Doug Kerr
Well-known member
We often hear that as we use a longer focal length lens, there is an increasing sense of apparent "compression" of distance in the image. Why should that impression depend on focal length?
Well, I can't tell you - cuz it isn't so.
Imagine that I shoot a scene of two telephone poles of the same height, in a line nearly going away from me, one almost behind the other, the distance between them being 132 feet (hey, that's the standard spacing!), and I being 33 feet from the nearer one.
The further pole is 5 times farther away than the near one. As a result, on the image, the further one will have a height of only about 20% that of the nearer one. Our experiential perception system will tell us that the second one is quite a way back from the first one.
Now, with the same lens aboard, we step back until we are 264 feet from the first pole and shoot again.
Now, the further pole is only 1.5 times as far as the nearer one. As a result, on the image, the further one will have a height of about 67% that of the nearer one. Our experiential perception system will tell us that the second one is only a little way back from the first one.
And that's the whole story, technically.
Why do we attribute this "apparent compression of distance" to the focal length? Well, its because if my objective was in each shot to have the nearest object fill some consistent fraction of the frame height, in the second case I would choose a lens with perhaps 8 times the focal length as for the first shot to get that framing.
I see a "more compressed distance", all right (because of the different distance ratios involved from my new camera position), but I say, "Wow, that longer focal length sure gives me 'compression of distance', all right, just like Bubba told me."
Best regards,
Doug
Well, I can't tell you - cuz it isn't so.
Imagine that I shoot a scene of two telephone poles of the same height, in a line nearly going away from me, one almost behind the other, the distance between them being 132 feet (hey, that's the standard spacing!), and I being 33 feet from the nearer one.
The further pole is 5 times farther away than the near one. As a result, on the image, the further one will have a height of only about 20% that of the nearer one. Our experiential perception system will tell us that the second one is quite a way back from the first one.
Now, with the same lens aboard, we step back until we are 264 feet from the first pole and shoot again.
Now, the further pole is only 1.5 times as far as the nearer one. As a result, on the image, the further one will have a height of about 67% that of the nearer one. Our experiential perception system will tell us that the second one is only a little way back from the first one.
And that's the whole story, technically.
Why do we attribute this "apparent compression of distance" to the focal length? Well, its because if my objective was in each shot to have the nearest object fill some consistent fraction of the frame height, in the second case I would choose a lens with perhaps 8 times the focal length as for the first shot to get that framing.
I see a "more compressed distance", all right (because of the different distance ratios involved from my new camera position), but I say, "Wow, that longer focal length sure gives me 'compression of distance', all right, just like Bubba told me."
Best regards,
Doug