All About Lenses - Part Two

The Long and Short of It

The focal length of a lens affects three important aspects of the image: angle of vjew; depth of field and perspecUve.

 

The angle of view gives the lens its name.

Lenses0405In Figure 5-4, a wide-angle lens (here an angle of 85 degrees) includes a great deal of territory. A normal lens (here 55 degrees) is less inclusive, and a telephoto lens has a very narrow angle of view indeed (here 12 degrees). So, at any distance from the subject matter, the wider the lens angle, the wider the field of view.

Incidentally, the angles selected for Figure 5-4 are only typical examples. Each category-wide, normal and narrow (telephoto)-includes a range of angles.

So while 12 degrees is a narrow angle, 9 degrees is also a narrow angle, though slightly more extreme.

As a videographer, you exploit the dif­ferences in lens angle of view all the time. For example: shooting a birthday party you may zoom out to your widest angle, to include more of the scene when the small room won't let you move the camcorder farther back from the action.

Going Soft

Earlier, we noted that lens aperture affects depth of field. Now let's see how lens focal length also affects depth of field.

As you can see in Figure 5-5, the wider the angle, the greater the depth of field.

In bright sunshine, a wide-angle lens will hold focus from a couple of feet to the horizon. At the other extreme, in dim light a telephoto lens may render sharp subjects through only a few inches of depth. Notice that we include the light conditions because aperture and focal length working together always govern depth of field. But the rule is, at any aperture, the wider the lens angle, the greater the depth of field, at any dis­tance from the subject.

Take special note of that last phrase, at any distance from the subject. When some photographers can't get enough depth of field they think, "Hey, no problem: I'll increase my depth of field by going wide­angle. "

Wrong! If you widen the angle you will increase depth offield, but you also reduce the size of the subject in the frame. To return it to its former size in the wide-angle view, you must move the camera closer. What's wrong with that? There's one last rule of focus we haven't mentioned yet: at any focal length (and any aperture too), the closer the lens is to the subject, the less depth of field in the image.

See the problem? Moving closer to com­pensate for the smaller image effectively wipes out the depth gained from going wide-angle. It's a wash.

We said that widening the angle decreases the subject size, and that leads us to the most dramatic effect that focal length has on the image: perspective.

Perspective and Focal Length

Perspective is the depiction of apparent depth-a phantom third dimension in a two-dimensional image.

In the real world, even people with only one functional eye can gauge distance, because the farther away objects are, the smaller they appear. Moreover, they dimin­ish in size at a certain rate because of the geometry of the human optical system.

But other optical systems, such as cam­corder lenses, may have very different
geometries, and objects may shrink much faster or slower than they do in human vision. The perspectives of different lenses depend entirely on their focal lengths.

As you can see, wide-angle lenses exag­gerate apparent depth.

Objects shrink quickly as they recede.

Normal focal lengths imitate the moder­ate perspective of human vision (which, of course, is why we call them "normal"). Telephoto lenses reduce apparent depth.

Background objects look much bigger and the space between them and the fore­ground appears compressed.

As the ground plans beside the draw­ings in Figure 5-6 show, you have to move the camera in order to achieve these dif­ferent effects. As youLenses06 change from wide­angle to telephoto, you must pull back so that the reference figure in the foreground (the man) remains the same size and in the same position in the frame. If you simply
zoomed in from the first camera position, you would instead get the effect shown in Figure 5 -4.
Wide-angle lenses can deliver very dra­matic results. People and vehicles moving toward or away from the camera appear to hurtle past. A roundhouse punch swoops toward the lens like an incoming meteor.

But since they exaggerate depth, wide­angle lenses have drawbacks as well. Get too close to people's faces in wide angle and their noses will grow to elephant size.

On the opposite side, telephoto lenses can make great compositions on the screen by stacking up pictorial elements. For instance, if you want to dramatize congestion and pollution, get an extreme telephoto shot of a freeway at rush hour, viewed head-on. Because you're squeez­ing a mile's worth of cars into 100 yards of apparent depth, you make a bad problem look ten times worse.
Telephoto shots are great for suspense.

Near the climax of Ferds Bueller's Day Off our hero must make it home through neighborhood backyards before his parents
arrive. In one suspenseful telephoto shot, Ferris runs straight toward the camera­and runs, and runs, and runs-without seeming to make any progress. It's the telephoto focal length lens, of course, that compresses the distance he's actually covering.

What's What Here?

So far we've talked about wide-angle, nor­mal and telephoto focal lengths without actually naming any. So what's a wide­angle lens, anyway: 8mm, 28mm, 90mm, 200mm? The answer: all of the above. For a full-size VHS camcorder, wide angle is 8mm; for a 35mm still camera it's 28 mm; for a 4 X 5 studio view camera it's 90 mm; and for an 8 X 10 behemoth it's 200 mm. In other words, the perspective delivered by a certain focal length lens depends on the size of the image it creates.

If you draw a picture of it, it looks like another dose of geometry; don't worry, it's really just common sense. The image created by a lens has to fill the camera's  frame, right? But the frame is rectangular  and the lens is round. That means that the lens diameter must slightly exceed the diagonal of the frame.  (Conveniently, lens designers discovered  long ago that for any size format, "normal" perspective is produced by a lens focal length slightly greater than the frame diag­onal. That's why a 15 mm lens is normal on a camcorder with a half-inch chip, but a 35mm still camera takes a 50mm lens instead. (On the larger camera a 15mm lens would be an ultra-wide.)

What does this mean to you and how do you interpret the lens markings on your camcorder? To understand the answer, you need to know what your camcorder lens is and how it works.

Zoom!

Unless you're using an older style, C-mount lens camera, or a surveillance camera discarded from a convenience store, your camcorder comes with a zoom lens. A zoom lens allows you to shift between focal lengths without changing lenses. In addition, it possesses two critical characteristics:

You can set the zoom lens at any and every focal length between its extremes. That means, if your camcorder lens ranges from 8 to 80 mm, you could, theoretically, set it at a focal length of 43.033 or 78.25mm.

The zoom lens remains at the same focus throughout its zoom range. Focus on your subject at any focal length and the subject will stay in focus if you zoom in or out. Note: some inner focus lenses do not have this capability.

Okay, so your zoom lens is marked, say, 8-80mm. What does that mean? What's wide-angle, normal and telephoto in that range?

8 mm would be wide-angle, about 15 mm would be normal and 80 mm would be telephoto. But regardless of what's normal for a given lens, the smaller the number (8mm in this case), the wider the angle. The larger the number (here 80mm), the tighter the angle.

Today many compact cameras use 1/3-inch CCDs, so their zoom lenses feature shorter focal ranges. In this format, a nor­mal focal length is around 10mm, a wide­angle setting would be 5 mm, and a strong telephoto would be 50 mm.

For example: the Canon XLl Mini DV camcorder has a 16:1 zoom that ranges from 5.5-88mm. By contrast, the Fuji H128SW Hi8 camcorder's 12:1 lens ranges from 4.5-54mm. Both have 1/3-inch CCDs.

As you can see, knowing what focal lengths mean can affect your choice of camcorder. The Canon offers you a longer telephoto; the Fuji a wider wide-angle. But to interpret the numbers, you have to start with the size of the CCD. 10mm is a "normal" focal length for a 1/3-inch CCD, while 15 mm is considered normal for a 1/2-inch CCD. Once you figure out your normal focal length, you can roughly cal­culate wide-angle and telephoto lengths as percentages of normal:

35 percent of normal: extreme wide-angle.
50 percent of normal: wide-angle.
70 percent of normal: mild wide-angle.
200 percent of normal: mild telephoto.
400 percent of normal: telephoto.
500 percent of normal: long telephoto.

As you can see, even the simplest lens on the simplest camcorder is a miracle of mod­ern optical technology. A long, long way from that accidental glop of molten glass.