Lenses: Distance, Speed, and Perspective

Lens focal length differences affect more than just the size of the image on the camera's target -- or in the case of a motion picture camera, the film. Also affected are: • • •

the apparent distance between objects in the scene the apparent speed of objects moving toward or away from the camera the relative size of objects at different distances

Compressing Distance A long focal length lens coupled with great camera-to-subject distance appears to reduce the distance between objects in front of the lens. The drawing on the right illustrates differences in the camera-to-subject distance and the photos below show the dramatic difference this distance makes in the appearance of the background.

The woman remained in the same place for both of these photos. But the fountain in the background of the photo on the right appears to be much closer to her.

Camera distance= 1 meter (approx. 3 feet) with wide-angle lens

Camera_distance= 30 meters (approx. 100 feet) with telephoto lens

However, the only distance that changed in these photos is the subject- (woman)-to-camera distance. (We didn't move her and we obviously didn't move the fountain.)

To keep the size of the woman about the same in each picture, the photographer

used different lens focal lengths: a wide-angle lens with a short focal length for the first photo and a telephoto with a long focal length for the second. This would make it seem that both depth of field and perspective are tied solely to lens focal length -- except they're not.

Contrary to widely held beliefs, the spatial relationship differences between objects in a scene that seem to accompany wide-angle and telephoto lenses (or zoom lenses used in the wide-angle or telephoto position) are not primarily a function of lens focal length, but camera-to-subject distance.

This gets a bit tricky to follow. In the setting above, if we used the wide-angle lens while standing at the distance used for the telephoto picture on the right (30 meters), the woman would obviously end up being rather small in the scene. But if we were to enlarge a section of that wide-angle picture to make the woman the same size as she appears in the second image, the result would have about the same fountain-to-woman distance perspective as the image on the right. Although you may think this is much to-do about nothing, it becomes important in understanding the effects of zoom lenses on subject matter -- not to mention legal cases involving "wandering road signs."

The Case of the Wandering Road Signs A group opposed to the addition of more billboards along a highway reportedly launched a court case a number of years ago -- a noble goal, unless you happen to be in the advertising business.

Advertisers defended the construction of new signs by saying the existing ones had been placed far enough apart that new ones would not create a cluttered appearance. The judge asked for "photographic evidence." Each side employed a photographer who understood the effect of subject-to-camera distance on spatial relationships. One of the photographers -- and one hired by the citizen group to show the close distance between the existing signs -- backed up a great distance and used a long lens, this compressing the distance between billboards, making them appear crowded together. (Note photo above.) The photographer representing the advertisers, however, moved in close to the first sign and used a wide-angle lens. That made all the signs appear to be far apart. (No sign clutter here!) Seeing the dramatic difference between the photographs (and possibly believing "the camera never lies"), the judge reportedly assumed fraud and disallowed all photographic evidence! Now you know more about these things than the judge did.

Changes in the Apparent Speed of Objects In addition to affecting the apparent distance between objects, changes in camera-to-subject distance and changes in lens focal length influence the apparent speed of objects moving toward or away from the camera. Moving away from the subject matter and using a long focal length lens (or a zoom lens used at its maximum focal length), slows down the apparent speed of objects moving toward or away from the camera. Filmmakers often use this technique to good effect. For instance, in The Graduate, Dustin Hoffman runs down a street toward a church to try to stop a wedding. The camera with a very long focal length lens conveys what he's feeling: although he's running as fast as he can, it seems as if he's hardly moving. Both he and the audience fear he won't make it to the church on time to save the girl he loves, thus, increasing the dramatic tension in the story.

Conversely, moving close to the subject matter with a wide-angle lens increases (exaggerates) the apparent speed of objects moving toward or away from the camera. You can easily visualize why. If you were standing on a distant hilltop watching someone run around a track or, perhaps, traffic on a distant roadway, they would seem to be hardly moving. It would be like watching with a long focal length lens. But stand right next to the track or roadway (using your visual wide-angle perspective), the person or traffic would seem to whiz by.

Perspective Changes The use of a wide-angle lens combined with a limited camera-to-subject distance creates a type of perspective distortion. If a videographer uses a short focal length lens shooting a tall building from street level, the parallel lines along the sides of the building appear to converge toward the top. (Note the photo on the left.) At this comparatively close distance, the building also appears to be leaning backward. Compare the photo taken with a wide-angle lens with the photo on the right taken at a much greater distance with a normal focal length lens.

You get even more distortion when you use an extreme wide-angle lens and get very close to subjects. (Note the two photos above.) The solution -- assuming this is not the effect you want -- is to move back and use the lens at a normal-to-telephoto setting.

Here's another example of perspective distortion. Note the convergence of lines in the photo of the video switcher on the right. A close camera distance coupled with a wideangle lens setting makes the rows in the foreground look much farther apart than those in the background. Again, you can eliminate this type of distortion by moving the camera back and using a longer focal length lens.

What's Normal? Psychologists have long debated what's "normal" in human behavior. But what's normal in terms of lenses and their focal length comes down to a simple measurement. First you need to know that the human eye has a focal length of about 25mm (approximately one inch) and covers a horizontal area of about 25 degrees. Since we're used to seeing the world in this perspective, this 25-degree angle represents a "normal" perspective for film and TV cameras. With cameras, however, "normal" also depends on the area of the camera's target or film. The larger the larger area the longer the lens focal length needs to be to cover it. Still photographers have a good rule of thumb. They consider a 50mm lens normal with a ▲35mm still camera, because this is the approximate diagonal distance from one corner of the film to the other. Using the same rule, we can define the normal focal length for a video camera as the distance from one corner of the target area to the opposite corner, as shown here. If the diagonal distance on the target of a video camera is 20mm, then a lens used at 20mm on that camera will provide a normal angle of view under normal viewing conditions.

Now, if we could just quantify normal human behavior as easily.