To determine the magnification ratio of a telephoto lens, the focal length should be divided by?

To determine the magnification ratio of a telephoto lens, the focal length is typically divided by the standard focal length of a 'normal' lens, which is approximately 50mm for full-frame cameras. This value serves as a reference point, as a lens with a focal length around 50mm is perceived to produce a natural perspective similar to that of human vision.

When you divide the focal length of the telephoto lens by 50mm, you can effectively calculate how much closer the lens brings the subject compared to that standard view. For example, a 100mm lens would yield a magnification ratio of 2x (100/50), indicating that the subject appears twice as large as it would through a 50mm lens.

This process helps photographers understand the relative magnification power of their glass, especially when comparing different focal lengths for specific types of photography, such as wildlife or portrait photography, where subject size and detail are critically important.

Choosing a value other than 50mm does not provide the same standard reference, which can lead to misunderstandings about how significantly a lens will magnify a subject in comparison to the natural perspective seen with a 50mm lens.