The original SLR cameras were 35mm film cameras and used a piece of film 36mm wide and 24mm high. SLR is an acronym for Single-Lens Reflex. What this means is that when you look through the viewfinder you are looking through the same lens the negative will be exposed to. The way this works is that there is a mirror inside and when you press the shutter release that mirror pops out of the way, the shutter opens and the film is exposed.
The size of that optical sensor varies in a DSLR. With a full frame DSLR the size of the sensor is the same as a piece of 35mm film. But with the DSLR sold for consumer use, the sensors are smaller. Hence, with the sensor being smaller, it crops off a portion of what a 35mm film camera would see through the same lens.
This is where the term crop factor comes from.
In the images below picture shown is the full frame representation of a 35mm SLR or a full framed DSRL such as a Canon 1DS or a Nikon D3. The green line represents a 1.3 crop factor of a Canon 1D Mark III. The yellow line represents the1.5 crop of the average Nikon, such as a D80 or a D200. The orange line represents a 1.6 crop factor of a Canon 40D or 50D
So, why aren’t all DSRLs made Full Frame? Evidently, the machine that’s used to make the digital sensors must make more than one pass to create a larger chip. In order to make cameras more affordable, a sensor that is made with a single pass is the necessary sacrifice.
Many people actually prefer the crop factor. I’m not one of them. While some people claim that a camera with a smaller sensor has better depth of field I don’t agree. After all, it’s a matter of preference whether a deeper depth of field is better than a shallow one. For certain situations it may be, such as in landscapes, but with portraits it isn’t.I also believe that by cropping off the outer edge of a lens you are changing its natural behavior. With an erratic wide angle lens that shows aberrations this probably isn’t a bad thing, but what if you want to experience a fish eye lens for the full 180 degree angle it may be capable of. Not any more. On today’s average consumer model DSLR you’re lucky if you’ll see 120 degrees.
On the other hand, if you are going out to shoot birds with the local Audubon Chapter that extra little reach a cropped DSLR can offer may be just what the doctor ordered.
In the end it’s all preference, but it is nice to know what alternatives are available.