Perspective

Choosing lenses can seem forbidding, but you don’t need a physics degree. Lenses giving the same sense of perspective as the human eye are called normal, while departures affecting magnification to either side of normalcy are classified as wide-angle or telephoto. Analogies from everyday life dramatize the basic differences:

These domestic devices suggest a range of dramatic possibilities. Compare telescope imagery to that of the security spyglass. The telescope brings objects closer; squashes together foreground, middle ground, and background; and isolates the middle-ground object in sharp focus while sending the foreground and background into soft focus. The security spyglass brings in a lot of the hallway outside and keeps all in focus, but produces a reduced and distorted image. If your visitor is leaning with one hand on the door, you are likely to see a huge arm diminishing to a tiny, distorted figure in the distance. This exaggerates distance, which is a function of the lens’s handling of perspective.

Perspective and Normalcy

Our sense of perspective comes from knowing the relative sizes of things and thus being able to judge how far apart they are. In a photo with a cat in the foreground and a German shepherd in the background, we judge the distance between them according to their relative sizes. In Figure 25-2 you judge the distance between the foreground and background trucks likewise—by their relative sizes. Many useful lenses, however, depart from this normality. The focallength of a lens, by the way, is the measurement in millimeters from its optical center to the film plane. Each camera format has its preferred focal length lens for perspective to appear normal:

FIGURE 25-2 Normal lens, where distances appear as the eye sees them.

These are for classic film formats, but those for video image chips (1⁄3 inch, 2⁄3 inch, and 1 inch) vary similarly according to target size. There is, as you see, a constant ratio between formats (width of film or image chip in use) and the focal lengths of the lenses. For illustration, the examples that follow discuss only 16 mm-format lenses, whose equivalent you sometimes find (though seldom calibrated) in small-format video cameras. For more on focal length, go to www.dvinfo.net//articles/optics/dofskinny.php.

Normal perspective (Figure 25-2) means the viewer sees an “as-is” size relationship between foreground and background trucks and can accurately judge the distance between them. The same shot taken with a wide-angle lens (Figure 25-3) changes the apparent distance between foreground and background, making it appear greater. A telephoto lens (Figure 25-4) does just the opposite, squeezing foreground and background close together. If someone were to walk from the background truck up to the foreground, the implications of their walk would be dramatically different in the three shots; all would have the same subject, all walks would last the same time, but each lens offers a different dramatic feel.

FIGURE 25-3 Wide-angle lens.

FIGURE 25-4 Telephoto lens, where the foreground to background distances appear different from those in Figures 25-2 and 25-3.

Perspective Changes when Camera-to-Subject Distance Changes

By repositioning the camera and using different lenses, as shown diagrammatically in Figure 25-5, we can standardize the size of the foreground truck, as seen in Figures 25-2, 25-3, and 25-4. Though background distances appear to have changed, the perspective changes actually result from changing the camera-to-subject distance, not from the lens itself.

FIGURE 25-5 Different lenses used from different distances in Figures 25-2, 25-3, 25-4 produced the same size foreground truck but altered the apparent distance to its background. Perspective changes when you alter the camera to subject distance, not because you change lenses.

Now examine Figures 25-6, 25-7, and 25-8. Each is taken with a different lens but from the same camera position. The proportion of the stop sign in relation to the background portico is identical in all three. Perspective (size proportions between planes) has not changed; we simply have three different magnifications. So, we have confirmed that indeed perspective is the product of camera-to-subject distance, for when this remains constant, proportions between foreground and background remain constant, too—even though the image was shot through three different lenses (that is, with three different degrees of magnification).

FIGURE 25-6 Wide-angle lens.

FIGURE 25-7 Normal lens.

Lenses and Image Texture

Compare Figures 25-6 and 25-8. The backgrounds are very different in texture. Although the subject is in focus in both, the telephoto version—by putting the rest of the image in soft focus—isolates and separates the subject from both its foreground and background. This is because the telephoto lens has a very limited depth of field (DOF), with only its point of focus being sharp. Conversely, a wide-angle lens (Figures 25-3 and 25-6) allows deep focus. This is useful if you want to hold focus while someone walks from foreground to background. Deep focus distracts, however, if it drowns its middle-ground subject in a plethora of irrelevantly sharp background and foreground detail. This has been the characteristic “video” look. Telephotos have a soft-textured background, while the wide-angle shot has one that is hard.

FIGURE 25-8 Telephoto lens. This shot and Figures 25-6 and 25-7 are taken from a common camera position. Notice that the stop sign remains in the same proportion to its background throughout.

Lens Speed

This deceptive term concerns a lens’s light admittance and has nothing to do with movement. A fast lens is one that lets in much light, making it good for low-light photography. A slow lens admits less light. Wide-angles, by their inherent design, tend to be fast (with a widest aperture, say, of f 1.4), while telephotos tend to be slow (perhaps f 2.8). In practice this means that, with a two-stop advantage, the wide-angle might give an acceptable exposure at one-quarter the light the telephoto would need. In practical terms, night shooting might be viable with one lens and out of the question with another. Prime (that is, fixed) lenses, having few elements, tend to be faster than zooms, which are multi-element. Primes also tend to have better acuity, or sharpness, because the image passes through fewer optics. The state-of-the-art RED ONE ™ modular digital camera (Figure 25-9; www.red.com) uses 35 mm lenses and behaves like the feature-film camera it really is, but with none of the bulk.

FIGURE 25-9 The RED ONE ™ modular digital camera in its basic form and fully equipped. (Photos courtesy of Red Digital Cinema.)

Manipulating Perspective

Using the magnifying or diminishing effect of different lenses allows us to place the camera differently and yet produce three similar shots (see Figures 25-2, 25-3, and 25-4); when the camera-to-subject distance changes, we can manipulate perspective. Wide-angle lenses appear to increase distance; telephoto lenses appear to compress it.

Zooming Versus Dollying

A zoom is a lens having infinite variability between its extremes (say 10 to 100 mm, which is a zoom with a ratio of 10:1). As we have said, if you keep the camera static and zoom in on a subject, the image is magnified but perspective does not alter. When you dolly in close with a prime (fixed) lens, however, the image is magnified, and you see a perspective change during the move, just as in life. Dollying is movement with a perspective change, zooming is magnification without a change of perspective. By the way, avoid lenses whose image size changes when you alter focus.

Getting a Film Look

Film stocks and video used to yield differing image aesthetics, but with the more sophisticated high-definition cameras and the control they allow over image black pedestal, contrast, and hue characteristics, it’s increasingly difficult to tell which was used. In the latest postproduction software you also have a high degree of control over contrast, gamma (black levels), and color saturation (color intensity).

Differences between film and video images also arise from the relative sizes of each camera’s image-collecting area. A film close-up of a man in a park will show his face in sharp focus, but the background and foreground will be agreeably soft. Using selective focus like this helps isolate the subject by sharpness as well as by framing. The same close-up shot with a video camera often has foreground, middle-ground, and background all in similar degrees of focus, and the eye is deluged with detail. Why?

At the root of these differences is the lens’s DOF. A 35 mm camera aperture is much larger than most video imaging chips. The larger image size requires more refraction (bending of light through the lens), and the net result is less DOF—that is, less acceptable focus behind and in front of the subject at the focus point. Were you to look at the image produced by a nineteenth-century plate camera, you’d see that because the imaging area is huge, the eyes may be in focus but the nose is out of focus! Now look at the results from a family 8 mm film camera, with its tiny image area. Refraction is so minimal that in daylight hardly anything is ever out of focus.

To emulate the restricted DOF of the 35 mm film camera lens with your cam-corder, you can always shoot off a tripod and use the telephoto end of your lens. This will isolate the subject from foreground and background planes, but camera movements must have steady handling, and of course you cannot handhold. Alternatively, you can use an optical attachment such as the P + S Technik’s Mini35 Digital Adapter (Figure 25-10). Using a two-stage optical operation, the adapter accepts different 35 mm lenses to produce an internal image that is then projected into the video camera’s imaging path. Now you have the focal length, DOF, and angle of view of the 35 mm format. At a stroke, the videographer has the lens choice and control of the feature film cinematographer. To see which cameras it fits, go to www.pstechnik.de/en/index.php.

FIGURE 25-10 The P + S Technik Mini35 adapter, which enables the videographer to realize all the depth of field and other qualities of a 35 mm film lens. (Photo courtesy of P + S Technik GmbH.)