Originally Posted by rustyhill
Can anyone tell me how the 12x optical zoom on one camera relates to the 200mm lens on the canon?
A focal length of 200mm is a physical characteristic of a lens. It's a measure of how much it bends light and thus its angle of view.
A 12X optical zoom is simply a ratio of the zoom lens' shortest focal length to its longest. It's a ratio between two real measurements, not a real measurement on its own.
A 12X zoom can mean that the lens has a focal length ranging from 25mm to 300mm, but it can also mean that it has a focal length ranging from 10mm to 120mm.
Because different cameras have different size image sensors, and therefore, different angles of view for the same focal length lens, in order to make a meaningful comparison, you need to use the 35mm equivalent focal length
. For instance, on your Canon Rebel, a 200mm lens has a 35mm equivalent focal length of 320mm. Meanwhile, the Canon PowerShot SX150 IS, which has a 12X optical zoom, has a physical focal length of from 5mm to 60mm. But because it has a much smaller image sensor than your Rebel, it has a 35mm equivalent focal length of from 28mm to 336mm. So the SX150, with its 5-60mm lens, could have the same angle of view as your Rebel with a 200mm lens, but the only way to know that is by comparing their 35mm equivalent focal lengths.
And, again, the 12X optical zoom specification doesn't tell us anything about the focal length (actual or 35mm equivalent), only its range of focal lengths.