Lenses: The Basics*
Apart from protecting it from the elements and occasionally cleaning it, the average person doesn't think too much about a camera's lens.
However, variables associated with camera lenses have a major effect on how a viewer will see subject matter. The cameraperson who understands this commands a significant amount of creative power.
To start our investigation of this "power," let's look at some basic information about lenses -- starting with the most basic of all lens attributes: focal length.
The focal length of a lens affects the appearance of subject matter in several ways.
Lens Focal Length
We define focal length as the distance from the optical center of the lens to the focal plane (target or "chip") of the video camera when the lens is focused at infinity.
We consider any object in the far distance to be at infinity. On a camera lens the symbol ∞ (similar to an "8" on its side) indicates infinity.
Since the lens-to-target distance for most lenses increases when we focus the lens on anything closer than infinity (see second illustration), we specify infinity as the standard for focal length measurement.
Focal length is generally measured in millimeters. In the case of lenses with fixed focal lengths, we can talk about a 10mm lens, a 20mm lens, a 100mm lens, etc. As we will see, this designation tells a lot about how the lens will reproduce subject matter.
Zoom and Prime Lenses
Zoom lenses came into common use in the early 1960s. Before then, TV cameras used lenses of different focal lengths mounted on a turret on the front of the camera, as shown on the right. The cameraperson rotated each lens into position and focused it when the camera was not on the air.
Today, most video cameras use zoom lenses. Unlike the four lenses shown here, each of which operate at only one focal length, the effective focal length of a zoom lens can be continuously varied. This typically means that the lens can go from a wide-angle to a telephoto perspective.
To make this possible, zoom lenses use numerous glass elements, each of which is precisely ground, polished, and positioned. The space between these elements changes as the lens is zoomed in and out. (Note cutaway view on the right below.)
With prime lenses, the focal length of the lens cannot be varied. It might seem that we would be taking a step backwards to use a prime lens or a lens that operates at only one focal length.
Not necessarily. Some professional videographers and directors of photography -- especially those who have their roots in film -- feel prime lenses are more predictable in their results. (Of course, it also depends on what you're used to using!)
Prime lenses also come in more specialized forms, for example, super wide angle, super telephoto, and super fast (i.e., it transmits more light).
However, for normal work, zoom lenses are much easier and
faster to use. The latest of HDTV zoom lenses are extremely sharp --
almost as sharp as the best prime lenses.
Angle of View
Angle of view is directly associated with lens focal length. The longer the focal length (in millimeters), the narrower the angle of view (in degrees).
You can see this relationship by studying the drawing on the left, which shows angles of view for different prime lenses.
A telephoto lens (or a zoom lens operating at maximum focal length) has a narrow angle of view. Although there is no exact definition for a "telephoto" designation, we would consider the angles at the top of the drawing from about 3 to 10 degrees in the telephoto range.
The bottom of the drawing (from about 45 to 90 degrees) represents the wide-angle range.
The normal angle of view range lies between telephoto and wide angle.
With the camera in the same position, a short focal lens creates a wide view and a long focal length creates an enlarged image in the camera.
The two images below shot from the same position demonstrates this.
Put another way, when you double the focal length of a lens, you double the size of an image on the target; and, as you would assume, the reverse is also true.
A Zoom vs. a Dolly
Another way to alter the area that the camera sees is to move (dolly) the camera toward or away from a subject. Although it might seem this would produce the same effect as zooming the lens in and out, that's not quite true.
When you zoom, you optically enlarge smaller and smaller parts of the picture to fill the screen. When you dolly a camera you physically move the entire camera toward or away from subject matter. The latter is how you would see the central and surrounding subject matter if you were to walk toward or away from it.
Some directors, especially in motion pictures, prefer the more
natural effect of a dolly, even though it's much harder to achieve
Zoom ratio is used to define the focal length range for a zoom lens. If the maximum range through which a particular lens can be zoomed is 10mm to 100mm, it's said to have a 10:1 (ten-to-one) zoom ratio (10 times the minimum focal length of 10mm equals 100mm).
That may tell you something significant, but it doesn't tell you the minimum and maximum focal lengths of the lens. A 10:1 zoom lens could have a 10 to 100mm, or a 100 to 1,000mm lens, and the difference would be quite dramatic.
To solve this problem, we refer to the first zoom lens as a 10 X 10 (ten-by-ten) and the second as a 100 X 10. The first number represents the minimum focal length and the second number the multiplier. So a 12 X 20 zoom lens has a minimum focal length of 12mm and a maximum focal length of 240mm.
The zoom lenses on most handheld field cameras have ratios in the range of 10:1 to 30:1. The photos below show the effect of zooming from a wide-angle position to a telephoto view with a 30:1 zoom lens.
Although one manufacturer makes a zoom lens with a 200:1 zoom ratio (the lens costs much more than the camera), the ratio used for network sports is generally 70:1 or less.
A camera with
a 70:1 zoom lens could zoom out and get a wide-shot of a football field during
a game and then zoom in to fill the screen with a football sitting in the
middle of the field.
Motorized Zoom Lenses
Originally, the cameraperson manually zoomed a lens in and out by push rods and hand cranks. Today built-in, variable-speed electric motors do a much smoother and more controlled job. We refer to these electric zooms as servo-controlled zooms.
Although servo-controlled lenses can provide a smooth zoom at varying speeds, directors often prefer manually controlled zoom lenses for sports coverage, because the camera operator can adjust them much faster between shots. This can make the difference between getting to a new shot in time to see the critical action -- or missing it.
Although most videographers work within the limits of the lens supplied with their cameras, it's possible to modify the focal length of most lenses (both zoom and prime lenses) by adding using a positive or negative supplementary lens. These generally go in front of the lens. Supplementary lenses, as illustrated here, can increase or decrease the basic focal length and coverage area of lenses.
Thus far, we've assumed that varying the focal length of a lens simply affects how close the subject matter seems to be from the camera. That's true, but we will see in the next section that focal length also affects the subject matter in a number of other important and even dramatic ways.
Video Projects Revision Information