3 Introduction to Exposure
Exposure, or iris adjustment, is another of the primary functions that must be mastered in order to produce video images of a professional standard. Adjusting the iris enables more or less light to pass through the lens. Obtaining correct exposure is not only about creating aesthetically pleasing images; it also helps to ensure the best quality images from your camera. Too little light and the picture will be under-exposed or or ‘noisy’ (grainy). Too much light will result in an over-exposed or ‘burnt out’ image.
Whilst many video cameras have an automatic exposure or iris setting, this should really only be used in situations where gathering any form of image is more important than gathering an image with perfect exposure. Although automatic exposure can be used to great effect, and is often unnoticeable to the viewer, it does leave the decision of what is the correct exposure to the camera and not the videographer.
Automatic exposure will only provide an estimate of what the iris setting should be, and is measured by the general light levels within the entire frame. The automatic setting will not allow for the correct exposure of individual items within the frame.
Likewise, if your shot includes tracking or panning with varying light levels throughout the movement, the automatic setting will continue to adjust the iris as the camera moves and this might not be your intended result.
It is therefore important to master manual control of exposure and to appreciate the impact that incorrect exposure has on the picture.
3.1 f-numbers (or f-Stops)
Regardless of whether the camera is set to automatic or manual, the amount of light falling through the lens and onto the camera’s imaging device is controlled by the iris. The bigger the opening, the more light is allowed through the lens.
On professional cameras, the iris setting is shown in f-Stops. In simple terms, f-Stops represent the size of the opening of the iris. This is shown as an approximate fraction (ƒ) of the size of the effective focal length of the lens (the smaller the f -number the larger the iris opening).
For example, f16 = 1/16th the size of the focal length of the lens – so if your zoom lens was set at 50mm the iris opening would be approximately 3.125mm wide (i.e. 1/16th of 50mm). Opening or closing the iris by one stop respectively doubles or halves the amount of light passing through the lens.
The f-numbers inscribed on the lens barrel of professional cameras are marked as multiples of the square root of 2 (approximately 1.414) to reflect this fact; hence typical f-Stop values of f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, etc.
3.2 Zebra Pattern
The Zebra Pattern function provides an aid to obtaining correct exposure. When engaged, a striped pattern will appear in the viewfinder or screen showing areas of the picture which are exceeding the selected threshold, therefore acting as a visual representation of over-exposure.
Cameras provide various Zebra Pattern threshold settings (shown as percentages) which relate to the percentage of signal voltage being generated by that part of the picture (the brighter the image, the higher the voltage). Although the whole video signal is 1 Volt, 0.3 Volts are taken up with synchronisation signals, etc. leaving 0.7 Volts purely for the luminance and chroma information of the picture. A Zebra Pattern threshold set at 100% will add the striped patterning to anything within the frame which is reaching 100% of the 0.7 Volts. The lower the threshold percentage the lower the voltage reading required for the patterning to appear.
3.3 Back Light
The Back Light function (or Back Light Compensation) is used in situations where the main subject has a strong light source behind it, causing the subject to be underexposed. This scenario often happens when filming during a wedding service, where the subject is filmed with a window to the rear of their position. The Back Light function opens up the iris slightly to expose the subject more accurately. Whilst this function can provide a quick solution to this problem – far greater control and more accurate exposure can be obtained using manual iris control.
3.4 Spotlight
The Spotlight function is used in situations where the main subject has a strong light source on them, whilst the surroundings are poorly illuminated. This scenario often happens when filming stage performances, whereby the character will have a spotlight on them and the stage behind will be in virtual darkness. The Spotlight function closes the iris slightly to expose the subject more accurately. Again, whilst this function can provide a quick solution to the problem, far greater control and more accurate exposure can be obtained using manual iris control.
3.5 Artificial Gain
Most cameras offer the means of adding ‘gain’ to the picture which artificially brightens the image and makes the camera more sensitive to light. This can be set to ‘off’, ‘automatic’ or ‘pre-set’ gain levels. In automatic mode the camera will determine at what light levels the gain will be introduced and will vary the level of gain according to the available light levels. In pre-set mode the operator determines the amount of gain to be added. Adding gain will enable the camera to produce pictures in very low light situations, but will also result in noise or grain being added to the picture (see also section 10. Filming in Low Light).