4. a lens that allows you to choose many different focal lengths is

Zoom Lenses. We have two type of lenses, zoom lenses and the fixed lenses. We need to adjust the focal length of the Camera for taking the best picture. And this is not possible with the fixed lenses. Though with the help of the Zoom lenses, we can adjust the focal length. And thus, we can take better pictures no matter how far or near the image is. However, both minimum and maximum distance is limited.

Explanation:

There are two types of lenses. The first one is the fixed lenses and the second one is the zoom lenses. Out of this zoom lenses can adjust to various focal lengths,

And we have lenses according to the focal lengths as well. Read in your prescribed book in detail. Its an essential knowledge for you.