All About Lenses

Lens00Without passing through a lens, the light falling on your camcorder's CCD would be as empty of information as a flash­light beam.

The camcorder's lens converts incoming light from a gaggle of unreada­ble rays to an ordered arrangement of vis­ual information-that is, a picture. It's the lens, then, that makes video imaging pos­sible. Without it, your camcorder would record an image of blank white light.

All videos are successions of individual images, each made by forcing light to form a recognizable picture on a flat surface. You can do it with just a tiny hole in the wall of a darkened room, but it's easier to use a lens.

A lens does far more than just render light into coherent images; it also deter­mines the visual characteristics of those images. For this reason, every serious vid­eographer should know how lenses work and how to use them to best advantage.

A Little Background

As long ago as ancient Greece, people noticed that when they put a straight pole into clear water, the part of the pole below the water line seemed to bend. The math­ematician Euclid described this effect in 300 Be. But it wasn't until 1621 that the scientist Willebrord Snell developed the mathematics of diffraction. Diffraction is the principle stating the following: when light passes from one medium to another­say from water to air or air to glass-it changes speed. And when light hits a junction between two media at an angle, the change in speed causes a change in direction.

Lenses, which refract light in an orderly way, were perhaps unintended side effects of glass blowing: if you drop a globule of molten glass onto a smooth, plane surface it will naturally cool into a circle that's flat on the bottom and slightly convex on top-an accidental lens. Look through this piece of junk glass and behold: things appear larger.

Now, hold the glass between the sun and a piece of paper and you can set the sheet on fire-but only if the glass-to-paper distance is such that all the sun's rays come together at a single point on the paper.

At some unknown moment somebody thought, "Hmmn, if it works with the sun, maybe it'll work with other light sources, too." In a darkened room, this someone held the glass between a piece of paper and an open window. Sure enough, at a certain lens-to-paper distance, a pinpoint of light appeared.

But then a bizarre thing happened. When the experimenter slowly increased the glass-to-paper distance, an actual pic­ture of the window appeared, small, to be sure and upside down, but so detailed that he could see that tree outside, framed in the opening. (You can try this yourself with a magnifying glass.)

Back to the Present

If you've ever seen a cutaway diagram of a modern zoom lens, you have a grasp on how far we've come from that first acci­dentally dropped blob of glass.

The camcorder zoom may contain a dozen pieces of glass or more. Some of these permit the lens to zoom, some make the lens more compact by "folding" the light rays inside it and some correct inescapable imper­fections called lens aberrations.

But since you didn't sign up for an advanced physics seminar here, we'll pre­tend that the camcorder zoom is a simple, one-element lens. We can do this because the basic idea is exactly the same: when a convex lens refracts light, the light's rays converge at a certain distance behind the lens, forming a coherent image on a plane still farther back.

The plane on which the focused image appears is the focal plane, the place where
the light rays converge is the focal point and the distance from the focal point to the axis of the lens is the focal length. Note: Contrary to common belief, the focal length is not the distance from the lens to the focal plane.

Your camcorder's image-sensing chip sits at the focal plane ofthe system, behind the actual lens.

Notice also that Figure 5-1 shows an additional measurement: maximum aper­ture, or, in plain language, the lens's abil­ity to collect light. Get comfortable with lens aperture, focus and focal length, and you've got everything you need to know about camcorder lenses. So let's run through 'em.Lenses01

Open Wide

The aperture of a camera controls how much light enters the lens. In one way, a lens is just like a window: the bigger it is, the more light it admits. But a lens isn't quite as simple as a window, because the amount of light that gets in is also gov­erned by its focal length (the distance from the lens to the focal point).

For this reason, you can easily deter­mine maximum aperture-the ability of a lens to collect light. Use this simple for­mula: aperture = focal length divided by lens diameter.

For example: if a 100mm lens has a diameter of 50mm, then 100 divided by 50 is 2. The lens's maximum aperture is 2, expressed as "f/2." Lens apertures are "f stops."

Since the amount of shooting light varies from dimly lit rooms to bright sunshine, all lenses have mechanical jrjs djaphragms that progressively reduce the aperture in brighter light. Your camcorder's auto expo­sure system works by using this diaphragm to change the lens's working aperture. In other words, the iris is changing the effec­tive diameter of the lens.

These changes occur in regular incre­ments called "stops," as noted. Each one­stop reduction in aperture size cuts the light intake in half. Most consumer camcorders fail to indicate these f stops. But some units-as well as most familiar single-lens reflex film cameras-indicate f stops by a string of cryptic digits: 1.4, 2, 2.8,4,5.6,8,11,16,22.

Why use these peculiar numbers to label f stops? Simple: long ago, lenses with max­imum apertures of f/2 were very common, so f/2 became the starting point. F/l.4 is the square root off/2; and if you look at the other f stop numbers you'll see that each is a multiple andlor root of another. (Some figures are rounded off: fill is not precisely a multiple of f/5.6.)

Just as confusing, these strange num­bers appear to work backward. As the f stop number gets bigger, the aperture gets smaller. F/22 is the smallest common aper­ture and f/l.4 (or even 1.2) is the largest.

Why should you care how big the hole is in your camcorder lens? Because the work­ing aperture has important effects on image quality and depth of focus. For critical applications, lenses create better images in the middle of their range of apertures. But for videographers, the crucial concern is the effect of aperture on focus.


Before we can explain how aperture affects focus, we need to see what focus is and how the lens does it.Lenses02

To start with, remember that the focal plane is the one and only plane on which the light rays create a sharp (focused) image. If you look at Figure 5-1 again, you'll see that the subject, the lens axis, the focal point and the focal plane are all in a fixed geometrical relationship. That is, you can't change one without affecting the others. You can't move the lens closer to the sub­ject without changing the path of the light rays. And if you do that, you change the position of the focal plane.

In Figure 5-2a, the subject is a long dis­tance from the lens, and its image appears sharply on the focal plane. Since the cam­corder's CCD is on that plane, the recorded image is in focus.

Figure 5-2b shows what happens when you move closer to the subject. The geom­etry of the light rays moves the focal plane forward away from the CCD. The result? When the rays do hit the CCD they no longer form a sharp image. You're out of focus.

The solution: change the position of the lens to compensate for the shift in subject distance. As you can see from Figure 5-2c, doing this returns the focal plane to the CCD's position and the image is back in focus again.

This is exactly what happens in your camcorder lens. Lens elements move for­ward and backward to focus the incoming light on the CCD. Most camcorder zoom lenses feature jnternal focusing: the lenses move inside a fixed-length lens barrel. Most still cameras use external focusing: you can actually see the lens grow longer as its front element moves forward for closer focusing.

What's in Focus?

Lenses03If you adjust the lens to focus on a subject near the camera, then the distant back­ground will often go soft. That's because every lens at every aperture and focusing distance has what's called a certain depth offield. Here's how it works. Strictly speak­ing, the lens focuses perfectly only on one plane at a certain distance from it. Objects receding from that plane-or advancing from it toward the lens-are all technically out of focus.

But in reality, objects up to a certain dis­tance behind or in front of this imaginary plane still appear sharp to the human eye. This sharp territory from in front of the focal distance to behind it is depth of field.

Two factors govern the extent of the depth of field: 1) the focal length of the lens and 2) the working aperture. Since we've already covered aperture, let's see how it affects depth of field.

Each drawing of Figure 5-3 represents a picture made with the same lens, at the same distance from the subjects, and focused on the same person, the woman. The only variable is the aperture. As you can see, the higher the f stop, the greater the depth of field.

In Figure 5-3a, the stop is very high (f/22) and all three subjects are sharp. In Figure 5-3b, the aperture widens to the middle of its range (f/5.6). Now the depth of field is more shallow and the man and the tree are at its front and back bounda­ries. They're starting to lose sharpness.

Open the aperture all the way to f/l.4 (Figure 5-3c) and the depth offield is quite narrow. Though the woman remains sharp, the man and the tree are just blurs. Once again, the higher (smaller) the f stop, the greater the depth of field, and vice versa.

As noted above, depth offield is also gov­erned by the focal length of the lens. But first, we need to see what that geometrical abstraction focal length really means to practical videographers.

Next month:  part two of Lenses