comparing point-and-shoot zoom factors with focal length

[jli]

I mentioned to my roommate that I don't have a telephoto lens (the most telephoto I can get is 55mm on my kit lens ). My roommate has a point-and-shoot with a 10x optical zoom.

How are compact camera optical zoom numbers calculated? One site claimed that for a 35mm camera, a 50mm lens is "normal", and so is considered 1x; 100mm would be 2x, etc. So a 55mm would be 55*1.6/50=1.76x?

A 10x zoom would then be 50mm*10/1.6=312.5mm. I'd need a 300mm telephoto on my 1.6 crop factor camera to zoom as much as my roommate's little point-and-shoot? Doh! That sounds plausible, though. Can anyone confirm that this is how the zoom numbers are calculated?

Digression: I understand that one can get the same perspective as a telephoto by just cropping a wider shot, sacrificing resolution in the process. But my roommate's compact has just as many megapixels as my 1000D/Rebel XS .

[waffles]

Take the longest zoom it has, divide that by the shortest zoom it has, and there you are. What they don't tell you is the actual zoom on there most of the time. And 10x doesn't tell you the range at all. You have a 10x zoom if you go from a 1mm to a 10mm on there, but you've got a better range if you have a 30mm at only 3x.

[Doug Pardee]

The zoom "x" number on a digicam is simply the ratio between the longest and the shortest focal lengths. No specific range is implied.

As a general rule, the shortest end will be somewhere between 24-35mm (equiv). Lenses with larger "x" numbers (super-zooms) usually start closer to 24mm, while lenses with small "x" numbers usually start closer to 35mm.

There is no industry standard for what a given "x" value means. For example, "20x" means 26-520mm (equiv) for Olympus and Casio, but 28-560mm (equiv) for Canon and Sony.

You need to look at the specifications for a specific digicam model to see what the focal length range actually is for that model.

Added in edit: As to your specific question, current 10x cameras generally run from 35-350mm (equiv) for Kodak, 36-360mm (equiv) for Canon except the TX1 is 39-390mm (equiv), and 38-380mm (equiv) for Olympus, Nikon, Sony, and Fuji.

So yes, you'd need over a 200mm lens on your XS/1000D to get the same "reach". If all you care about is reach, then you should look at one of the superzooms. DSLRs are about the quality of the images, and the photos from your XS/1000D will be sharper and, in low light conditions, a lot less noisy.

You can stick an 18-200 lens on your XS/1000D and get 29-320mm (equiv), which is pretty close to the 10x digicam range. But the photos would look a lot better.

[jli]

Thanks Doug and waffles - exactly what I was looking for!

[inkista]

One more thing. P&S cameras will have their actual focal lengths printed on the front.

If the camera has a 1/2.5" or 1/2.3" sensor, it has a 6x "crop factor." The crop factor is how the 35mm-film-equivalancy is being calculated.

For example, the Canon SX120IS has a 10x zoom.
Its "35mm-film equivalent" focal lengths are 36-360mm.
But the actual focal length range of the zoom lens is only 6-60mm.

And there are significant differences between having 36-360mm in SLR lens terms vs. the 6-60mm on the P&S camera. The magnification is actually different. The "zooming" is done more by cropping/sensor size, not optically through the lens. What's identical is the field of view; i.e., the scene coverage.

The tiny 6mm-60mm of the SX120IS is why it will rarely ever have anything out of focus, and also why the camera can have a macro mode--something a dSLR cannot do without additional special equipment. A smaller focal length gives you a much deeper field of view than a longer one. 6mm is tiny enough that you can focus closely on something without having to swap the lens.

The other thing to note is that while your 18-55 kit lens might not have as much "reach" as a superzoom bridge P&S, it can go wider. 35mm-equivalency wise, the P&S only has 35mm, while a dSLR with an 18mm lens is about 28mm.