The long and short of it
How the angle of view changes with different focal lengths and how this affects your image
Take a look at the following images to understand the difference in perspective and angle of view that changing your focal length has. These images are all taken from the same location, on a tripod and with a range between 14mm to 500mm. The obvious change is the size and magnification of the subject in the frame, but also pay attention to the perspective and apparent distance between the subjects. This changes as the focal length increases and the angle of view becomes more narrow. This is only one approach to using focal length to change the impact of your images. If you were to move forward or backwards while maintaining the same focal length and subject framing, you again will alter the perspective and size of the subjects in the frame.