Zoom comparisons - X factor vs mm size

[Philip Colmer]

Traditional 35mm cameras talk about the zoom capability in terms of the mm value of the lenses, e.g. 75mm, 200mm, etc.

Digital cameras typically don't - they often just refer to a multiplier, e.g. 10x optical zoom. I believe that I understand the reasoning, which is that a 35mm lens stuck onto a digital SLR doesn't give you the same zoom factors. Reviews often talk about an effective mm range.

Does anyone have a means of converting between the two? The main reason I'm enquiring is because my wife has a lens on her Canon camera that she is very happy with in terms of its zoom capability. However, I'm trying to see what the zoom capabilities of digital cameras are, by comparison, because I'm not entirely convinced that she really needs to stick with a model that allows for lens swaps - she's only got the one :-)

--Philip

[Bob12]

In the 35mm film world, a 50mm lens is generally considered to be "life-size" and the basis for the multiplier. Thus, a 100mm lens would be a 2x, a 150mm 3x, and so on. Just to keep us on our toes though, often a fixed lens camera will specify zoom magnification based on the lens's lowest setting. For example, a fixed lens camera with a 28-200 lens might be advertised as having a 7x zoom when in reality, it's closer to a 4x zoom. In the 35mm film world, a lens such as this would be a wide angle below 50mm and a telephoto above.

[Lee Yuan Sheng]

No, a 100mm lens is a fixed focal length (prime) lens. It doesn't have any zoom (variable focal length) to speak of. A 600/4 and a 20/2.8 have the same zoom factor - 1x.

The X rating on zoom lenses for consumer cameras have always been a bunch of nonsense to me. Read and compare lenses based on focal length and effective focal length. It's more meaningful that way.

[Bob12]

I was referring to the magnification factor, not zoom factor.

[Littleshmee]

Back to your original question... there isn't really a fixed method to convert between the two, because the "X" number multiplies by the widest angle, so a 3x zoom on one camera could be equivelant to 35-105 mm, and on another it could be 38-115 (the most common). A lot of manufacturers will list the 35 mm equivelant, which would probably be your best bet.

[Jason Kravitz]

I was wondering the same thing when looking at binoculars... I was wondering what lens for my dSLR would be equivilent magnification to 10x binos. I somehow came to the conclusion that my 400mm lens was similar to 8x so I guess that is consistent with what Bob was saying.