Lens Techniques

If you’re like me, it’s easy for you to become enthralled with your technologically wondrous camcorder. So many buttons, so many effects–never before has videography been made so easy. But that doesn’t mean that your camcorder does everything for you. There are some effects that require a deeper understanding of (gulp!) light and optics.

Don’t run away scared. We’re not going to get into a physics lesson or anything. We’re simply going to look at the one item that is absolutely critical to proper camcorder functioning–the lens. Without it, your camcorder would not be able to see, and would be little more than a box of gears and fancy electronics.

This month, we’re going to review the lens and what it does to affect the image you see later on videotape. By learning how to work effectively with your lens, you can control the images you record with great skill and precision.


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The Focal Point of Your Camcorder

What does your camcorder’s lens do? Well, not only does it provide an opening for light to pass–light that is reflected off your subject–but it focuses that light so that the image you view on tape is sharp and recognizable.

In autofocus mode, your camcorder focuses the lens automatically. That’s not to say, however, that it always focuses exactly the way you want it to. Sometimes you may want your entire shot to be in focus. Other times, you may want to selectively focus on certain portions of your shot. That’s when it’s time to turn the autofocus off and take control yourself.

Accomplishing special focal effects requires an understanding of depth of field. Simply put, depth of field refers to that area in front of your lens that will be in sharp focus when you shoot. A number of things affect the depth of field, such as the distance between the lens and your subject, the focal length of the lens and the amount of light passing through your lens (see diagram). You can learn to manipulate these variables for just the right focal result.

Let’s begin with a shot that requires little manipulation–one in which all aspects of your shot are in focus: a subject shot against a landscape, perhaps. In this case, you want your depth of field to extend from your lens all the way back to the horizon. Fortunately, depth of field increases as the distance between your subject and the lens increases. Simply position your camcorder at a reasonable distance from your subject and when you focus your shot, all components–foreground, your subject and the background–should be in sharp focus. If they are not, simply move back a bit more and try again.

If you have trouble focusing, zoom in on your subject as tightly as you can. Get the subject in focus and then zoom out. The rest of the shot should be perfectly focused.

Now, let’s use depth of field to selectively focus on certain things. We’ll demonstrate selective focus by using a technique called "rack focus." This is a method of shifting focus from one object to another–usually from the foreground to the background or vice versa–to call the audience’s attention to those objects as appropriate.

To begin, position your camera fairly close to the object in the foreground on which you want to focus. As I mentioned before, the depth of field increases as the distance between the lens and the subject increases. Thus, by moving the camcorder closer to the subject in the foreground, you are severely limiting the depth of field. In this case, however, that’s a good thing.

Begin by focusing on the foreground object. You will notice that because the depth of field is very short, your entire background will be out of focus when the foreground is sharply focused. Now, manually re-focus your shot so that the background begins to come into focus. You will see the depth of field shift to the background and throw the foreground object out of focus. This way, you can toggle your focus between the background and foreground of your shot.

It’s a good idea to practice this technique before you start rolling tape. Otherwise, your audience may giggle as you try desperately to focus your shot.

The Lighter Side of Focal Effects

The depth of field is also subject to the amount of light entering the camcorder.

As you may know, your camcorder has an iris control–also called an aperture or f-stop, for you photography buffs–that works like the iris in your eyes; it opens and closes to adjust the amount of light entering the camcorder through the lens. Most camcorders adjust the iris automatically, while a select few have an override available should you want to fiddle with the setting manually.

You might think that by opening the iris and letting more light into the camcorder you will get greater depth of field. Actually, it’s quite the opposite. The more open the iris is, the shorter the depth of field becomes.

What are the ramifications of this reality? Well, when shooting in low-light conditions, you will most likely have to contend with a short depth of field. Why? First, your camcorder will try to compensate for the lack of light by automatically opening the camcorder’s iris. Then you may try to take control as well and open the iris as wide as it will go. When this happens, you reduce the depth of field, regardless of whether or not your shot is ultimately as bright as you wish it to be.

Low light may also become an issue if you decide to use your high-speed shutter to shoot something that is moving very quickly. Because you are increasing your shutter speed to get a decent shot of your quickly moving subject, the amount of light picked up by your camcorder’s image-sensing CCD will be reduced. If you try to compensate by opening the iris, as before, you will shorten your depth of field.

If depth of field becomes a significant problem in situations like these, such that you can’t seem to get enough depth to keep your entire subject in focus, first try situating your subject(s) in the same plane across your shot. That is, make sure that your entire subject is at a uniform distance from the lens. Also, try to shoot in good light conditions, so that your iris doesn’t have to open as much.

Your Light Filtration System

You can play with your image even further, not only by adjusting the lens itself, but by intercepting the light before it reaches the lens. Objects that were designed to do this, called filters and lens attachments, can change the quality of the light entering your lens and make it easier to create unusual effects.

There are more kinds of filters available than we can adequately discuss here. For a complete discussion of filters, see the June 1999 issue of Videomaker. Suffice it to say that there are filters that diffuse light, filters that color light, filters that reduce the amount of light entering the lens and special effects filters that create streaks or patterns.

These filters usually attach directly to your camcorder’s lens or are held in front of your lens by some sort of mounting device. Because they modify the light that enters your lens, they may require you to experiment with them to learn exactly how they affect the depth of field, color and brightness of your shot.

Some lens attachments modify the light entering your camcorder by framing it in some way. You’ve no doubt seen scenes from old war movies in which the ship’s captain puts the binoculars to his eyes. The next shot not only shows what the captain sees, but also shows the shot framed by a cutout of the view through the binoculars. The net effect is that the audience feels as if it is looking through the binoculars.

You can create your own effects like these by creating cutouts of keyholes, binoculars and telescopes and then holding them in front of the lens when you shoot. These cutouts will really make your audience feel as if they are a part of your show, viewing things first hand.

All of the bells and whistles on your camcorder can only help to a certain degree when you want to create special effects. Focal effects require an understanding of how your camcorder’s lens works with light to create the images you see on tape. By taking control of your lens–and the light passing through it–you can do some amazing things that you might not be able to do otherwise.

The Videomaker Editors are dedicated to bringing you the information you need to produce and share better video.