Images are recorded on a DSLR’s sensor. The size of the sensor is important in two major ways.
The larger the sensor the more room there is for pixels. Camera designers can use this space for more and larger, less tightly packed pixels. More pixels mean higher resolution and larger pixels with more space between them mean less noise. All of this means bigger enlargements or more cropping flexibility when you print your photos. Sensor technology has been improving steadily and all DSLR sensors, large or small, are generally able to produce images with less noise than two or three years ago. Smaller sensors cannot effectively use traditionally 35mm professional lenses. I’ll talk about this issue next.
Read along and follow the diagram below. Lenses produce circular images. If a DSLR has a full frame (35mm-film size: 36 x 24 mm) sensor, it can capture the full image from a lens designed for 35mm film. A smaller sensor with the same lens can capture only part of the full image. This phenomenon determines what I call a sensor’s "angle of view magnifier." Some call it a "focal length multiplier", but that is incorrect as you’ll see below. A DSLR with a 36 x 24 mm sensor has an angle of view magnifier of 1.0. Currently only Canon makes DSLRs with 35mm-film size sensors. Most DSLRs without full frame sensors have an angle of view magnifier of 1.5x (Nikon, Fujifilm, Pentax and Sony) or 1.6x (Canon). So for example a 50mm focal length lens made for a 35mm film camera acts in terms of angle of view magnification like a 75mm (1.5x) or 80mm (1.6x) lens on a sub full frame DSLR.
Some photographers love the angle of view magnifier because it allows lower focal length lenses to masquerade as higher powered telephoto lenses. But it is, after all, just a masquerade because a 50mm lens’s focal length is still 50mm even though an angle of view magnification factor of 1.6x makes it seem like an 80mm lens. Lenses in the 80mm - 100mm range are great for taking portraits because they provide a nice perspective for photos of the human face and have more limited depth of field which causes backgrounds to be attractively thrown out of focus. However, the perspective and depth of field of a 50mm lens is that of a 50mm not an 80mm lens. So when you use a 50mm lens at a 1.6x magnification you get a very different perspective and sharper backgrounds, both of which are less suited to portrait photography, than you would with a real 80mm lens.
While the angle of view magnification factor allows a lower focal length lens to masquerade as a telephoto lens, it also reduces the sweeping coverage of a wide angle lens. A 12mm lens has the angle of view of an 18mm lens with a 1.5x sensor. Though 18mm may seem wide, its images don’t cut it when you need the kind of dramatic images you can take with a 12mm lens.
There’s one key advantage with smaller DSLR sensors. As noted above, you don’t need as large a circular image to cover the sensor as you do with a full size sensor. This allows manufacturers to produce lighter, smaller and potentially less expensive lenses with smaller image circles for small sensor DSLRs, than the lenses required by full sensor DSLRs. At this point there are tons of such lenses on the market, some of which produce great results.
You’ll find three types of sensors in most of the current crop of DSLRs: CCD CMOS and MOS. Nikon’s D2Hs uses a fourth type, a JFET LBCAST sensor.
CCD (Charge Coupled Device) CCDs have traditionally been more sensitive to light, which usually produces a better dynamic range along with better overall uniformity. They consume more power than CMOS devices and are usually more expensive to produce. Nikon and a number of other DLSR manufacturers use CCD sensors. CMOS (Complementary Metal Oxide Semiconductor) In the past, CMOS sensors were more susceptible to noise, causing poor low-light performance, but DSLR sensor makers have significantly improved their CMOS products in this area. CMOS devices have faster data transfer rates and consume less power for better battery life. As proof that CMOS has a significant place in DSLR technology, Canon uses CMOS sensors in all its current and coming DSLR cameras. MOS (Metal Oxide Semiconductor) MOS-based DSLR sensors are similar to CMOS devices, except that they don’t have the power saving capabilities of CMOS sensors. Olympus uses MOS sensors in most of its DSLRs. JFET (Junction Field Effect Transistor) LBCAST (Lateral Buried Charge Accumulator & Sensor Transistor array) JFET LBCAST sensors are similar to CMOS sensors, but can move data faster than traditional sensors. As I noted above, this sensor is available only on Nikon’s high speed multi-image capturing DSLR, the D2Hs, which I also discuss in the next section.