How Is Optical Zoom Measured?

2013.01.31

With all these new ultra-zoom boasting now up to 50X optical zoom. The big question is: How Much Is An X? Plus, what difference does an extra X or two make?

The optical zoom, measured in Xs, is simply the ratio between the longest focal-length and the shortest focal-length of the camera's lens. Thus, if a camera has no lens, it should have no zoom. This is why no optical zoom is given for DSLR and Mirrorless cameras. When it does have a lens, the calculation is very easy. For example, the Fuji Finepix F900 EXR launched yesterday has a 25-500mm equivalent lens, so its zoom is 20X because 500 / 25 = 20 while the recently-reviewed Fuji Finepix SL280 shown below has 28X optical zoom because 676 / 24 = 24.

It does not matter if this is computed based on the actual or equivalent range because the answer will be the same. More importantly though, two cameras with 10X zooms can have very different ranges, say 25-250mm and 35-350mm, which is why it is better to look at the actual equivalent focal-length instead of zoom ratio. Between these two examples, one camera is much better of architecture (starting very wide at 25mm) and the other for wildlife (reaching much longer at 350mm).

Each X therefore represents a different increment depending on the camera, usually between 24 and 28mm for a modern digital camera. As the focal-length increases linearly, the angle-of-view decreases in proportion. However, the image area decreases slower. So, while there is a huge difference in framing from 2X to 4X, the difference is much smaller from 28X to 30X. All this means that an X makes much more difference for small zooms than for long zooms.

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