In perspective

Want to start a fight? Ask a few photographers what effect the choice of lens has on perspective. Perhaps not a fight, but you will get a lot of hearsay rules and theories. But, as we know, a proper theory is one that can be falsified and makes testable predictions.

So a camera is basically a pinhole sitting at some distance from a sensor or film. The angle of view is set by the size of the sensor and how far away it is from the hole. A big sensor further away from the hole can have the same field of view as a smaller sensor closer to the hole.

Since the distance between sensor and hole is basically the focal length, this explains why different sizes of film or sensor need different focal length lenses to get the same angle of view. For example, a 50mm lens would be a telephoto (narrow angle of view) on an APS-C sensor, a normal lens on 35mm and a wide angle on medium format. Or, to put it the other way round, a standard lens on a small sensor would have a shorter focal length than one for a larger sensor – see A and B in the diagram above.

Let’s ignore the sensor size for now and just look at the angle of view of the lens.

The lens we have fitted has a wide enough angle of view to take in both the person in the foreground and the building in the background. If I stay in the same position and fit a lens with a narrower angle of view, it gets just the person’s face and a small section of the background.

What you will notice though, is that the relative sizes of the person and the building do not change, you just get a narrower slice of the wider version. You can test this by taking the same picture from the same position on both a wide and a narrow angle lens. Or use a zoom. Enlarge the wide-angle shot so that the central portion matches the narrow-angle picture and you will find that they match perfectly. This is the falsifiable test. This means that perspective, in the sense of the relative sizes of objects in the frame, does not change with your choice of lens if you stay in the same position. All that changes is how much stuff you get in the frame.

So what does a wide-angle lens change? If you get close to the subject, it changes the relative sizes of the nearby subject and the distant background.

In this diagram I shoot a head and shoulders with a narrow-angle lens (the dotted lines). It can see a narrow section of the background, so the background looks quite large in relation to the subject. If I get close to the subject with a wide-angle lens, so that I still get a head and shoulders (the solid lines in the diagram), the subject is the same size but I see more of the background and it looks smaller in relation to the subject. What changes is called the diminuition – the rate at which objects get smaller as they get further away.

This is why we usually avoid shooting portraits with wide-angle lenses. If we get close enough to fill the frame, the relative size of things near and further from the camera changes. These could be the nose or an arm or leg. But we do use wide angle lenses for landscapes, where we want to give prominence to a foreground object (like a Joe Cornish rock).

So there you have it – the angle of view of the lens only controls how much you can fit into the picture, providing you stay in the same place. You can use the angle of view of the lens to control how big the background is in relation to the subject, but only if you move nearer or closer to the subject.

What does this mean? That perspective is controlled by position, not the field of view of the lens.

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