I'm just getting into the world of photography, stepping up from a point-and-shoot. I've just purchased a Nikon D3100 and it should be here in a couple of days.
I've done lots of research, but there's one thing I don't understand.
Why can a 200 dollar compact point and shoot zoom 10x or greater optically but to get anything over 200mm-300mm for a DSLR is going to cost an arm and a leg?
Is it sensor size that allows that kind of distance? Does something like the SX120IS and other compacts just have that small of a sensor so even zoomed all the way out, its going to be alot more magnified than something like the D3100 (which yes, I know, don't have a full size sensor either)?
Answer
It has to do with the sensor size - large sensors require correspondingly larger lenses. For example, a lens on a small sensor with field of view equivalent to 400mm on full-frame is quite small; 66mm real focal length if I did my math right, and needing to cover a much smaller image circle - both factors which lead to a smaller lens. On the other hand, a 400mm lens for use on a full-frame sensor is a giant $5000 beast. Those really big lenses are expensive because they require huge, precisely manufactured chunks of specialty glass along with the machinery & electronics to focus, zoom, and VR them.
http://en.wikipedia.org/wiki/Image_sensor_format is a good comparison of various common sensor sizes. Your Canon SX120 has a 1/2.5" sensor (5.8x4.3mm), while the D3100 has an APS-C sensor (24x16mm).
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