What is the X in digital camera zooms?

The lens of a digital camera has a certain focal length, expressed in mm. For example, a 50mm lens has a field of view that approximates what we see with our eyes looking straight ahead. A 25mm wide-angle lens has a field of view that approximates what we see with our eyes when we turn our head from side to side.

The shorter the focal length of a lens, the wider the field of view. The longer the focal length of a lens, e.g. 300mm, the narrower the field of view. Of course, a narrow field of view is equivalent to zooming in to a far object, like when we look into binoculars.

A lens can either have a fixed focal length, e.g. 25mm, 50mm, 300mm, etc. or can have a range of focal lengths, e.g. 25mm - 100mm, 50mm - 300mm, etc.

A lens that has a range of focal lengths is called a zoom lens. The zoom factor of the lens is calculated by dividing the maximum focal length by the minimum focal length. That zoom factor is the "X" you are referring to.

For example:

1. A 25mm - 100mm lens is a 100/25 = 4x zoom lens.

2. A 50mm - 300mm lens is a 300/50 = 6x zoom lens.

3. A 50mm - 200mm lens is also a 200/50 = 4x zoom lens.

4. A 100mm - 400mm lens is also a 400/100 = 4x zoom lens.

So knowing the zoom factor of a lens alone does not tell you what its minimum and maximum focal lengths are. Both examples 1, 3 and 4 above are 4x zoom lenses, but example 1 has a minimum focal length that is a wide-angle and allows us to take wide landscape shots or include all our classmates in one shot. Example 4 has a maximum focal length that is a long telephoto and allows us to bring far object near.