Zoom factor

The so-called zoom factor describes the magnification power of a zoom lens, that is, the ratio between the smallest and largest adjustable focal length. If the shortest focal length, also called the starting focal length, is 35 mm and the longest, also called the ending focal length, is 70 mm, the zoom factor is 2. This is referred to as 2x zoom. The calculation is:

Ending focal length : Starting focal length = Zoom factor

70 : 35 = 2

If a lens has a starting and ending focal length of 35 mm and 350 mm, it is a 10x zoom. However, the zoom factor alone says nothing about the smallest and largest focal lengths, and therefore nothing about the magnification factor or the type of lens. Both wide-angle and telephoto lenses can, for example, have 3x zoom while covering very different focal length ranges:

100-300 mm = 3x zoom with a telephoto lens

3.4-10.2 mm = 3x zoom with a wide-angle lens

Cameras with built-in lenses (compact and bridge models) can be used more flexibly the larger the zoom factor is. For this reason, such cameras are often advertised with the zoom factor. However, a larger zoom factor is always also a compromise between focal length and the size and quality of the photos. Often, the aperture is significantly lower. With DSLRs, by contrast, interchangeable lenses are used, so the compromise here is more about carrying a large bag of equipment and lenses than about photo quality.