Depth of field is controlled by the aperture, or the opening of the lens. For several decades aperture was expressed as a function of the size of the lens. So f2 is the focal length of the lens divided by two. (In other words, the lens opening is half the diameter of the lens.) Naturally this varies with the actual focal length and on a 7mm lens the actual size of the opening at f2 is 3.5mm. But the 35mm equivalent in field of view is a 35mm focal length, and f2 on a 35mm film camera lens is actually 17.5 mm.
Now the tricky part of this is that while the aperture of the lens is relative, the depth of field is determined by the actual size of the opening. If in fact you were to stop down the 35mm film lens to an opening of only 3.5mm, the aperture would be an f8. Extending this to the minimum aperture, an f8 on my G1 would be equal to f42 on the 35mm focal length film lens. That’s extraordinary, and the reason that the depth of field was significantly enlarged.
The camera manufacturers have made allowance for this phenomenon and all the newer models (from 2000 and on) offer a choice of scenarios. And if you’re a casual shooter you’ll definitely want to use the portrait mode, or macro mode, if you want the bokeh in the background; and landscape mode if you want the entire scene in focus. If however, you aspire to understand this better you can do a search on "depth of field" and the "circle of confusion" which is the zone where the image appears to be in focus.
In addition to the lens and aperture issue, consumer cameras also exhibit a higher noise level than the SLRs. Because their sensors are very small, the pixels are closer together. And the more pixels they pack into a sensor the more noise they generate. This has led to noise reduction algorithms in the camera’s firmware and essentially negates some of the advantages of the extra pixels.
So the advantage an SLR offers is that the sensor is larger, allowing more space between pixels, and therefore has less noise per pixel. In addition, some SLRs like my Canon 10D have a CMOS sensor rather than the CCD or charged coupled device used in the consumer models. It’s the primary reason that Canon is able to reduce noise and other artifacts in their digital SLRs to a level that has become the benchmark for all others.
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