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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GAIN, NOISE & SENSITIVITY

The sensitivity of the camera will usually be reference to 4 interlinking element by camera manufacturers.
1.A subject with peak white reflectivity.
2. Scene illumination.
3. F number.
4. Signal-to-noise ratio for a stated signal.

For example: A peak white subject that has 89.9% reflectance lit by 2000 lux quoting the signal or noise ratio will be expressed as the resulting F number when it is exposed. From that example: most of the current digital cameras will be F8 or a better with a signal or noise ratio of 60dB. This does not indicate how much light the camera should use but instead, it allows different camera sensitivity.

NOISE
Greater amplification of weak signals could increase the sensitivity of the camera but it may degrade the picture by adding noise which would be generated by the camera circuits. Contour or gamma correction will not necessarily be needed when measuring the signal or noise ratio. Manufacturers may vary in the way that they will state camera sensitivity, and because of this, comparing the differences between the models will require a conversion of the specification figures. If the camera has the same F number, then the higher the signal or noise ration and the lower the scene illuminance or lux, the more sensitive the camera will be.

GAIN
If there is insufficient light that exposes the picture, then the gain of the head amplifiers may need to be increased and any additional gain will be calibrated in dBs. An example of this would be adding +6dBs which would double the amount of light that is available to the sensors. The amount of switch gain that is available would usually depend on the camera. Some cameras however, will automatically increase gain if the light were to decrease when a specific F number is selected. This could increase noise to an unacceptable level without the cameraman being aware. Cameras may also have a negative gain setting which would reduce noise and control depth of field without the use of filters.

GAIN AND STOP COMPARISON
+3dB will be equivalent to opening up 0.5 stop
+6dB will be equivalent to opening up 1 stop
+9dB will be equivalent to opening up 1.5 stops
+12dB will be equivalent to opening up 2 stops
+18dB will be equivalent to opening up 3 stops
+24dB will be equivalent to opening up 4 stops

Any extra gain in the amplification is a corresponding decrease in the signal to noise ratio and will result in an increase in noise in the picture.

CALCULATING THE ASA EQUIVALENT FOR A VIDEO CAMERA
A video broadcast camera will have a reliable light meter if it has a good auto-exposure system. Most cameras will use a combination of three different exposures: 1. Manual exposure 2. Instant auto-exposure 3. Zebra exposure Some cameramen with a film background sometimes feel more comfortable when a using light meter to check exposure level; they would be able to do this by an equivalent ASA rating which would be logarithmic. There are several methods to determine the rating:
- The sensitivity of the video camera uses a stated light level, signal to noise level, a surface, with a known reflectance value and with the shutter set at 1/50.
- Japanese camera manufacturers use a standard reflectance of 89.9% as peak white while UK television practice is to use a 60% reflectance value as peak white therefore an illuminance level of 3000 lux must be used when transposing a rating of 2000 lux with 89.9% reflectance to 60% peak white working.

Camera sensitivity has increased over the years and in the last few, cameras have begun to include a negative gain setting.

IMAGE INTENSIFIERS To boost sensitivity, image intensifiers are fitted between camera and lens while shooting is occurring in low light levels. The end picture may lack contrast and colour but it will produce recognizable images.

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THE ZOOM LENS

The zoom lens is a complex and sophisticated component of any video camera. lts performance may be described in brochures in impenetrable techno-speak that many, at first glance, may feel has little or nothing to do with the ‘real’ business of filming. To take this view is a mistake, as above all else, the lens characteristics are the most important visual influence on the appearance and impact of an image.

Altering the settings on focal length, lens angle, zoom ratio, aperture or depth of field will have a significant influence on the perspective and the depiction of space and composition when setting up a shot. It is a fallacy to believe that the camera will truthfully record whatever is in shot whenever you put your eye to the viewfinder and press to record. If you are unaware of how the choice of focal length, etc., will affect the chosen image then this important creative decision will be left to chance and accident.

Prime lens or zoom?

Before the introduction of colour television, video cameras usually had four, individually turret-mounted prime lenses (a lens with a fixed focal length) on the front of the camera. By rotating the turret, the correct lens for a specific focal length would be chosen for a shot. Selecting a prime lens to match the needs of a shot is still common film practice. For technical reasons, such as faster working methods and the option to zoom on shot to capture close-ups without adjusting the camera, meant the wide majority of colour video cameras were fitted with a zoom lens. With the ability to change the lens angle quickly and easily, it can be easy to forget what focal length is being used which could compromise the space or perspective of the shot so be wary.

Image size

With CCD (charged couple devices) coming in different sizes (2/3 inch, 1/2 inch, etc.), the variation in flange-back distance the mechanical connection between lens and camera, and the variation in cable connections with the camera, it is often impossible to interchange lenses between different makes or models of cameras. The image formed by the lens on the face of the CCDs is called the image size of the lens. This must match the size of the camera sensor. Lenses designed for different sized formats (pick-up sensor dimension) may not be interchangeable. The image size produced by the lens may be much smaller than the pick-up sensor and probably the back focus (flange _‘back) will not have sufficient adjustment. A common lens on a broadcast video camera is a 14 x 8.5 f1.7 zoom with a minimum object distance of 0.8 m or below.

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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: