A DSLR camera lens is not very different from a refracting telescope or even a microscope.We need the lens to gather light and focus it while transmitting the light to the image sensor.
Here is an example of a simple lens:
It gathers light to a focus. Sort of. There are some inherent problems with a simple lens.Lets first talk about Chromatic aberration.
Because the lens is thicker in the middle than at the edges all colors of light do not focus in the same place.