On Perspective and Focal Length

A few days ago, I had the unfortunate idea of engaging in an online debate about the effect of focal length on perspective in photographs. By now you’d think I would have learned that those exchanges rarely result in anybody changing their minds, even when presented with unequivocal visual demonstration of how wrong they are.

Anyway, I had all but forgotten about that discussion, when Benoît Felten left me a comment on a photo of the basilica of Sacré-Coeur in Paris that I had posted on 500px.com. What Benoît wrote is that he had a similar shot, taken from almost the same position, but that the two photographs looked “radically different” in terms of perspective, due to his usage of a different focal length (20mm instead of my 11mm).

Now, the two perspectives didn’t seem too different to me, as the converging lines appeared to slope at pretty much the same angle in both pictures. To verify this, I loaded each photo on its own layer and proceeded to align them as best as I could.

Perspective comparison

As you can probably see, the images do not align completely, mostly due to the fact that Benoît’s photo was taken from a spot a few meters to the right of mine, but the angle of converging lines and the relative sizes of near and far objects are very similar in both.

If you ever needed a demonstration of the fact that focal length does not affect perspective, only the relative distances between camera and subject do, this is as good as any you can find.

Thanks to Benoît Felten for allowing me to use his photo as an example. Check out his work on 500px and Google+, it’s good!

 

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