system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What the numbers on my camera lens mean ?

I'm very amateur photographer . I'm use differents lens : standard , wide angle , zoom lens , macro ? But ...**what the number on my lens mean ?**

By way of illustration : 

-Canon EF **100-400mm** f4.5-5.6L IS USM
-Canon Fixed Focal Length EF **24mm** f/1.4L II USM
-Canon EF **300mm** f/4.0L IS USM

**Edit :** I'm very happy for all the answer , is dinner time here in Ireland . After my dinner I going to spend time reading **every post** here in ELI5 . Thanks a lot and if you **want to see** some of my picture //goo.gl/oOO8Ay
 [ I'm 100 % amateur ]
gpt
The numbers in mm refer to a focal-length in millimeters. This is a measure of how far from the lens (in mm) the light converges to a focused point (on the sensor, for a properly focused image). For a telephoto lens, the number is high (and the lens body longer), and for a wide-angle lens the number is low (and the lens body shorter).

The f-number refers to the minimum aperture. It is the ratio of the focal length to diameter of the aperture (wide-open) of the lens. The lower the number, the more light gets in to the sensor. Low numbers are good because they mean that you have a wider range of possible exposure and you can work in lower-light conditions. Zoom lenses have a range of f-numbers because you physically change the distance of the lens from the sensor while zooming (so the ratio changes).