So I was thinking about how does the focal length of a lens actually manifest itself to how much is my camera able to ‘see’. For example, if I have a 50mm lens, and I replace it with a 85mm, how much more/less is the camera able to capture. One measure of this is the ‘Field of view’ (FOV) or ‘Angle of View‘. Now given that the camera sensor is not square, the FOV in horizontal and vertical direction will be different. Without any reason to choose one, I focus on the horizontal FOV.
Shown below is the horizontal FOV (in degrees) as it changes with the focal length of the lens. The blue curve shows the FOV for a full frame camera (36mm wide sensor) and the red for a 1.6 crop factor camera (22.5mm sensor width). I have a Canon 30D which has 22.5mm sensor. So, a 50mm lens gives me a 25° horizontal FOV. Buying a 85mm lens would decrease that to about 15°. Although still not very intuitive, these numbers do give me something to visualize.