Zoom Factor of a Lens
The so-called zoom factor describes the magnification power of a zoom lens, i.e. the ratio of the smallest to the largest adjustable focal length. If the smallest focal length also known as the initial focal length is 35 mm and the final focal length, which is the largest focal length is 70 mm then the zoom factor is 2. This is called a 2x zoom and below you can find the formula for this:
Final focal length: initial focal length = zoom factor
70 : 35 = 2
If the initial and final focal lengths of a lens are 35 mm and 350 mm then it is a 10x zoom. However, the zoom factor alone does not tell you anything about the smallest and largest focal length, i.e. about the magnification factor or the type of lens. For example, both wide angle and telephoto lenses can have a 3x zoom and cover very different focal lengths in each case:
100-300 mm = 3x zoom for a telephoto lens
3.4-10.2 mm = 3x zoom for a wide angle lens
The greater the zoom factor the more flexible fixed lenses can be used on compact and bridge cameras. For this reason, they are often advertised with a zoom factor. However, a larger zoom factor is always a compromise between focal length and size and the quality of the photos. Often the light intensity is considerably lower. DSLRs on the other hand, work with interchangeable lenses, which means the compromise here is more the large camera bag with lenses rather than the quality of the photos.