ELECTRONIC SHUTTERS

A frame in the PAL television signal is made of two interlaced fields with a repeated rate of 50 fields per second. The charged couple device (CCD) will scan an image 50 times every second which is the average shutter speed of a PAL video camera. You can adjust it to reduce the time it takes to collect the light from a field; you can also reduce the length of the read-out pulse which is equivalent to the increase of the shutter speed. The shutter will increase the time by switched steps, improving the reproduction of motion but also reducing sensitivity.

SHUTTER SPEEDS

There are different steps that a shutter speed can be altered, such as;

• 1/60 of a second
• 1/125 of a second
• 1/500 of a second
• 1/1000 of a second
• 1/2000 of a second

Some cameras will be continuously varied in 0.5 Hz steps. With older CRT computer displays, black or white horizontal bands will often appear across the screen because the scan frequency of most computer display will differ from the television system of PAL (50Hz). However, altering the shutter speed will allow the camera exposure interval to match the computer refresh scanning frequency which will reduce or eliminate the horizontal streaking. Flatscreen LCD monitors do not have this problem and as CRTs have become more or less obsolete this issue does not generally affect today's filmmaker.

MOVEMENT BLUR

The PAL shutter speed is usually set to 1/50 of a second. This speed will make a fast moving subject that is in front of the camera appear blurred. The image definition of a moving object can be improved by reducing the time interval of the exposure if there is an increase in the electronic shutter speed; this is useful when slow motion replay is required. However, reducing the time interval will reduce the amount of light that will be captured by CCD scans and therefore increased shutter speed will require the aperture to be opened.

PULSED LIGHT SOURCES AND SHUTTER SPEED

Short pulses of light at a frequency that is depended on the mains supply can include: fluorescent tubes, HMI discharge lamps and neon signs. If the correct settings are not used, the screen will produce severe flicker.

A high shutter speed can be used with a HMI/MSR light source but it may not coincide with the shutter open and colour drift; usually a cycle of blue and yellow but can be eliminated by switching the shutter off but not with FT sensors as they have a mechanical shutter and cannot be switched off.

TIME-LAPSE CONTROLS

This is when the camera will be programmed to make brief exposures at specific intervals. The time interval, movement of the shot and time of the recording is dependent on the captured shot. Time lapse is an animation technique where objects are repositioned between each brief recording. It is needed to estimate how long the sequence will run in normal speed, the real time event completion and the duration of each shot. Some shots will require more complicated time lapse sequences than others.

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EXPOSURE

It is very easy to believe that the two dimensional images when you watch a television programme or film are believable reproduction of the original scene. There are many manipulated productions on television that have the audience believing that they are watching an honest representation of what was filmed at the time.

It is an important technique to manipulate exposure to control the look of a shot. Video cameras are not as adaptable as human perception; subtle tonal differences can be easily detected by the eye, the brain automatically adjusting and interpreting the information without us having to consciously think about it.

The signal on the television is designed to handle no more than approximately 40:1. However, there is another difference between viewing a television image and observing a subject; a television image is attempting to tell a story and also creating an atmosphere or emotion designed to manipulate the response of the viewer(s). On the other hand, human perception is conditioned by psychological factors which sometimes delude us into often seeing what we expect to see as opposed to what we really are seeing. For this process, exposure plays a crucial part of camerawork. A lot of thought needs to go into what is required including; tones recoded, use of lighting, staging, stop number etc. all to affect the observer and how he relates to the image(s) shown, this is important and a key production tool. Factors which can affect decisions on exposure are:
1. Contrast range of the recording medium and viewing conditions.
2. Choice of peak white and the details in the shadows is to be preserved.
3. Subject priority – the principal subject in the frame.
4. The electronic methods of controlling contrast range and which one are used.
5. The lighting techniques applied in controlling the contrast.
6. The staging directions – where someone is placed affects the contrast range.

EXPOSURE OVERVIEW
- An overall system gamma of around 1.08 is required for an accurate conversion of a range of tonal contrast going from light into an electrical signal and back again.
- The use of additional lamps or graduated filters or compression of highlights will be required when necessary when the scene contrast range can’t be accommodated by the five-stop handling ability of the camera.
- The camera man will decide the choice of compressed tones by altering the iris by either the use of highlight control or the transfer of the camera’s characteristics.
- Automatic exposure cannot judge what may be the important subject in the frame. It gives exposure for the average light levels and some the weighting to centre frame and continuously adjusts to any change in the light level in the frame.
- It achieves exposure by a combination of F number, shutter speed and gain setting. The noise will increase with the increasing gain and the shutter speed will be dependent on the subject content. The F numbers control the depth of field and are based on shot content.

DEPTH OF FIELD
When a lens aperture is chosen to shoot in daylight, it will be dependent on the choice of the required exposure. The depth of field is proportional to the F number. Other factors may affect exposure if the in focus zone is a significant consideration in the shot composition. Several things may need to be adjusted such as:

1. Neutral density filters.
2. Altering gain which includes the use of negative gain.
3. Altering the shutter speed.
4. Adding or removing light sources.

More often than not, there are opportunities to achieve required aperture when shooting interior shots by the choice of lighting treatment, however, daylight is at many times more powerful than portable lighting kits. Some of the time, the depth of field on shots that are similar in size will need to be intercut in order to be matched. The higher lens specification that is required for digital cameras usually ensures that the slight loss of resolution is not noticeable even when you are working wide open, however the lens sharpness may decrease while the lens opens up. Definition problems may occur with the auto-focus and anti-shake devices, even more when you are attempting to extend the zoom range electronic magnification.

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VIDEO PRODUCTION TECHNIQUES: COLOUR TEMPERATURE

When considering the basics of the mixing of colour and light, we are traditionally taught through the three-filter system (add red and green together to make yellow etc). A common assumption made about filming, however, is that cameras work in the same way – just as the human eye does. Whilst similar in its basic operation, there is a bit more to a camera’s method of perceiving light than perhaps what we are used to.


Consider the idea that the camera is without a brain – when converting light signals into electricity, it does so on an entirely mechanical level, without any subconscious or set pattern in mind. The human brain, on the other hand, filters sensory information such as light perception with automatic additions and adjustments on such a natural level that we don’t even know it is happening.

The difference between the human eye and the camera lens is evident when changing from one type of light to another. When shifting between synthetic indoor lights and the natural hue of daylight, the human eye recognises the difference and quickly adjusts to the change in conditions (this adjustment is barely noticeable to us humans). A camera in this situation, however, will not handle things quite as well. It may be initially calibrated to interpret conditions for indoor lighting accurately, but when shifting to natural outdoor light an incorrect perception of colour will result with the camera presenting an image that is overly blue.

When thinking of this in film terms, the colour balance may need to be corrected while shooting or printing to achieve a neutral colour print. This is where the ‘white board’ step comes into play – by placing a plain white sheet in front of a camera before filming, it can then recognise that ‘version’ to be the ‘natural’ white. It then adjusts its perception of other colours based on that starting point, allowing the camera to avoid any colour disorientation.

In terms of film and video, the term ‘colour temperature’ refers to the range of light sources available to be filmed, measured traditionally using the ‘Kelvin Scale’. For instance, one of the ‘hotter’ light sources - blue skylight, is measured between 9500-20,000k. When compared to something smaller such as a match flame, which measures around 1700k, one can easily see the vast differences available concerning colour temperature and the fidelity in which a camera reproduces light.

Comparison between varying degrees of colour temperature

With this in mind, a basic understanding of how a camera interprets light is important in learning how to control it. What may not seem an integral element to the typically amateur film enthusiast, is in fact a vital facet that can mean the difference between a novice and a professional. For further help in understanding colour temperature, follow the links below:

VIDEO PRODUCTION TECHNIQUES: FOCUS

As an integral element to film and video, focusing determines the sharpness of an object or image by adjusting the lens, providing an acceptable visual perspective known as the depth of field. The art of focus lies in knowing what part of the frame needs it most, and the avoidance of over-shooting. For example, interviews or ‘talking heads’ within television are an excellent example, with the eyes of the given subject being an important point for the viewer’s attention to be drawn to. However, much of this will lose meaning if you don’t learn the behaviour of your camera itself and the way it captures distance, sharpness, movement and zoom in relation to the focal plane (the area in which the eye or camera draws attention to). Without any of these elements, the image within the frame will appear ‘flat’, with no sense of immersion or depth.

Depth of field: Notice how the subject is sharp, while the background is soft

To avoid this, and to gain an understanding of how integral focus can be to your shot, practice is a crucial element. For instance, even with the most professional equipment, manual focus is a critical skill with any level of video production. Here are a few ideas to experiment with and try out:

Pulling Focus:

Within the frame, attention is drawn towards the subject with the sharpest focus. This attention can be changed through the use of focus pulling. For instance, a foreground subject (such as a person) can move away from the frame, with focus instantly thrown back to the background, containing a second subject.

Differential Focus:

By using a deliberately narrow depth of field, emphasis on a principal subject can be increased. This is down to the contrast between the sharp focus of the principal subject, and the heavily out of focus background (and sometimes a foreground subject is used as well which is also out of focus).

Zoom lens and focus:

Provided the back focus has been correctly adjusted, this is designed to ensure the focal plane is maintained throughout its range. By zooming in without manual focus, images can lose focus and become distorted.

By becoming familiar with these ideas, you can be more artistic and manipulative about what you want the viewer’s attention to be drawn to. As an important but often overlooked aspect to filming, these techniques are most often mastered through practice and experimenting. 

To find out more about the different techniques with focus, check out these links:

www.mediacollege.com/video/camera/focus/

en.wikipedia.org/wiki/Cardinal_point_(optics)

www.techradar.com/news/photography-video-capture/cameras/how-to-use-your-new-digital-camera-1049684/8

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