Focal Length and Perspective
We saw in the previous sections how the angle of view varies depending on the focal length. Changing the focal length, however, allows you to control other aspects of your picture such as the depth of field and the perspective.
Should I zoom with the lens or my feet?
Obviously there are times when you canít just walk closer to the object you want to take a picture of. The photographer of the picture on the right was probably glad of the distance between himself and the flames shooting out the back of the space shuttle (picture courtesy of NASA).
However if you are taking a picture of the love of your life (be it a girlfriend, boyfriend, Springer Spaniel or vintage Leica M3) against a scenic background, you may have more options open to you. So first letís look at the differences between walking closer and zooming in.
You have a camera with a 18-200mm lens on it and your subject is standing 40m in front of you with some wonderful mountains in the background (about four kilometers away). You put your lens to about 18mm to take a beautiful wide angle shot to get in all the scenery. The problem is that your subject is a tiny dot in the middle. So you can either walk closer to them, or you can zoom in. Which should you do? Is there any difference?
Well yes, there is a huge difference in the resulting picture. Let's consider both options.
Option 1 (get closer). You decide to walk up to your subject leaving your lens set to the wide angle setting. Walking forward 35m, so that instead of your subject being 40m away, he/she is now only five metres away. In doing this you have also walked 35m nearer the mountains which are now 3965m away. Your subject is now a lot bigger in the picture, but the image of the mountains has not really changed a huge amount. The difference between 4000m and 3865m away is fairly insignificant.
What you have done in walking closer is changed the relative perspective of the image. Your subject is nearly 90% closer to the camera than they were before, however the mountains are only 1% closer.
Option 2 (zoom in). You stay exactly where you are but you zoom in on the camera so that your subject appears eight times bigger (about the same size in the frame as they were in Option 1 when you walked closer). The lens achieves this by narrowing the angle of view - it knows nothing of how far away things are, and everything in the scene is magnified by eight times. Therefore the mountains 4000m away now appear about 500m away.
The above image shows a subject being taken by the photographer standing at four different positions. He is using a different focal length for each picture and is changing his position so as to keep the subject the same size in the frame. The amount of background visible in each image is different (much more background in the wide angle image where the photographer is close to the subject).
Everything in the zoomed-in image will appear to be a lot more compressed. It is because of this that many photographers talk about how different lenses change the perspective in an image. Do you really think a piece of glass on the front of a camera can warp reality is such a way?
A lens does not change the perspective in an image. Perspective
is a consequence of the photographer's position and the relative
positions of the visible objects in the scene.
It is actually the photographer changing his position relative to all the objects in the scene that creates the different perspectives.
Having said this, an experienced photographer can often recognise what type of lens (which focal length) was used to take a picture because the distances between foreground and background objects are often greater than or less than what would be expected. If, for example, the hills seem incredibly close behind a subject, there's a good chance it was taken by a photographer standing some way back from the subject and zooming in.
Here are three pictures of Phil (one of the authors of this website). In each case the photographer (Colin, the other author) used a different focal length lens but changed his position to keep Phil approximately the same size. The perspective is clearly different in each image, but remember it is the photographer changing his position that alters the relative distances between camera, subject and background.
||18mm focal length (29mm in 35mm equivalent terms)|
||50mm focal length (80mm in 35mm equivalent terms)|
||120mm focal length (192mm in 35mm equivalent terms)|