Choosing A Focal Length
Even in the days before I truly was interested in photography I understood that zooming out and taking a picture near someone’s face makes their nose look big. However, I never really understood the full impact the choice of focal length makes on an image. I decided to write a simple post for my “more newbie than me” photography friends which will hopefully get you thinking about focal length with each shot.
Even the most amateur photogs know that a wide-angle (short focal length) lens will expand your field of view (zoom out = fit more in the picture). They also know that a telephoto (long focal length) lens will allow you to get closer to a subject. What many new photographers don’t understand is how the focal length can completely change the perspective of an image. They don’t understand the concept of “compression” or how lines completely change as focal length changes.
Let’s examine the two images below. In both of them the two subjects are standing 10 feet apart and the nearer subject fills up approximately the same percentage of the frame. In the wide-angle image on top, a focal length of 10mm was used and in the telephoto image, 200mm was used.
The far subject appears very distant in the wide-angle shot yet the subjects appear practically next to each other in the telephoto shot. We say that the telephoto lens (long focal length) compresses the scene (ie makes things look closer together). Why does this happen if the subjects haven’t moved? Ignoring the background for the moment, think of the total depth of the image as being the distance from the lens to the far subject. In the wide-angle shot this distance was 12′ and in the telephoto shot, about 80′. The 10′ distance between the subjects is roughly 84% of the depth in the wide shot (a large percentage of the total depth — therefore exaggerating the distance) vs. about 12% in the telephoto (a small percentage of the total depth — effectively compressing the subjects together). That’s a very crude explanation but hopefully makes some sense to you.
Note also how, in the wide-angle shot, you almost lose the fact that the far subject is standing slightly downhill but in the telephoto shot it appears that the ground just drops away — you can’t even see her feet anymore.
I remember noticing the effect of compression while watching the Olympic marathon trials in 2008 (Of course I had no idea about focal lengths or compression at the time — no idea *why* I was seeing it). A good friend of ours was competing so we took the time to watch the coverage. At one point in the race the leader had opened up some distance from the others. However, on one stretch of the course the camera focused on the runners as they approached and it appeared that her competitors had completely caught up to her — they appeared to be maybe 10′ behind. We were actually seeing the effects of compression from a telephoto lens. Footage from a helicopter showed that the leader still held a significant lead.
The choice of focal length also has a dramatic effect on perspective and appearance of lines in an image. Using a wide angle has the effect of making lines converge more quickly. Go shoot something with strong lines (a brick wall for example). Use your shortest and longest focal lengths to experiment.
One can also have some fun with the wide angle. This shot was taken right in front of this boy’s face with a 10mm focal length. Note the apparent huge eyes/head and tiny feet. There are all sorts of games you can play with perspective.
Do you simply choose a focal length on the basis of filling your frame with a subject? Or, do you take the time to choose a focal length and move your feet based on what you want to portray or emphasize? It can have a dramatic effect on the final image.