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‹ Photo Notes · 6 září, 2014
In the April article about the relationship between focal length and perspective I committed a silly mystification when I wrote that focal length determines perspective. It isn’t so. Or is it?
The relationship of focal length to perspective
I won’t drag it out – from the point of view of optics the focal length of a lens has no influence on perspective. None at all. In photographic practice, though, it really does compress or enlarge perspective. Let’s explain why.
For experiment no. 1 I used in turn lenses of four different focal lengths, two longer (200 and 135 mm) and two wide (23 and 14 mm). At first sight it seems that the perspective changes markedly, but the opposite is true. Only the angle (width) of the scene and the degree of enlargement change. In the photograph taken with the 200mm telephoto the model fills the whole photo, with each shorter focal length she recedes, and in the photograph taken with the ultra-wide 14mm lens she is barely visible. But what if we crop the photographs so that the model is equally close?
Now compare the proportions of individual objects in the crops from the photographs. The proportions of the model’s body, the rubbish bin on the left, the traffic signs etc., everything is in the same ratio regardless of the focal length used.
So when and how does perspective change?
A crop as proof is a nice but impractical thing. When I photograph a model with a wide lens, I probably don’t want her centimetre-sized somewhere in the distance, I need her visible across the whole photo. Then nothing remains but to go closer with the shorter lens.
The main subject – here the model – is about equally large in all the shots, but because with each shorter focal length I had to come closer and closer, the perspective between individual objects grows, because my distance from them changes.
For a better overview, each of the photographs separately at a larger resolution.
At the next nude photography workshop we’ll talk about the use of different focal lengths when photographing portraits and the body.