Manual Exposure Control

First, a warning–most of the time, automatic exposure works pretty well, especially if you don’t shoot regularly. Even though it seems pretty silly that the light meter thinks everything should reflect ten to eighteen percent of the light, it doesn’t get it wrong as often as you might think. What’s more, aperture control in an overly enthusiastic hand can be quite noticeable. However, there are some fairly common situations that will fool your light meter, time and time again. When you run across those, you really do want to be able to make the necessary adjustments.

When you set your camera to manual, it will probably give you an option in your viewfinder or LCD, which allows you to adjust either the iris or the shutter. (I say probably, because some consumer models don’t let you know how your controlling the exposure, the screen just gets brighter or darker. Check the manual for specific instructions on your unit.) Iris priority mode allows you to control the size of the aperture and consequently, the intensity of the light. Shutter priority mode controls the time of exposure. To fully understand manual exposure, you need to understand how to use both iris priority mode and shutter priority mode.

Iris Priority
Manipulating the iris or aperture is pretty straightforward. Open the iris wider, more light enters the lens and your image gets brighter. Close down the iris, less light enters the lens and your image gets dark. Like I said, it’s pretty straightforward. The only thing that might be confusing here is the f-stop numbers, which represent the size of the iris or aperture opening. Counter intuitively, the larger the f-stop number, the smaller the iris and vice versa. If the f-stops are not displayed during manual iris control, then this is not an issue. If they are, just remember that f/22 gives you the smallest iris opening, and f/1.4 gives you the largest.

In addition to controlling the intensity of the light entering the lens, the size of the iris also determines your depth of field. Depth of field refers to a specific range of distance–how far away from and how close to the lens an object can be and still remain in focus. A narrow iris, f/22 for example, gives you a long depth of field; a wide iris, f/1.4, gives you a shallow depth of field. Generally, the more light in the scene, the longer your focal depth. Richer tones, less grain and a long depth of field are all fringe benefits of a well-lit scene.

If you want or need to control how much of the frame is in focus and how much is not, then you’ll need to control the depth of field. In order to control the depth of field, you have to have some control over the light in your scene. It’s the only way to be sure you can adjust the iris to suit your focus needs and still have the right amount of light to achieve correct exposure. But that’s another lesson.

In addition to controlling the intensity of the light entering the lens, the size of the iris also determines your depth of field. Depth of field refers to a specific range of distance–how far away from and how close to the lens an object can be and still remain in focus. A narrow iris, f/22 for example, gives you a long depth of field; a wide iris, f/1.4, gives you a shallow depth of field. Generally, the more light in the scene, the longer your focal depth. Richer tones, less grain and a long depth of field are all fringe benefits of a well-lit scene.

Shutter Priority
The illusion of motion in video is created by a successive number of still images, flipping by one after another. For film the images go by at a rate of 24 images or frames per second. For video produced in the United States, the rate is just about 30 frames per second. The maximum amount of time that the shutter can be open without causing a distortion is 1/60th second. Which as it happens, is also the default rate for video cameras.

As in still photography, a faster shutter speed, say 1/120th second, will give you crisper images because it freezes the action and cuts down on motion blur–the blurring of the image caused by movement within the frame during the period of exposure. If your subject is moving very fast, a racing car, basketball players, running water, motion blur will be obvious so long as your camera is set on automatic exposure, in other words at a shutter speed of 1/60th second. By manually adjusting the shutter speed to 1/120th or 1/250th per second, or by putting the camera in the AE (automatic exposure) mode for sports, you can get crisper images. Some motion blur is okay. In fact, if you cut the motion blur down too much, your action will appear jerky as in the fight scenes in Gladiator (directed by Ridley Scott, released in 2000). In this case, the faster shutter speed not only allows us to see each droplet of blood as it splurts out of various gashes in the bodies of the fighters, but also captures the fast-paced, frenetic feeling that we imagine combat to be like.

On the other hand, it can also be cool to exaggerate motion blur, rather than eliminating it. By setting the shutter speed to a slower rate, say 1/30 second, waterfalls can be made too look more like fluffy blankets and headlights from cars send out beams of light way far ahead of them as they speed past the camera. Slowing up the shutter speed also increases the amount of light that makes its way through the lens, which can help you achieve the correct exposure in low light. However, it will cause a trailing effect and not just with headlights, but for anything that moves. At 1/60 second, the camera scans one image per frame, at 1/30 second or slower you end up using the same image for two or more frames. This is what causes the trailing effect. The image simply stays on the screen longer than it should in order to achieve animation that looks natural.

Even though both shutter speed and iris together determine the exposure, you should try to control the brightness of the image by manipulating the iris only. Ideally, you need only adjust the shutter when you’re recording fast moving subjects or for certain special effects. When the light is extremely low and you’ve opened the aperture as wide as it will go, then you may have no other option than to slow down the shutter speed. However, if you must decrease the shutter speed below 1/60 second, which is the standard rate for video, then you’re probably going to notice a trailing effect. There’s no way this can be mistaken for normal. Whether or not it’s worth it, depends on how much the trailing effect will detract from the content of your video or, if you’re lucky, be an appropriate enhancement to the mood you’re trying to achieve.

Manipulating the iris or aperture is pretty straightforward. Open the iris wider, more light enters the lens and your image gets brighter. Close down the iris, less light enters the lens and your image gets dark. Like I said, it’s pretty straightforward. The only thing that might be confusing here is the f-stop numbers, which represent the size of the iris or aperture opening. Counter intuitively, the larger the f-stop number, the smaller the iris and vice versa. If the f-stops are not displayed during manual iris control, then this is not an issue. If they are, just remember that f/22 gives you the smallest iris opening, and f/1.4 gives you the largest.

In addition to controlling the intensity of the light entering the lens, the size of the iris also determines your depth of field. Depth of field refers to a specific range of distance–how far away from and how close to the lens an object can be and still remain in focus. A narrow iris, f/22 for example, gives you a long depth of field; a wide iris, f/1.4, gives you a shallow depth of field. Generally, the more light in the scene, the longer your focal depth. Richer tones, less grain and a long depth of field are all fringe benefits of a well-lit scene.

Practice
If you want to get good at adjusting exposure manually, you have to know what correct exposure looks like in your viewfinder or LCD. This requires practice. While the camera is in automatic exposure mode, frame a shot with nice, even lighting. A landscape or wide shot taken outdoors is usually a good bet. Notice how bright the picture is then set the iris to manual. (We’re only using the iris here, because as stated earlier, the shutter speed should only be adjusted when you want to control motion blur.) Open and close the iris, so that the exposure changes drastically, then try to set the exposure where it was before. Check to see how close you’ve gotten by putting the camera back into automatic exposure mode.

Once you get the wide shots down, try the same thing with medium shots and close ups. If you’re shooting something that’s bright, open the aperture a stop or two, and allow more than the prescribed amount of light to enter the lens. This allows objects that have a greater than eighteen percent reflectance to come out looking realistically bright rather than too dark. Do the opposite for objects that are dark.

Now, try to set the exposure for a scene that the camera is sure to get wrong. Automatic exposure often reads scenes in which there is very little contrast incorrectly. If your frame is mostly white, say when you’re looking at a field of snow, then the automatic exposure on your camera is going to make your snow look gray. If you’re frame is filled with dark objects, say the waves of the ocean as seen from a boat, then you’re going to get something that looks rather muddy or gray rather than dark and foreboding. In these cases, you’ll need to trust your eye more than your camera. So, it’s good to have some practice with these conditions, before you go out and shoot something that’s important to you.

Too much contrast is another cause for concern. When you try to compensate for a scene that’s very bright in some places and rather dark in others, you end up losing details in the highlights, the lowlights or both. High contrast in video means an inevitable loss of detail. If you must shoot an object that is relatively bright or dark, and you want to pick up the details without washing out some portion of the frame, then you must control the contrast between your subject and everything else. In fact, the key to capturing consistent and professional-looking video is the careful control of the contrast in the frame. But that’s another lesson.

While your practicing, record a few seconds of each shot while the camera is set to automatic exposure and a few seconds of the shot after you’ve set the exposure manually. This way, you’ll be able to compare your eye to the automatic exposure. If your footage comes out uniformly darker or lighter than you anticipated, your LCD or viewfinder may be calibrated incorrectly. After a while, you’ll be able to predict when the camera’s going to get it wrong and will automatically make the necessary adjustments.

The subject of the next page is Manual Focus Control.