Video Camera

The Video Signal

The North American video standard is called NTSC. It formats the video signal as:

  • 525 scan lines
  • 29.97 frames per second
  • 2 fields per frame, interlaced
  • Drop Frame Time Code

A video frame is composed of two fields: an even field and an odd field. These fields are interlaced that is each field contains half the information to form a complete picture. The odd fields contains all the odd scan lines and the even fields contain the even scan lines. These fields are projected at our eyes 60 times a second (59.94 times exactly). Through persistence of vision of brains merge the two fields together to form a complete picture (or frame of video).

On videotape, each frame of video can be uniquely identified with its Time Code. Time Code is used to identify timings on a video signal. Time code looks like this:

HH:MM:SS:FF

where:

HH Hours
MM Minutes
SS Seconds
FF Frame Number

The frames count from :00 to :29 then cycle back to :00. In North America we use A Drop Frame Time Code. As you see, we don't get an exact number of frames each second: 29.97 to be exact. This makes the timing system fall behind 3/100 of a second every frame. Over a little time it adds up enough to make a difference, so the time code skips (or drops) a frame number to catch up. It doesn't drop the frame itself, just adjusts the numbering system to catch up.

DV (Digital Video) Cameras

In a DV camcorder light is focused through a lens and shone onto a device called a CCD (or Charge Coupled Device). On better cameras the light from the lens is shone through a prism first. This breaks up the light into three different component colors: Red, Blue and Green. Each of these colors shines onto its own CCD. The information from the CCD's is converted to compressed video data and stored on the tape in a digital format. While some data is lost during the compression process, once this process is completed the digital data can be copied without further losses. DV (Digital Video) cameras provide two major advantages over conventional analog cameras:

  • Image quality: by storing the video in a digital format instead of analog, it is possible to attain excellent image quality on moderately priced gear. Furthermore, as a digital signal, it does not degrade with time, or suffer generation loss.
  • Video capture: The video is digitized as it's being filmed (at the source), and is stored as files on a tape in the camera. Video "capture" simply involves transferring those files to the computer. There is no additional digitizing to be done.

There are several variations on DV. "DV" is often used to mean MiniDV, the popular consumer format. DVCAM uses the same bitstream, but uses a larger, higher-grade tape for greater reliability. DVCPRO uses a professional-grade tape as well, and supports a higher quality mode (DVCPRO 50) that uses twice the data rate to achieve higher color resolution and less compression. MiniDV's color subsampling is fine for most projects, but is not optimal for high-quality bluescreen work. Bluescreen should use the higher quality DVCPRO 50 format, or an analog pro format such as BetaCam SP. Color subsampling Specs: DV, miniDV, DVCAM = 4:1:1, DVCPRO 25 (25MBps/sec) = 4:1:1, DVCPRO 50 (50MBps) = 4:2:2

The Cameras Parts

The Neutral Density Filter

This filter is used to cut the amount of light that is entering the lens. It does not change the temperature of the color only the amount of light. You would use this filter on very bright days.

The Auto Features

Most cameras have auto features that let the shooter just concentrate on what they're shooting. Being professionals though, we must regain all the control of the camera. In doing so we get back a great deal of creative control. So turn off all of the cameras auto features. These typically include the Focus the White Balance and the Exposure (or Iris).

The White Balance

When editing a video sequence together with different shots taken at different times, the color can be off from shot to shot. This is called color balance. Video looks funny when a clip in a scene has a warm orange quality to it the next has a cold blue feel. Same scene, two completely different feels from the clip. We must maintain this color balance when we shoot so that a scene can have the same lighting quality throughout.

Different light sources throw out different spectrums of light. Natural sunlight is quite blue. Incandescent light is very orange and fluorescent lights throw out a lot of green light. Our eyes automatically compensate for these changes in the light spectrum when we change from sunlight to incandescent light to florescent light. We are unaware that our brains are compensating. The camera must do the same thing. It needs to know what true white looks like in the light that it's shooting in. To accomplish this, you hold up something that is pure white in front of the camera. It should occupy most of the viewfinder. Push the white balance button located on the camera. This tells the camera it is "seeing pure white. It adjusts the camera electronics to properly represent all the other colors. You must repeat the white balance every time you move the camera or you go from inside to outside or vise-versa.

The Lens

Every video lens has a length that gives it a normal view; a picture closest to the size, point-of-view and proportion that your eyes see. This is usually around 50mm. Any lens setting shorter than normal (50mm) creates a wide-angle view and a greater sense of depth. Lenses longer than normal create a telephoto view and makes an image narrower in view and flattened in depth. There are two ways to change the length of a camera lens. You can use different lenses of varying lengths (wide angle, normal, telephoto) or one zoom lens that adjusts from short to long for an infinite range of wide angle to telephoto. A setting longer or shorter than normal effects your image in predictable ways. The table shows the results of setting the zoom length at its short and long position.

WIDE-ANGLE TELEPHOTO
Broad view Narrow view
Smaller image Larger image
Good for close shooting Good for distant shooting
Deep depth of field Shallow depth of field
Increase object speed to camera  Decrease object speed to camera
Minimizes camera shake Maximizes camera shake

The Focus

Your brain is the best auto-focus mechanism ever invented. Despite advertising claims, there is no electronic camera system that is as precise and controllable. You can use auto-focus to make sure your image is crisp or for a visual effect, but under normal shooting conditions make sure it's turned off.

Don't use auto-focus because:

  • the constant focusing adjustment drains the battery.
  • focus will unpredictably shifts if elements in your image move.
  • you can't shoot through windows, fences, trees, fabric, mirror shots, etc. Focus will shift constantly between the foreground image and the background action.

Focus manually for maximum control. What you see in the viewfinder is what you get. If your image is out of focus during shooting, it won't look better during playback.

To focus:

Adjust the lens to the full telephoto position so the subject looks as big as possible in the viewfinder.

Turn the lens focus ring till the subject is in focus in the viewfinder.

Zoom out to compose your shot and create a point-of-view.

If you try to focus in the wide angle setting, you will see little difference in the viewfinder because the wide angle setting creates a long depth-of-field.

Unless you have "pre-focused" on your subject in full telephoto position, you will not be able to zoom in from the wide angle to the telephoto setting without losing focus because the telephoto setting creates a short depth of field.

The Iris

To get a properly recorded video image, you must control the amount of light coming into the camera through the lens. Too little, and your picture will be dark and full of video noise (grainy with little red, blue, and green dots). Too much, and it will be washed out and grey.

The iris controls the amount of light entering the camera. The bigger the opening in the iris, the more light that is allowed through the lens. The iris is adjusted either automatically by an electronic circuit inside the camera (auto-iris) or manually by changing the exposure setting on the lens barrel. You should turn off the auto-iris feature if you can. Manual iris gives you more creative control over the look of the video.

The numbers on the exposure ring are called f/stops. The simple way to understand how they work is to think of them as fractions with the f/number on the bottom and 1 on top: f2 = 1/2, f4 = 1/4, f8 = 1/8. Since 1/2 is bigger than 1/8 it's easy to see that a lens opening of f2 is going to let in more light than f8.

This difference in "contrast ratios" fools the camcorder's exposure setting and gives poor results. The sensing circuit cannot deal with big differences between bright and dark. It chooses one extreme or the other depending on where you aim the light sensor. The sensors expect your main subject to reflect around 18% of the entire light that it is seeing. You can see the effect of camera placement in the viewfinder. Moving the camera a few inches can give the right exposure setting for your subject.

Backlighting

You will find a backlighting feature on cameras that don't allow you to turn off the auto-iris. Backlighting is used when you are shooting dark objects against a well light background. This makes the objects stand out more against the background. It opens up the iris a little more to expose the darker areas.

Image Stabilizer

The camera may have an image stabilizer setting. This will help minimize the shakes and jitters of hand held shooting. It is a clever use of artificial intellegence in the cameras circuitry. The frame of video that is scanned from the CCD is reduced so that it doesn't use the entire CCD surface. This magnifies the image slightly. The background is then memorized and the next frame of video scanned. The electronics tries to line up the background information between the two frames. It adjusts the frame so that backgrounds are more inline. This compensates for the camera jitters.

Cons are that there is a small loss of image quality and there may be a slight smear of the image quality when the camera is first panned or titled. Also the pan and tilt don't follow the camera right away.