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Getting the most from your Tennis Video, Part 2

Shutter Speed, Aperture, and Exposure

The shutter speed and aperture settings on your camera will have a large impact on the look, feel, and quality of your tennis video. The basic idea is that by working together, shutter speed and aperture determine how much light comes into your video camera, in other words how bright the picture is. We will first talk about shutter speed, then aperture, then examine how the two work together to determine the "exposure" of the final picture.

Shutter Speed

In plain English the term "shutter speed" describes the length of time that the camera's electronic "eye" (the shutter) opens to capture a frame. Shutter speed is also called "exposure time" because in a film camera, how long the shutter is open determines how long the film is exposed to light. The longer the camera's shutter is open the more light enters the frame, and the brighter the picture becomes.

On many consumer-level camcorders, the shutter speed is manually controllable.

Shutter speed is usually described as a fraction of a second, so for example, a shutter speed of 1/2000 means that the camera will open its electronic eye for one two-thousandth of a second, then close it again. The number to the left of the "/" sign is always 1. The number to the right of the "/" changes, and the smaller that number is, the longer the exposure time is. For example, 1/30 is a longer exposure time than 1/2000, because 1/30 means that the camera shutter (the eye) opens for one thirtieth of a second (three hundred milliseconds), which is a lot longer than one two-thousandth of a second (two milliseconds).

It is important not to confuse shutter speed with frame rate. We know from Part 1 of this article that a typical camcorder will record 60 interlaced fields (or 30 frames) per second, but the camera may only open the shutter for 1/500th of a second to capture the image for each one of those fields or frames.

When you are shooting live video, there is a limit to how long the shutter can stay open. That limit is the frame rate. If the camera must record 60 interlaced fields each second, then the camera cannot possibly leave the shutter open for longer than 1/60th of a second. If you were shooting 30-progressive format, then 1/30 would be the lower limit to the shutter speed. So a shutter speed of 1/10 is not possible with a normal consumer video camera shooting live footage.

Motion Blur

So how does shutter speed affect tennis video? To answer this, we need to examine what exactly it means to open and close your eye in, say, 1/2000th of a second, versus, say, 1/60th of a second.

The longer the camera's shutter stays open, the more possibility there is that something in the scene moves. Let's say you are filming someone swinging a racket very fast. If you open the shutter for 1/60th of a second the racket will move across the frame some distance before the shutter closes. Over the course of 1/60th of a second (about 17 milliseconds), a professional tennis player with high racket-head speed might swing the head of his racket a foot or more. The camera will gather information during this entire time, and it will make the racket head look like a streak across the picture. This is what we call "motion blur."

But if the shutter only opens for 1/2000th of a second the racket will not travel nearly as far during that time (in fact it will barely move), resulting in much less motion blur. So the faster the shutter time, the more the action becomes "frozen."

Realistically speaking, what shutter speeds are required to freeze tennis action? I have found that 1/500 will freeze an amateur player because they don't swing too fast. More advanced players with higher racket head speed require at least 1/1000, and preferably 1/2000 to completely freeze the action.

Image Brightness

Shutter speed is not simply a tool for freezing high-speed action. It also directly affects how bright the video picture is. Again, the longer the shutter stays open, the more light enters the camera, making the picture brighter. Conversely, as you shorten the shutter time, the less light can enter the camera, making the picture dimmer.

There can be real limits on how fast of a shutter speed you can use before the image becomes too dim to make anything out. On a sunny day with no clouds, there is practically no limit on your shutter speed because it is so bright that even using a shutter speed of 1/15,000th would result in an acceptably bright picture. Cloudy days may limit you to 1/2000 or less. If you are trying to film at night under the lights of a well-lit tennis court (think center court) you will be limited to more like 1/320th of a second. Even an extremely well-lit court at night is still far, far less bright (several thousand-fold) than that same court during on a sunny day. At 1/320th, there will be noticeable motion blur. There is no way around this, so film during the day if possible.

I know what you're thinking right now: "This is obviously not true because a lot of tennis matches are filmed by ESPN at night, and the picture looks fine." Firstly, they are using a shutter speed of around 1/500th, which is close to the limit. Second, those $100,000 broadcast cameras they use are extremely sensitive to light, meaning that the "eye" inside them gathers more light at the same shutter speed than a consumer camera can.

An analogy would be that it's sort of like how a hawk's eyes are simply better (sharper) than a human's, or how dogs and cats' eyes see much better than ours at night because they pick up more light (even though cat's eyes are actually smaller than ours, they are more sensitive).

Key points:

• Shutter speed means how long the camera opens its "eye" for each frame.
• Faster shutter speeds freeze the action more, but can be dimmer because less light can enter the camera in a shorter time.
• It can be hard not to get motion blur at night, so film during the day.

Aperture

Inside the lens of your video camera, there is something called an aperture stop, or "F-stop." It is a circular ring of metal or plastic that can shrink or expand, making the ring smaller or larger. At the center of the ring, like a donut, there is an opening that the light coming through the lens passes through. By changing the diameter of that opening, you can control two things: How much light comes through the opening, and something called the "depth of field." Most consumer camcorders allow you to manually control the size of the lens aperture.

Image Brightness

Let's first examine how changing the aperture changes the brightness of the image. This is relatively straightforward. The smaller you make the opening, the less light can come through, making the image dimmer. Enlarging the ring allows more light to enter, making the image brighter. For the purposes of tennis video, the main use of the aperture setting will be to control the amount of light entering the camera. But, you ask, wasn't the purpose of shutter speed to control the amount of light entering the camera? Why do you need to control the aperture, then?

After the explanation of Depth of Field, I will go into why you need both shutter speed and aperture to control the amount of light in the picture.

Depth of Field

The aperture setting also affects the Depth of Field of the video picture. For many people, this is a confusing concept, but the definition is rather simple. The depth of fiend describes how far (how deep) in front and in back of your subject will be in focus.

Depth of Field is also not really critical to video work for very technical reasons (but it is extremely important in film work), so I will try to keep the explanation brief and in plain English.

Light bounces off every object in a scene, and some of those light rays enter the camera's lens. However, the light coming from objects in the background of a scene enters the lens at a different angle than light coming from objects in the foreground. Because of the laws of physics, it is impossible to make a lens that can focus all those different angles to exactly the same place.

This means that you have to settle for some part of the scene being in focus, and some part being out of focus. Only objects at a certain distance will be in focus. Things in front of or behind that distance will be out of focus. Of course, you normally want your subject (the tennis player!) to be in focus, and objects in the foreground and background can be out of focus.

You can do this simple experiment to demonstrate to yourself the concept of depth of field. Your eye is a lens just like a camera lens, so your eye has a depth of field, too. First, close one eye. Now focus on an object about ten feet away from you. Then bring your hand up and hold it about six inches in front of your face. Your hand will be out of focus. Now shift your focus to your hand, and you will notice that the bookshelf behind you goes out of focus. No matter how hard you try, you can't make the two be in focus at the same time, because you can't break the laws of physics.

So how does the aperture ring in a camera's lens affect the depth of field? When the aperture is at its widest (largest) setting, light can enter and pass through the lens at the largest number of angles. Because these angles can are so different and cannot be focused to the same place, this results in the foreground and background being completely out of focus.

If you shrink down the aperture ring, it cuts out many more of the angles that light can enter from, meaning that almost all the light coming in is traveling at nearly the same angle. Thus, almost all of it can be focused at the same spot. So with a very small aperture, almost the whole picture can be in focus at the same time.

Again, because of some very technical reasons, depth of field in video is different from depth of field in film, and I really don't want to get into an explanation of this here. Basically, with a consumer video camera, almost everything remains in focus throughout a scene, so anyone putting a video camera up on a tripod to film some tennis video should not have to worry too much about depth of field. I still think it is very important to understand, though.

In filmmaking, depth of field is a powerful tool. For example, sometimes using a wide aperture to make much of the scene out of focus can be advantageous. You could focus the camera on an actor's face, but have everything in the background go completely out of focus, thus forcing your attention to his face. You can also use a narrow aperture to make the entire depth of a scene focused, such as in panoramic shots of landscapes. The next time you watch a movie, notice how the director uses depth of field in each scene to control the "visual story."

Aperture as an F-number

On every camera lens, how wide or narrow the aperture ring is set is described by a number, which we call an F-number. If you've messed around with your camera and/or lens, you may have seen numbers like F/5.6, or /F11. I don't want to get into math here, so I will just say that the F number is a ratio of the focal length of the lens to the diameter of the aperture opening. Let's stay away from further mathematical explanation here, because all you really need to know is that lower F numbers (ex: F/3) mean a wider aperture, and higher numbers (ex: F/16) mean a narrower aperture.

F/1.4, the widest most normal lenses will go, is usually the lower limit. This setting lets the most light through, but also decreases the depth of field (only the subject is in focus, everything in front and back of him is out of focus).

F/32 is about as narrow as film and photography lenses get. Video lenses usually cap at about F12. This setting lets the least amount of light through, and increases the depth of field (everything is in focus).

Interesting side note: As I mentioned earlier, your eye is a lens just like a camera lens. The focal length of the eye is the distance from the lens to the back of your eye, the retina. Your eye also has an iris (the aperture ring) which naturally shrinks and opens depending on how bright the view is. Thus, the human eye has an F-number of about F/2 in the dark, to F/8 in bright sunlight!

Combining Shutter Speed and Aperture to Correctly Expose the Picture

Simply put, you need to be able to control both the shutter speed and the aperture to be able to fully control how bright your image is. If the picture is too bright, we say it is "overexposed," and if it is too dim, it is "underexposed."

Let's take a look at some practical examples of how the two work together to control image exposure. On a somewhat cloudy day, I go out to film some tennis video. I want to use a shutter speed of 1/1000 to freeze the action. But during that 1/1000th of a second, exactly how much light enters the camera will depend on how bright it is outside.

That's why we also need the aperture ring to control the amount of light entering the camera. You can use a very fast shutter speed with a very wide aperture… or a slow shutter speed with a narrow aperture. Both result in the same amount of light coming through to the camera.

In fact, you can use any combination of aperture setting and shutter speed that results in the correct amount of light entering the camera, i.e., proper exposure. However, you need to remember that changing the shutter speed will change how much the action is "frozen," and changing the aperture or F-number will change the depth of field. This trade off always exists.

Shutter Speed and Aperture on your Camera

If you are comfortable controlling both the shutter speed and aperture at the same time, then you can operate your video camera in fully manual mode. That of course assumes that your video camera will let you manually control both these things.

If you aren't comfortable with controlling both the shutter speed and aperture at the same time, many cameras have shooting modes that can help. Some cameras have what is called Shutter Priority mode. In this mode you manually set the shutter speed and the camera's internal computer automatically determines the best aperture size to use. The camera will automatically change the aperture each frame if it needs to (such as when the sun dips behind a cloud) while maintaining your chosen shutter speed, keeping the overall image brightness the same even as lighting conditions change. On the camera control wheel this mode is labeled SP, Tv, or S mode.

In Aperture Priority mode, you can manually force the aperture size, and the camera will automatically set the shutter speed. This mode is usually labeled AP, Av or A. This is less desirable for tennis video, since we are mostly interested in being able to force the shutter speed.

If you have a very cheap camcorder or just a cell phone camera that only has a fully automatic filming mode, you may be out of luck. Try your best to get a hold of a camcorder that has a manual setting for shutter speed.

One final note on forcing shutter speeds. You may be asking: Why would I ever want motion blur in any video, ever? Why not just always set the shutter speed to say, 1/1000 and let the camera adjust the aperture even when filming something like a wedding? The answer is that to the eye and brain, motion blur is actually much more visually pleasing than a choppy series of frozen pictures separated by 1/60th or 1/30th of a second. Try recording a movie of a friend or family member talking and moving around, first with the camera set to fully automatic mode, and second set to a shutter speed of 1/2000. The fully automatic mode (where the shutter speed likely will not be above 1/120) will look much more smooth and natural. Only force extremely high shutter speeds when you are trying to freeze sports action.

Key Points:

• Aperture affects how much light enters the camera and the depth of field.
• Combining shutter speed and aperture correctly results in a properly exposed picture that isn't too bright or dim.

Read Part 3 »

Read Part 1 «