera, megapixels aren't everything. People who admire the pictures I take often ask me how many megapixels my camera has, and they're mildly surprised that it's an old 2MP job. It seems as if the megapixel count has become the equivalent of megahertz in the PC industry (or kilobytes of RAM back in the eighties): the premier "mine's bigger than yours" number. It shouldn't be.

Mind you, they're not completely insignificant. Other things being equal, the capability for more pixels is usually better. If you're just going to use your pictures on the Web or in e-mail, two megapixels are more than enough; you'll end up scaling or cropping the pictures down anyway. 2MP produces pretty good prints, too, at typical sizes. But the more pixels you have, the more cropping you can do when composing your image after shooting it, and still get decent-looking results. Also, because of the interpolation done to get a full-color image out of the camera's filter mosaic, digicam images displayed onscreen at the original resolution usually look a little soft, so an image scaled down from a higher resolution can actually be somewhat sharper and more detailed.

But in practice, other things are not equal. As Reid mentions, a larger number of pixels crammed into the same physical sensor size can lead to greater sensor noise and make grainier-looking images. (I've heard that this is a problem with the Nikon 3500, the higher-resolution sibling of my 2500.) Bigger images take up more memory on the card. Also, if you've got a finite amount to spend, often you can spend it either on higher sensor resolution or on other things, and the other things might be better. The size and quality of the lens is important; optical zoom is good. The most limiting thing about my little Nikon is not that it's two-megapixel, but that its lens is kind of small and doesn't do that great in low light, which limits what I can do indoors.

The most important thing to realize, though, is that there's an effect of diminishing returns. The pixel count is a kind of area: horizontal resolution times vertical resolution. That means that the number of pixels per centimeter in your image (however you display it) only increases as the square root of the pixel count. Two megapixels are 1.41 times as good as one, but four megapixels are only 1.15 times as good as three. Before long, you need tremendous increases in megapixels to get an equivalent increase in image quality. Meanwhile, the amount of memory you need to hold the images goes up more or less linearly with the pixel count.

It seems to me that, once you've got the money to buy a higher-end camera (which I can't personally justify doing at this point), one of the most important tradeoffs has nothing to do with megapixels, but is lens size versus portability. I'd love to have a camera with the kind of lens that you can use to shoot reliably by dim indoor light, but better lenses are usually bigger, so they tend to make cameras harder to carry around and operate on a whim. Conversely, there are expensive and fancy cameras that easily fit in a shirt pocket, but slowish lenses are typically their weak point.

Comments

[kerri9494.livejournal.com]

Are there digital cameras with 3 CCDs yet? Because I think that would make a huge difference in image quality.

Also, I too have a 2.xMP camera. It does a GREAT job in bright light, but the pics it takes with a flash...why bother? The thing can't seem to focus in half-light, never mind darkness. When I get my next camera, the thing I'm going to concentrate most on is clarity of the pic in the dark.

[mmcirvin.livejournal.com]

You mean separate sensors for each color channel? It would probably be hard to get the registration exactly right.

On the other hand, there is an unusual sensor made by a company called Foveon that senses red, green and blue channels for every individual pixel, which eliminates the Bayer mosaic and the interpolation needed to get around it. It's currently used in an expensive digital SLR from Sigma. I've heard that the first model that came out was kind of a dud (sharpness was amazing but there were weird artifacts when the brightness became saturated), but the second generation is much better.

Entry-level digicams all seem to have some trouble in low light, and you have to spend major bucks to do much better. More of them now at least have some sort of auto-focus assist light to help with the focusing. There are those fancy Sony ones that actually have a laser doohickey on them for this purpose. To do really well in low light I imagine you'd want to get either one of those big Sonys that are almost all lens and look L-shaped, or go the whole hog and get a digital SLR. But that's around a thousand dollars right there at the very least.

[jecook]

The Nikon D100, used by journalists, is a 6 MP digital SLR with the traditional F style lens mount. The body runs nearly 2 grand by itself. Trust me on this. :)

[mmcirvin.livejournal.com]

Yeah, I was thinking of the Canon Digital Rebel, which isn't as capable as the professional ones but is almost consumer-priced-- about $1000 for the kit with the lens. I think Nikon is coming out with a similar low-end DSLR soon.

[jecook]

There are DV camcorders that use 3 seperate CCDs, but they are quite expensive ($2800 for a Canon GL1 at one site), but they are professional grade. JVC has a single CCD unit out which is more expensive ($3000+), but HDTV certified.

[davetheinverted]

Thanks for the link, and good point about lens size. I recently picked up a Canon S50 and I've already run into the issue of not being able to get high-quality indoor shots (even in fairly well-lit rooms) without the flash. I'm still pretty happy with it, and the compact form factor is a big plus, but I'm casting covetous eyes towards the G3....

Dav2.718

Lens quality is really the primary factor

[(Anonymous)]

[I don't have a LJ account, hence the "anonymous" post]

Unless you're into printing things at fairly large sizes (read: 11x14 and up), the quality of the lens is the primary factor once you're shooting with a camera with a sensor size of greater than three megapixels. And, as the CCD bin size gets smaller, noise does go up. This is generally a problem only on smaller cameras; all of the current crop of DSLRs have, in my opinion, sensors of adequate size.

For an example of what three megapixels looks like through a good lens, consider:

http://www.wherry.com/photos/2002-10-11-africa-highlights/IMG_2112.jpg

This is a picture my wife took with a Canon EOS D30, a 3-megapixel DSLR accepting Canon EOS-mount lenses (the one she used for this shot was a 28-105mm zoom, one of Canon's better consumer-grade lenses).

High-resolution sensors *are* useful, though, when you decide after the fact that the image would look better cropped. For example, consider:

http://www.wherry.com/photos/2003-11-27-sfo/E7DU0349.jpg (warning: large file!)

There are some neat architectural details in the image that are usable primarily because the resolution of the sensor is so high. The camera used, a Canon EOS 1Ds, has an eleven-megapixel sensor; the lens used here is a 70-200mm pro-grade zoom. This particular combination really illustrates Matt's point about the size-versus-quality tradeoff, though: the camera+lens combination used here is pushing ten pounds.

Phil Wherry
psw@wherry.com