CNET editors' digital SLR buying guide: All about sensors

What you need to know, from the kind of user you are to current technologies.

Sensor types

Start shopping for a digital SLR and you quickly find yourself entangled in a rat's nest of claims about sensor types and sizes, not to mention an entire alphabet of acronyms. Here's a breakdown of the basic sensor types.

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.

Sensor sizes

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.

Four Thirds
The specified sensor size for the Four Thirds format, an open digital SLR standard created by Olympus and Kodak. 
Dimensions: 13.5mm by 18mm
Example cameras: All Olympus dSLRs, Pansonic Lumix DMC-L1

APS
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 XTi, Nikon D80, Sony Alpha DSLR A100, Canon EOS-1D Mark III

35mm-film format
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 II, Canon EOS 5D


Geek note
Larger sensors are the secret to why 5 megapixels from a digital SLR beat 5 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.
Step 1: Figure out the required output resolution.
Our inkjet printer produces best results at a resolution of 240 pixels per inch or greater.

Step 2: 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

Step 3: Multiply vertical by horizontal.
1,920 x 2,400 = 4,608,000, or 4.6 megapixels, is our minimum required resolution.

Step 4: 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.

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