What do the measurements on my lens mean

Lenses are measured in terms of their focal length, which broadly describes the effect they have on incoming light and the way it is focused on the sensor. A short focal length, such as 24mm, doesn t have a very high level of magnification, so will focus a broad vista on the sensor. A long focal length, such as 240mm, has a high level of magnification, like a telescope, and so will fill the sensor with just the central part of the view.