All About Lenses - Jim Stinson (Videomaker)

Aperture: how much light gets in. 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 a 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' maximum aperture is 2, expressed as "f2." Lens apertures are "f stops."

Since the amount of shooting light varies from dimly lit rooms to bright sunshine, all lenses have mechanical iris diaphragms that progressively reduce the aperture in brighter light. Your camcorder's auto exposure system works by using this diaphragm to change the lens' 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 one-stop 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 f2 were very common, so f2 became the starting point. F1.4 is the square root of f2; and if you look at the other f stop numbers you'll see that each is a multiple and/or root of another. (Some figures are rounded off: f11 is not precisely a multiple of f5.6.)

Just as confusing, these strange numbers appear to work backward. As the f stop number gets bigger, the aperture gets smaller. F22 is the smallest common aperture and f1.4 (or even 1.2) is the largest.



What's In Focus?

If 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 of field. 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.

As noted above, depth of field 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 videomakers.

The Long and Short of It

The focal length of a lens affects three important aspects of the image: angle of view, depth of field and perspective.

The angle of view gives the lens its name,

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


As a videomaker, 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.

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 depth of field is always governed by aperture and focal length working together. 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 wide-angle."

Wrong! If you widen the angle you will increase depth of field, 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.


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.


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, wide-angle 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 Ferris Buehler'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 camera--and 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 wide-angle 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 28mm; for a 4x5 studio view camera it's 90mm; and for an 8x10 behemoth it's 200mm. In other words, the perspective delivered by a certain focal length lens depends on the size of the image it creates.


This drawing 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, as you can see from the three diagrams.

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 15mm 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:

1) 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 80mm, you could, theoretically, set it at a focal length of 43.033 or 78.25mm.

2) 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?

Again, it all depends on how wide the diagonal of your image is. If your camera has a 1/2-inch CCD, then 8mm would be wide angle, about 15mm would be normal and 80mm 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 wide-angle setting would be 5mm, and a strong telephoto would be 50mm.

For example: the Canon ES1000 Hi8 camcorder has a 12:1 zoom that ranges from 5.2-62mm. By contrast, the Fujix 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 Fujix 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 15mm 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.

Videomaker contributing editor Jim Stinson makes industrial videos, teaches professional video production and writes mystery

Rob Barron's photography tutorials on aperture, dof, shutter speed.
http://www.youtube.com/watch?v=VGAUIBSLJhI
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