How do I determine the focal length on a digital SLR camera lense?

That is, when a camera says it has 12x optical zoom, what does that translate to in mm?

If, for example, I am looking at a Sony camera that says its has 12x optical zoom, what is that the equivalent of in an SLR telephoto lense?

Panacea is correct. The box or even the official web site will usually tell you the "35 mm equivalent." I think more cameras start at more of a wide angle equivalent, such as 28 or 35 mm. This would make the 12X zoom the equivalent of 28-336 or 35-420. I doubt you'd find one that's 50-600 and you couldn't use it without image stabilization anyhow.

It depends on the brand of camera. For example... a Nikon you will need to multiply the focal length by 1.4 and a Canon by 1.6 ie. Nikon 28-105mm = 39.2-147mm. Canon 28-105 = 44.8-168mm.
Are you using normal lenses on a digital body?
12x optical means that it will bring the object 12 times closer to you. There is no set rule, it depends on the camera and where the CCD sits in the camera.

Usually, the "n-x" zoom designation is left for "consumer-level" cameras, i.e., non-SLR cameras, since the target audience for a camera with the "n-x" label would be confused with the whole "mm" factor. SLR cameras (you know, the kind with interchangeable lenses) generally use the "mm" scale when talking about zoom, since persons who use SLR cameras generally understand that "lingo" better.

But, that said, there really isn't a good "chart" that shows the relation between "n-x" and "mm" designations. However, you can guesstimate knowing that:

A 50mm lens is essentially a 1-to-1 focal length. That is, a 50mm lens sees what you see with your naked eye. Anything BELOW 50mm is wide angle (the lower the number, the wider the angle), and anything ABOVE it is a "zoom" lens (the greater the number, the further you can zoom).

So, a 100mm lens could be equated to a 2x, 200mm would be 4x, and so on.

Not an exact relation, but it may help you understand your camera more.

Just look on the side of the box. It usually gives the focal length equivalents. However, be aware that a dSLR camera, is usually not comparable to that standard because of the size of the sensor. The equivalent focal length is usually higher than what the lens used with the camera is rated as.

http://www.dpreview.com/learn/?/Glossary...

That figure just tells you the zoom ratio from one extreme (widest wide angle) to the longest telephoto point. A 10x could be from 18 to 180mm or 20 to 200 or 25 to 250. If the lens itself is not marked, go to the instruction book. Other than that, go to the camera manufacturers web site and look it up.