Module 19

    

Updated: 02/12/2017

module 19

  

 

 

 

 

 

 

 

Creative Control

Using Shutter Speeds  

 

In addition to the focus, iris, and color balance adjustments on camcorders, many video cameras have an adjustment for shutter speed. How to use shutter speed is another example of an important creative control that can separate the amateurs from the professionals.

Unlike the shutters used in still cameras, the shutter used in most video cameras is not mechanical.

Video camera "shutter" speeds simply represent the time that the light-induced charge is allowed to electronically build in the imaging chip before being discharged.

With speeds as high as 1/12,000 second in some consumer camcorders, almost any movement can be "frozen" without blur or smear -- speeding cars, golf balls, or hockey pucks. 

If you think of dividing a second into bullet through apple12,000 parts and then allowing the image to be exposed for only one of those 12,000 intervals you get an idea of just how fast this is.

When we move to these speeds we can see things in new ways. For example, an ultrahigh speed exposure (achieved with special equipment) was used to show this bullet going through an apple. 

 

Shutter Speeds and Resulting Exposure

By setting a video camera to a "normal" shutter speed of 1/60th second (the time it takes to scan one video field in the NTSC standard), the electronic sampling is done at the maximum time allowed by the field rate of the TV system. This represents the maximum exposure possible with normal sampling. Shutter Speed and Exposure Graph

In this graph the bars represent total exposure at various shutter speeds.

In very low light conditions the shutter speed on some video cameras can be slowed down to allow much more light to register on the chip  (the bars below the blue 1/60th second in the graph).

Although this results in much brighter video, if there is any action, a jumpy, stroboscopic effect will be obvious.

If there is a need to stop (freeze) action, faster shutter speeds than "normal" can be selected.

Most professional video cameras have a series of shutter speeds from of 1/60 second (normal), to 1/2,000th second. Many go beyond this to 1/5,000, 1/10,000th, and, as we've noted, even to 1/12,000 second.

At the same time, note in the graph above how this reduces the total light on the chip (exposure). To compensate, the iris of the lens  must be opened up. freeze-frame image

The higher speeds (1/1,000th and above) make possible clear slow-motion playbacks and freeze-frame still images, such as the one we see here.

 

Shutter Speeds

and F-Stops

Just as in traditional still photography, with video cameras there is a direct relationship between shutter speeds and exposure.  Each of the combinations in the table below represents the same exposure (total light on the chip).


Typical Relationship Between
CCD Shutter Speed and Exposure
 


CCD Shutter speed:

Normal

1/100

1/250

1/500

1/1,000

1/2,000

1/4,000

1/8,000

1/10,000

 Corresponding f-stop (or T-stop):

16

11

8

5.6

4.0

2.8

2.0

1.4

1.2


You will note from the table above that each time the shutter speed is doubled the lens must be opened up one f-stop to provide the same net exposure. (The increased shutter speed cuts the exposure time in half, but opening the iris one f-stop lets twice as much light through the lens to compensate.)

The combinations shown in the above table are for a scene with a specific amount of light. A different scene may call for an exposure of 1/100 at  f/4 instead of f/11, for example, and then all of the other shutter speed and f-stop relationships would need to will shift accordingly.

The variable we are holding constant in this example is the light sensitivity of the camera.  However, keep in mind that this can also be changed on some cameras with a video gain boost control.

These f-stop and shutter speed numbers may seem confusing at first, but once you get them in mind, they will serve you well in video, still photography, and even in motion picture work. It's all the same and it has remained essentially unchanged worldwide for well over a century. (And, in case you need it, you will recall that - earlier we explained how to determine the IE sensitivity of a video camera.)

 

Shutter Speed and Stroboscopic Effects

A stroboscopic effect(where you see a rapid sequence of images associated with movement) can occur in video cameras set to very high (above 1/250th second) and very low (below 1/60th second) shutter speeds.

 

Low Shutter Speeds

As we've noted, it's possible for some video cameras to use exposure rates below the normal 1/60th second. This allows the effect of the light to build in the chip beyond the normal scanning interval.

But the catch is that in this process some of the normal fields and frames must be omitted (ignored) at regular intervals.

If no movement is involved, the loss of frames will go unnoticed. stroboscopic effectHowever, with movement the loss of frames results in a discontinuity in action and a jerky, stroboscopic effect.

Note the separate images in photo on the left and the jerky motion in the reel jerky motion animation on the right.

Besides the (somewhat questionable) special effect this provides, there are occasions -- primarily very low light news and documentary situations -- where imperfect video is better than no video at all.

 

High Shutter Speeds

Now let's consider the other end of the shutter speed range.  With shutter speed intervals shorter than 1/250th second, action tends to be recorded in terms of sharp, discrete, images.

Without the slight blur that helps smooth out the transition between successive frames, we may notice a subtle stroboscopic effect when we view rapid action.

Video cameras capable of slow motion (slo-mo) can be speeded up to between 100 and 200 frames per second with correspondingly high shutter speeds. By slowly playing back footage taken at this speed you can study sharp, discrete slices of the action.

To see the difference that shutter speeds can make in stopping action, study the following sequence of roller coaster photos. The first was taken at 1/30 second, the second at 1/100th second, the third at 1/500th second, and the final photo at 1/1,000 second.
 

shutter speeds and blurr examples

A final note on shutter speeds.

When taping under fluorescent lights, it's advisable to stick to a 60th second (normal) shutter speed. Using a faster shutter speed typically results in a flickering effect in the video as the chip exposure interval interacts with the normal flicker of fluorescent lights.  

 

Variable Frame Rate Cameras

As we noted in Module 8, the normal frame rate for video cameras is 30 per-second or 25 per-second, depending on the country. However, the latest generation of professional video cameras can be overcranked and undercranked.

motion picture camera 1900The "cranking" term originated with film back in the early 1900s when motion picture cameras had to be cranked by hand. One of the earliest motion picture cameras is shown on the left.

Since film projectors were supposed to be operated at a set speed, if the film camera was overcranked and the frame rate was speeded up, the action was effectively slowed down when the film was later projected at normal speed.

In the process of speeding up the frame rate, the shutter speed of the camera was also shortened, requiring either more light or a wider f-stop. (Recall our earlier discussion on shutter speeds and exposure.)

If the motion picture camera was undercranked, the frame rate was slowed down. This means that the action was speeded up when it was played back at a normal speed. 

Even when electric motors replaced hand cranks on motion picture cameras the terms "overcranked" and "undercranked" were retained. Film personnel seem to hang on to their terms throughout the years, even though new technology comes along; whereas, video personnel seem to be willing to, or maybe have to, adjust to new terms on a regular basis.

Although we often see obvious slow-motion effects and even scenes that are sped up for effect, sometimes the creative effect is meant to be more subtle.

For example, a scene may be slightly undercranked (resulting in a slight increase in action speed) to add intensity and frenzy to a scene. Or the camera may be overcranked to slightly slow down action to add a subtle fluid motion to a scene.



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