All about digital SLR sensors
CCD (charged-coupled device)
Currently the most common type of digital SLR sensor. Almost every dSLR manufacturer offers at least one CCD-based model.
Pros: Traditionally, the highest image quality, pixel for pixel. Current sensors include innovative chip architectures designed to enhance dynamic range or speed.
Cons: Most expensive. Most power-hungry.
CMOS (complementary metal-oxide semiconductor)
Initial implementations took advantage of CMOS' on-chip electronics to make cheap but noisy sensors. Stripping off the extra circuits increased each pixel's light-collecting area, thereby boosting sensitivity and quality to surpass that of many CCDs.
Pros: Theoretically, lower production cost. Uses less power than CCD.
Cons: CMOS sensors tend to be bigger than their CCD equivalents, resulting in larger cameras.
The bottom line on sensor types
Ignore the theoretical claims and judge the cameras, not the sensor types. We've seen great photos--and crummy ones, too--from digital SLRs with every technology.
Nearly all digital SLR sensors are much bigger--and consequently much more expensive--than the thumbnail-size imagers in point-and-shoot digicams. Current digital SLR models use these sensor sizes.
The specified sensor size for the Four Thirds format, an open digital SLR standard created by Olympus and Kodak.
Dimensions: 17.3mm by 13mm
Example cameras: All Olympus, Panasonic dSLRs
A loose term for imagers that are approximately the size of the APS-C or APS-H film formats. Most digital SLRs use this size.
Dimensions: Varies, APS-C (ranges from about 14mm by 21mm to 16mm by 24mm), APS-H (28.7mmx19.1mm).
Example cameras: Canon EOS Digital Rebel XSi, Nikon D80, Sony Alpha DSLR-A350, Canon EOS-1D Mark III
Often called full-frame, to indicate that the sensor is the same size as a standard frame on a roll of 35mm film. Sensors this big are very expensive to build, but they eliminate the so-called focal-length magnification factor (see page 4 for more on this).
Dimensions: 24mm by 36mm.
Example cameras: EOS-1Ds Mark III, Canon EOS 5D, Nikon D3
Larger sensors are the secret to why 10 megapixels from a digital SLR beat 10 megapixels from a consumer digicam. To spread the same number of pixels over a larger sensor area, the pixels (technically, photosites containing diodes) must be bigger. These bigger photosites gather more light, so they produce less-noisy images, capture greater dynamic range, and perform much better at high ISO settings.
So what sensor resolution do I need?
The short answer
Any current dSLR has sufficient resolution to handle Web or newspaper reproduction. For magazines and large print sizes (16x20 inches and greater), especially where you'll be cropping for detail, 8 megapixels is a good starting point. Fine-art landscape photographers and others seeking maximum detail should consider 10-megapixel-plus digital SLRs.
The long answer
Do the math. For example, let's say you're making an 8x10-inch print on an inkjet printer.
Figure out the required output resolution.
Our inkjet printer produces best results at a resolution of 240 pixels per inch or greater.
Multiply the required output resolution by the linear dimensions of your final print.
8 inches x 240 pixels per inch = 1,920 pixels required for the vertical dimension
10 inches x 240 pixels per inch = 2,400 pixels required for the horizontal dimension
Multiply vertical by horizontal.
1,920 x 2,400 = 4,608,000, or 4.6 megapixels, is our minimum required resolution.
Accommodate some overhead for cropping.
If you think you'll usually crop out about 30 percent of a photo, add 30 percent more pixels to the minimum required resolution.
4.6 megapixels x 1.3 = 5.9 megapixels
Keep in mind:
The math outlined above isn't written in stone; you can usually get good large prints even when you scale an image's resolution up in a program such as Photoshop.
What about dust?
Every time you change the lens, you run the risk of dust getting on the sensor, which at best can result in bad pixels you'll need to retouch out of your photos and at worst can muck up your camera's insides. Unless you shoot in very dusty, sandy, or otherwise particulate-heavy environments, and if you take reasonable care when you swap lenses, dust shouldn't be a huge worry; most cameras include a combination of antidust technologies, such as vibrating the sensor on startup to dislodge particles. If you are partial to shooting around dirt and sand, then you may want to search for discussions about the efficacy of various systems. Some sites, such as Chasseur d'Images, ran comparative tests in 2007 and concluded that Olympus had the only decent performance in this area. However, a year (or more) is a long time in a camera product cycle, and the results may have changed with subsequent generations of cameras.