The shutter is but one of several important camera components used to exercise greater control over the look of your video images. Well begin our exploration with a discussion of the benefits the shutter brings us. Before we open a camcorder to take a look at the inner workings of the high-speed shutter, though, lets review the more general function of all shutters.
A cameras shutter controls the length of time that light is permitted to enter the lens. The other major exposure control on any camera (still or video) is its iris, which affects the "space" through which light must pass, so our discussion today will be about space and time. Dont worry, its not rocket science. All you really need is to understand a few simple terms and relationships.
Simply stated the shutter and iris work together to control the amount of light entering the lens of a camcorder. The shutter controls the duration of the exposure of light to the CCD (Charge Coupled Device). The iris controls the size of the opening through which the light is allowed to enter the lens. While this is simple enough, each affects the final image: each gives us control over the look and feel of our video images.
Expanding Your Focus
The iris in a camcorder works much like the iris in your eye. If the light is bright, the iris in your eye closes down, making the opening through which light passes smaller, so the light doesnt blind you. In low light the iris in your eye "dilates", opening wide to allow more light to enter. The same is true of your camcorders automatic exposure system. The wonderful thing about the iris is that it lets you control the depth-of-field in your images. In low light, your camcorders iris opens up. In bright light, it closes down. But as the opening becomes smaller, the depth-of-field, or range that is in sharp focus, increases. In fact, if the iris is closed down far enough you dont even need a lens to focus the image. The entire world comes into sharp focus when the opening is extremely small.
The first camera, the camera obscura, didnt have lens, shutter or even film. Long before anyone thought of such things, the ancients noticed that when light passed through a small opening into a darkened room, an image was projected at the point where the light struck the wall or floor. With a little experimentation it was discovered that the smaller the hole through which the light passed, the sharper the image. This resulted in the development of the camera obscura, which uses a small hole (and in later versions, a lens) in the wall of a darkened room or box, to project images on a drawing surface where they would be traced, producing highly accurate and detailed drawings.
The pinhole camera is a more modern example of the same approach to imaging. All you need is a shoebox, a pin and some film. If youd like to know more about pinhole photography, you can find loads of information on the Web.
The Problem with the Pinhole
While using a small hole instead of a lens can produce images that are sharply focused from nearly the surface of the lens to infinity, it has one big disadvantage. The tiny hole cant pass much light. To get more light, you could make the hole larger, but then the sharpness of the image is lost. By using a lens in place of the pinhole, you can collect light over the entire surface of the lens, and still get an image that is in focus at the CCD or film.
The lens allows more light to enter the camera, but the area that will be in sharp focus is reduced. The larger in diameter you make the lens, the worse the problem becomes. With very large diameter lenses, the area in focus, or the depth-of-field as its called, can be reduced to a fraction of an inch. Another factor that affects the depth-of-field is the focal length, or magnification, of the lens. Telephoto lenses produce a shallow field, while wider-angle lenses produce a deeper field.
Enter the Iris
If you make your lens large enough to deliver quality images in low light, you need a way to reduce the amount of light entering the camera under brighter conditions. When the light is too bright the iris is used to mask part of the lens (see Figure 1). This not only allows the photographer or videographer to adjust the amount of light entering the lens, but also allows him to adjust the depth-of-field, and thereby control the portions of the images that are in focus, and the portions that are not. By carefully selecting the iris setting, you can focus sharply on a portion of the frame, while throwing other elements out of focus, giving your images a three-dimensional look that cannot be achieved any other way.
The iris helps control the space within the image that is in focus. The focus of the lens itself determines the location of that space, and the iris controls its depth. So much for "space", but what about time? The length of time during which the CCD is allowed to collect light also affects the total exposure. For instance, if you let in half as much light by reducing the iris opening, you can still get the same exposure by allowing the light to enter the camera for twice as long.
Enter the Shutter
To control the length of time during which its film or sensor is exposed to light, cameras use some form of shutter. In the simple pinhole camera, the shutter is a piece of dark paper, or other opaque material, which is opened and closed by hand to time the exposure. This works fine for very long exposure times, because you can simply check your watch or count the seconds. However, modern film emulsions and CCDs can be exposed in tiny fractions of a second, so we need something a little more sophisticated than a simple piece of black construction paper to control our exposure times. By allowing a wide range of accurate exposure times, the cameras shutter works with the iris to give you complete control of the time and space of your image.
Many camcorders have shutters that support exposure times as slow as several seconds down to as fast as 1/10,000th of a second. The slower (longer duration) shutter speeds allow you to use a small iris setting, to get greater depth-of-field while still getting plenty of light to the film. You may have a problem though, if the camera or subject moves during the exposure. Anything that moves will be blurred in the final image, and if the camera moves, the entire image will be blurred. Of course, this blurring can be used for effect, as is often seen in still photos of waterfalls, where long exposure times are used to give the images a more dreamy and idealistic look. Higher shutter speeds (short duration) allow you to use a larger iris opening, even in bright light, reducing the depth-of-field, and producing the nice 3-D look illustrated previously (see Figure 2). In addition, high shutter speeds allow images to be made when the subject is in motion, without the subject appearing blurred. The shutter can open and close so quickly that the subjects position hardly changes at all, and a perfectly sharp still image of a rapidly moving subject is possible if the shutter is fast enough.
The shutters in most still cameras are mechanical contraptions that open to let in light for a time and then close again. Some shutters sweep across the film rapidly, and others are in the lens itself, but the idea is always the same. Open briefly, then quickly snap closed again, just at the right time. Mechanical shutters have mass though, and that mass limits the speed with which they can be opened and closed. This generally limits the maximum shutter speed (or minimum exposure time) to something around 1/8000th of a second on still cameras.
Some still cameras get away from the mass problem by using electronic shutters. For this little bit of techno-magic, two polarized lenses are used. A polarized lens passes light that is polarized in one direction while blocking light of other polarization. If you put two polarized lenses together, oriented so that they both pass light of the same polarization, light that passes through the first lens will also pass through the second lens. But if you turn one of the lenses so that its polarity is opposite of the other, the first lens will pass light that is the wrong polarity for the second, and no light will pass through the lens pair at all. You could therefore make a shutter from two polarizing filters that could be "opened and closed" by simply rotating one of the lenses. Better still, it is possible to build a polarizing filter that will flip its polarization when an electrical signal is applied, so you can design a shutter that doesnt move at all. You simply change a voltage to open and close the shutter. Cameras with this type of shutter system dont make the clicking sounds that cameras with mechanical shutters make, and their shutters can be opened and closed again in 1/10,000 second or less. Shutter speeds this high can stop the motion of a golf swing or speeding car, and produce crystal clear images of fast action, perfectly frozen in time.
With the advent of the Charge Coupled Device, or CCD, all this mechanical and electro-optical messing around is rendered unnecessary. The CCD itself doubles as a virtual electronic shutter that can be opened and closed as quickly as an electronic signal that can be switched on and off, without any additional parts and very little additional complexity. Instead of using a physical shutter to control exposure time, the modern CCD-based camcorder simply discards the light that strikes its surface, simulating a closed shutter. To open the virtual shutter, the CCD is allowed to start collecting light energy. When its time to stop collecting the light, the CCD is simply stopped, and the virtual shutter is closed. So one of the surprising things well find when we look at the shutter in our camcorders is that there isnt one.
No Shutter at All?
Thats right. The modern camcorder doesnt really have anything that you could identify as a shutter. Instead, at the time a mechanical shutter would close, the CCD is simply made insensitive to light. Lets take a closer look at the CCD itself, and see how this is accomplished.
Inside the CCD there are a large number of "cells" that are sensitive to light. The pixel count for a CCD tells you how many cells the CCD contains. When light strikes one of these cells, an electrical charge is generated within the cell. The more light that strikes the cell, the greater the charge that accumulates. The cells dont actually collect "light", but instead convert light into an electrical charge that is proportional to the light striking the cell.
Notice also that the photosensitive cell has no sensitivity to color. It accumulates its charge regardless of the color of light that strikes its surface. To produce color images, filters are placed in front of the cells so that some of them see red, some see blue and some see green light. This can be done with a single CCD and a complex red, green and blue lens-filter assembly, or by using three separate CCDs with a single color filter over each (see Figure 3).
Initially, each cells output is shorted to ground, so that the charge generated by any light striking the cell is drained immediately. In this state, the cells collect no charge, and the virtual shutter is "closed." To capture an image, the drain from the cells is closed, and charges begin to accumulate in proportion to the light striking the cell. At the end of the exposure period, the charges in each cell are passed out to a Vertical Transfer Register, which then passes each charge, bucket brigade style, down to a Horizontal Transfer Register. There, they join the parade of charges from the other Vertical Transfer Registers and are marched out single file to the video processing and recording circuitry (see Figure 4).
The "moving" pictures that we see as video are actually a series of stills displayed rapidly. All NTSC camcorders produce 30 still frames per second. Each frame is composed of two fields, for a total of 60 fields per second. There is, however, a trend to producing cameras capable of recording 30 frames per second without fields sometimes called progressive scan or "Frame Movie" mode. In this mode these cameras do not record 60 discreet fields, but they do output to a TV or VCR in the standard NTSC 60 field/30 frames per second format. This mode is generally used to gather images that will be exported to a computer as stills.
At "normal" speed the CCD collects its charge for the full 1/60 second field time, making 1/60 the default shutter setting for normal shooting. This is fast enough to avoid most blurs caused by camera movement and slow movement within the frame. But our camcorders now let us choose shutter speeds either faster or slower than the frame rate of the camcorder. Slow shutter settings simply allow the CCD to remain active for several (or many) field intervals, and record the previously captured image during the exposure period. When you shoot with a slow shutter speed of 1/15 second, the CCD is kept active for the period of four fields. Its charges are then marched out for recording, and the same data is recorded for four frames, while the CCD fills its cells for another 1/15th of a second. This way the frame rate is kept constant, even though the shutter is operating at an entirely different speed.
The super-quick and silent high-speed shutter operation is even simpler. Instead of removing the drain from the cells at the start of the 1/60 field interval, the charge is allowed to continue to drain its charge away for a portion of the total field time. If a shutter speed of 1/120 is selected, for example, the CCDs will continue to drain for the first half of the frame interval and collect light during only the second half. For higher speeds, the cells are simply drained for a larger percentage of the field time.
In the end, we have a virtual shutter with no moving parts, and no mass. In fact, our shutters have no real existence at all. And yet they still give us control over the space and time of our images and exposure.
Putting it All to Work
All this theory is interesting, but the real value of the high-speed shutter lies in the control it gives you over the look of your images. When combined with the iris, you get control over depth-of-field, motion blur and the brightness of the image itself. A review of the relationships seems in order.
Smaller iris diameter settings admit less light, but give greater depth of field. A slower shutter speed must be used if the brightness of the image is to remain constant.
Larger aperture settings reduce the depth-of-field, accentuating the subject and creating a more three dimensional image. A higher shutter speed is required to keep image brightness low enough to keep the iris open to a wider diameter.
Higher shutter speeds freeze motion, giving sharp images in spite of any relative movement between the subject and the camera. To get enough light at the higher shutter speeds, the iris must be opened wider to admit more light, and as a result, depth-of-field is reduced.
Very slow shutter speeds can be used to allow the CCD to collect more light in low light situations or to allow a smaller iris opening greater depth-of-field. The trade-off is that the same image will be repeated over several fields, potentially resulting in jerky strobe-like motion. Also, the slow shutter speed will allow more motion blur, which may be either a problem, or an artistic effect.
Finally, the iris and shutter together control the total amount of light that is used to build a charge in the CCDs cells, and each or both can be adjusted to produce an image that is either darker or lighter than the original scene.
Learn by Doing
Now you know the theory behind the operation of your camcorders high-speed shutter. But when it comes to selecting the best exposure mode, or shutter and aperture settings for a particular situation or effect, there is no substitute for experience and practice. Take the time to experiment with the different exposure modes and settings on your camcorder, and youll gain much greater control over the look of your finished product.