Focal length and perspective

This post is my learning log for the exercises in TAOP project 2: Focal lengths.

The project falls in two parts:

  • Assess how (and whether) the focal length of a lense impacts the character and perspective of an image
  • Assess how (and whether) the viewpoint impacts the character and perspective of an image

I went to Weald Country Park to check on the autumn colours of the leaves, and picked a quiet view of the picnic area by the Bluebell Pond (not many bluebells at this time of the year!). I took three shots, with 24mm, 40mm and 70mm focal length.

Fig 1: 24mm view of the picnic area

At 24mm, the camera has taken in much of the environment. There is a lot of (empty, apart from fallen leaves) foreground and the angle of view emphasizes the distance between the camera and the bridge and picnic tables

Fig 2: 40mm view of the picnic area

At 40mm, the view is close to “normal”, giving a sense of the viewer being part of the scene

Fig 3: 70mm view of the picnic area

Using a short tele (a bit of a contradiction in terms, really) of 70mm has brought the bridge a lot closer, and it now feels like the viewer can almost reach out and touch it

The view in Fig 1 looks cluttered – there is simply too much going on, but for the purpose of this exercise, the main thing to notice is the feeling that the camera is outside the scene looking it; it gives distance and isolation.

The normal view in Fig 2 is close to what I saw at the time – the curving angle of the bridge, the golden leaves of the overhang, the almost dried out pond and the green grass in the background. I feel the picture invites the viewer to walk across the bridge and become part of the scene.

Fig 3 feels cluttered again. The compression of the perspective (more about that in the next paragraph) means it is hard to tell which tree each of the groups of leaves belong to. The purpose of the three pictures was to show serenity, but everything in this short tele view is too much “in your face” for that. On the other hand, it works as a picture of the leaves yellowing as winter grows nearer.

I mentioned the compression of the perspective above, and most beginners books on photography will recommend using a long lens to compress the perspective and a wideangle to emphasize depth and distance. But let’s look at whether it is really the focal length or simply the viewpoint that makes the difference.

I have cropped the centre part of Fig 1and adjusted the exposure to match the one in Fig 3 (the camera was on aperture priority, and the different tones the camera could see at different angles of view meant the automation chose a difference of 2/3 stops between the two images).

Fig 4: The central part of Fig 1

The same image as Fig 1, but cropped to match the width and height of Fig 3

Fig 5: Same image as Fig 3

The same image as in Fig 3, repeated here right next to Fig 4 to make it easy to compare the two

If it hadn’t been for the explanations in the right hand column, it would have been impossible to tell which picture is which. This demonstrates that the focal length does not have any effect whatsoever* on the perspective, contrary to popular belief.

Having established that, I went onto the second half of the exercise, namely to assess the impact of the viewpoint on the perspective. For this, I shot two pictures of a birch tree with beautiful, yellow leaves:

Fig 6: Birch tree, 200 meters away (crop, 145 mm lens)

From about 200 meters away, the golden leaves appear squashed right up against the other trees and bushes in the picture, and it appears to be planted very close to the edge of the path.

Fig 7: Birch tree, 5 meters away

From about 5 meters distance, it’s clear that the birch is much nearer to the camera than the other trees, and it is also quite clear that it is some distance behind the path.

It is very clear that the perspective in the two pictures is different: In Fig 6, all the elements appear to be squashed together, whereas Fig 7 shows there is plenty of distance between them. This is most notable by looking at the distance between the birch and the path, or by the distance between the birch and the green tree to its left.

Although the picture captions give the focal lengths used, we saw in the first part of the exercise that the focal length doesn’t have any impact on the perspective. So it must be the viewpoint that does it. What comes into play is the differences in distance between the picture elements compared to the distance between the camera and the nearest element. The distance between the birch and the green tree on the left is maybe 10 meters, so Fig 6 shows the two elements at 200 meters and 210 meters – almost the same distance, and hence very compressed perspective. In Fig 7, on the other hand, they are at 5 and 15 mters distance, so they appear to be set very far apart.

This brings back the common advice from many photography books: Given the same scene and the same framing (as far as possible), a long-distance shot with a long lens will compress the perspective so that individual elements in the picture will appear close to each other, and a short-distance shot with a wide-angle lens will emphasize (or sometimes even exaggerate) the apparent distance between objects. If the objective is to fill the frame, then the distance dictates the focal length, which leads to the (incorrect) perceived wisdom of the focal length’s influence on the image. However, it is the viewpoint rather than the focal length that determines the perspective.

Footnote: Having reached a nice, simple, black-and-white conclusion, it’s worth pointing to this long debate on Flickr. As it turns out, it is not the location of the shutter, the battery (d’oh!) or the sensor plane of the camera that is important; it is the “input aperture” or “entrance pupil” of the lens. The input aperture is the point where the rays of light reflected off the subject intersect – or in other words, the location from which the camera sees. This point can be at different places within the lens, and even outside it. Input aperture displacement (having the point outside the  lens) is one of the consequences of a telecentric lens  design, so  although it can be safely ignored for normal shooting  situations, the  choice of focal lenght (which often dictates the lens design) can  actually have some impact on the perspective. Sorry about  this less  than clear conclusion.

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