Have you ever wondered why the selfie that you are taking with your phone is so bad while when you taking a photo a decent Nikon or Canon DSLR you get a nice portrait?
This happens due to the size of the sensor and focal length. Since the sensor in your cell-phone is very small a lens with a very small focal length is needed to collect the light. You can see a nice demonstration of changing focal length on the same scene:
An other demonstartion:
