light (cinematography) Light is the part of the electromagnetic spectrum that can be perceived by the eye, it acts as illumination, making things visible to sight. To set out the use and control of light as it relates to narrative film, this can be looked at from three aspects: continuity, realism and narrative. In practice, when filming, they intermingle, but to identify their importance they can be looked at separately. To begin with lighting for continuity.
For continuity a consistency of lighting has to be maintained. It is what enables one shot to be connected to another shot to in editing to maintain a sense of temporal and spatial coherence. The photographic factors that control continuity of light are exposure, contrast range and colour temperature.
Exposure is the relationship between the level of illumination of the subject being filmed, in relation to the sensitivity of the recording medium, which is a variable in both digital technology and film technology, and this variability in the responsiveness of the recording medium to light is represented in ISO and/or ASA, with the higher ratings being more sensitive: 800ASA is more sensitive, more responsive to light than 400ASA which is less sensitive. In digital imaging the sensitivity of the photo-electric chip, the CCD, the charge coupled device, is a factor of electrical current across different designs and sizes of chip, and in photo-chemical film it is a factor of film emulsion: the sensitivity to light of silver nitrate particles: small grain particles being less sensitive to light and large grain particles being more sensitive.
The camera has three features that factor into the passing of the light from the subject to the recording medium. The intensity of the light reaching the recording medium is set by an opening in the lens, the aperture, which is typically understood as a small circular opening within the barrel of the lens. The aperture is rated in F Stops, which as full F Stops represents a halving or a doubling of the light at each stop: F8 to F11 represents a decrease in light passing through the lens by half and F16 to F11 represent an increases in light to double it. Full F Stops are set as: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Digital cameras offer settings that can indicate half stops or thirds of a stop.
The aperture opens and closes to let in more or less light and the numbering of the F stops is explicable as part of the calculation of the area of a circle, which is π multiplied by the R2, the radius of the circle squared. The camera aperture is a circle and if the radius is understood to be 1 and it is multiplied by π, then as this is repeated the rounded up figures will be: 1, 1.4, 2, 2.8, 4, 5.6, 8, with each F stop lowering the light level.
The aperture controls the intensity of the light passing through the lens and the camera shutter controls the length of time the light will reach the recording medium. The camera shutter is set in seconds and fractions of a second, with 1/10 second being a faster, quicker shutter speed than one second. The shutter speed ratings half and double speed as do the F Stop ratings, so that a camera set to F11 at 1/60 second is the same exposure at F8 at 1/120 second or F16 at 1/30 second.
Both F stop and shutter speed are factors in setting exposure for both still and moving image with frame rate being applicable only to the moving image. The frame rate is the number of images recorded per second. In a still image a single frame is exposed to produce a single image and in filming a concurrent set of images are recorded to reproduce movement in the subject in the film. A frame rate of 50 frames per second will require twice as much light to reach the recording medium than a frame rate of 25 frames per second, so that in combination with the frame rate, ISO/ASA, F Stop, and shutter speed, the exposure is set.
The setting of the exposure will change how the subject is reproduced in terms of tonality, which in photography is understood as a graduation of tones from white to black in the recorded image, and perceived as light and dark in relation to the viewing of the image. The term normal exposure is used in photography, and this term indicates that the image produced will be comprehensible as realistic, in the sense that if the subject were viewed in life this would correspond to the viewing of the film image. So, in the subject areas of brightness are also lighter in the image, and areas of darkness in the subject are also darker in the image, with a tonality, a tonal range overall that is perceived to reproduce the subject realistically. Realism is a mimetic quality in photography and film: the image produced is limited by the medium, but is perceived as a representation of real life. The setting of exposure determines the reproduction of the image.
Exposure is set in the camera through the relationship between, ISO/ASA, F stop, shutter speed, and frame rate, and underpinning all of this is the measuring of the light level of the subject. Digital film cameras have built in light meters, and traditional film cameras, filming with silver nitrate stock do not. A light meter measures the light level of the subject in relation to the ISO/ASA of the recording medium and this indicates the settings for the camera, and the light meter will be calibrated to a mid-grey tone: 18% grey.
What this calibration means is that if the light level is measured from a mid-grey card and the readings given by the light meter are set on the camera then the resulting image will be corresponding mid-grey. So that with a subject that has varied light levels the aim is to meter the subject and set the exposure based on the mid-grey tone. On digital cameras there are different kinds of metering that will set exposure, automatically taking over the procedure for the settings on the camera. A spot meter will measure the light level on a small areas of the subject, centre metering will measure the central area of the subject, and multipoint metering will measure several points to produce an average reading for exposure, but all these different metering systems are based on light metering calibrated to a mid-grey tone.
As mobile phones meter the light and set the exposure without reference to exposure settings and because digital cameras have light meters which operate when the camera is turned on and the will function automatically, how a light meters measures light in relation to the recording medium, and the importance of tonal range in understanding of exposure is not self-evident.
A person working as a cinematographer or Director of Photography, DOP, will need to ensure that they are in control of exposure, and that exposure of the subject does not change in terms of tone, from shot to shot, and so they will meter elements of the scene as the camera moves to new positions, and then the cinematographer will make a decision to set the exposure based on the meter reading. Using an automatic meter would produce inconsistencies in exposure, changes of exposure in a scene, but the cinematographer will assess the exposure, understanding how the tonal range in the image will be reproduced in the digital image. Exposure levels, brightness and darkness in the image, can be changed in the grading of the image, during post-production, but this is only to a degree: controlling exposure throughout the filming is essential or problems will occur and shots will not match.
As an example of where problems can occur: the intensity of light changes during the day, this is sometimes consciously noticed, but as a precise measurement has the light changed by one stop, two stops, or three stops: what F Stop should be set on the lens for exposure in the same filming location across the day so that the shots taken at different times will match? For filmmaking careful metering of light levels for continuity is required. For still image photography this is not necessary because a single still image does not need to match and edit to another image. One reason for filmmaking taking place on interior, studio built sets, even when they represent exteriors is so that lighting and exposure can be controlled, for the design of lighting and also the continuity.
In still photography and filmmaking setting exposure and the image produced by this is variable and is often set for aesthetic effect. For example a scene shot in daylight can be underexposed during filming or graded in editing to appear to be a night scene, or a scene can be graded or exposed to produce mainly high tones, light tones from mid-grey to white, and be high key image, or if low tones predominate in the exposure, this is a low key image. How the subject appears during filming will not match how the image appears in the film: the realism of the image is an element of control of the light.
Exposure is one important aspect of continuity and another is contrast range. Contrast range is the ratio between light and dark in relation to the subject and also in relation to the recording medium. Here the perceptual ability of the human eye and the characteristics of the recording medium need to be understood. The human eye can perceive a high level of contrast: when looking at a subject it can be too dark to see or so bright it’s necessary to squint, but in most environments the brightest and the darkest elements of the subject can be viewed by the eye. The human eye has a very high contrast range compared to film and video.
The recording medium of film and until the second decade of the millennium, and the recording medium of video had a very limited contrast range. What this meant in practice is that the contrast range of the subject and the contrast range of the recording medium had to be matched or the film image would not produce the full range of tones in the scene. A subject that had such a high contrast range that it could not be reproduced within the contrast range of the recording medium results in an image that is overscaled, and a subject that has so little tonal range that it will produce only a limited tonal range in the image, is a flat image, It is underscaled. The use of the scaling is a reference to the tonal scale, or greyscale, a graduated tonal range between black and white.
With analogue video, the image recording had a particular problem in that when the exposure reached a particular high point, then any tone brighter than this would be a crushed, very bright white as the video chip could not produce any graduation of tones at or above this level of exposure. The zebra bars that appear in a digital film camera indicate this overexposure. Digital cameras offer a range of codesc for producing the image, different file types to control and compressions of data, and some camera’s offer specialised settings that change the tonal characteristic of the camera, which is a change to the contrast range that the recording medium can reproduce in terms of its gamma characteristics. These contrast range changes will have titles in a camera menu such as SLOG2, SLOG3, CINELOG. Understanding how each of these settings alters the contrast range for the recording are part of the work of the cinematographer. When there was nitrate film stock this had different gamma curves and a cinematographer might choose different film stocks, but in digital video there are different gamma settings.
The gamma curve is a diagrammatic representation of contrast range. At the bottom of the curve is where low exposure levels will produce black, then the slope of the curve indicates the number of stops that the recording medium will take to produce a visual white tone. A steep game curve indicates a limited contrast range, two or three stops of light from black tones to white, and a flatter curve indicates that the camera will record a greater contrast range. The gamma is a curve because at a certain point the curve flattens out with a maximum exposure ending the rise of the curve and higher levels of brightness in the subject cannot produce a brighter image as the upper limit of the contrast range has: the recording medium’s ability to produce tonality has been reached. Analogue video cameras typically had a 3 to 5 stop range that is very limited, because natural lighting conditions produce subjects that have an 8 to 12 stop range on average.
Some contemporary digital film cameras and DSLR cameras can now be set with a 14 or 16 stop contrast range, which will allow for the filming of a subject with wide contrast range, and they are intended to produce a flattened under scaled image. The subject contrast range will be less than the contrast range of the recording medium, and then in grading the contrast of the flat image will be expanded to make the subject appear more realistic in comparison to the subject. The wide contrast ranges make digital camera more practical to use, particularly because this range avoids the problem with the gamma curve, where the exposure of the brightest tone of the subject reaches the shoulder of the curve, the top of the curve which is the brightest level of tonality can be produced in the digital image.
Using high contrast range cameras settings can lead to the assumption that issues with exposure and contrast range can be fully controlled in grading, so that visual continuity will enable shots to cut together, but if exposure varies in the filming and the contrast range also changes the likelihood of being able to match shots in editing diminishes. Exposure and contrast range need to be understood and carefully controlled during filming. The improvements in technology make cameras easier to use to produce a usable image, but reduce the control of the cinematographer unless they understand the specific operation of the camera they are using, particularly the gamma curves for the recording.
In filming for lighting the practice to control contrast range was and still is to filter light sources through diffusion to dim strong light to lower contrast in the scene, and add small amounts of lights to act as fill in, increasing light levels for shadows and also to use key lights to produce brighter tones to ensure that the image is not flat with a limited contrast. In terms of filmmaking and continuity what is crucial here, is that what the human eye sees in terms of visual contrast range and what the camera medium records as the tonal contrast range does not match, so that the cinematographer has to understand and control this.
In everyday experience there is little or no conscious perception of changes in contrast range, but the cinematographer actively controls the contrast range through different methods, choosing the location, the direction of filming, the time of day for filming, and by lighting the set so that the exposure and contrast range will be consistent and controlled: grading of the shot material takes place in editing, but the filming still needs to be of a standard that small changes in exposure and contrast can be graded to match. To some extent inexperienced filmmakers can be successful in making a film without having full control of exposure and contrast range, they can just trust to the automatic settings, but this will not be satisfactory for a long production and there will be circumstances where the shots produced will be underexposed, the shots change in contrast, and the habit of inexperienced filmmakers is to discard these shots or scenes in editing, losing narrative elements of the story or limiting the dramatic range of the film.
Colour temperature is the third factor that needs to be controlled to ensure visual continuity for editing. Every light source emits light with a colour temperature. At different times, in everyday life this is consciously perceived: it's a warm sunny day and the light has amber yellow tones, it is a cold day and the light is washed out with cold tones of light, bluish compared to a sunny day. However, there is a colour temperature to the light at all times and how this appears in the film image and this can be controlled can be approached in different ways.
Colour temperature is set on the camera and is independent of the controls for exposure: setting colour temperature is identified as white balance on a camera, and the operation to do this is known as white balancing. A digital film camera will have preset colour temperature settings and manual white balance controls. The colour temperature is set in degrees Kelvin with lower numbers that appear warmer to the human eye and higher numbers appearing bluer and colder to the human eye. In terms of setting colour temperature for filming if the colour temperature of the light source and the colour temperature of the camera are matched, then there will be neutral colour tones in the image, and it will not appear to have a warm or cold tone overall in the image. The term white balance is used as the matching of colour temperature of the subject, and to the camera, and it will result in the white tones in the scene being a neutral white: they will not appear warm or cold.
If instead of matching the white balance there is a difference between the colour temperature of the scene and the camera settings the image will have a tint, either warmer or colder. The practice of white balancing can be used for continuity purposes: all the shots will have a neutral tone, and will edit for continuity of light, but then this looses realism in the sense that there is a colour temperature to all light sources so that colour temperature differs in real life. In practice experienced cinematographers will use different colour temperatures for different scenes and this will indicate times of day, type of location and so support the narrative.
What may seem counterintuitive here is that colour temperature control is essential for continuity filming, but at the same time the audience will rarely be aware of this aspect of light. Just as in life, when watching a film, there are only a few moments where there is a conscious perception of the tone, tint, colour of scene: warm, cold. This also makes colour temperature and aesthetic tools. It can be controlled to indicate narrative connections and narrative changes, which are perceived as realistic.
When a film narrative with scenes that have different colour temperatures is shown this will often pass unnoticed to the audience as this is the habit or perception in relation to changes in colour temperature in real life, we don’t consciously assess our surrounding in degrees Kelvin, so a film narrative will have a greater realism if colour temperature changes rather than being neutral throughout. In editing, colour temperature is part of the grading, enabling shots to edit, but decisions on this needs to be made during filming: in editing a shot can’t just be given a colour temperature, because in life the colour temperature effects the saturation of different colours in the image, so that in a high colour temperature, a blue light, then a red tomato will be paler and in a lower colour temperature, a tomato will appear warmer, redder. The digital process of changing colour temperature of the image may change the entire image giving it a single tone, but this will not reproduce how the light during filming, having different colour temperatures, changes the subject. This will be particularly noticeable because scenes often have different colour temperatures: sources of light that produce different colour temperatures, which effects the colours of elements in the scenes.
Visual continuity of light is an aspect of narrative film that passes relatively unnoticed by the audience: scenes take place and the shots cut and the scene appears to take place as a continuous event, but this is only possible because exposure, contrast range and colour temperature are controlled. To do this the functions of the camera needs to be understood, and this is specific to different cameras in terms of gamma ranges, and also, the light at the time of filming needs to be understood: the level of illumination, the contrast range, the colour temperature. Being able to control these factors are usually identified as technical control, but they effect how the image will be perceived as realistic and with colour temperature this has the potential to indicate times of day, time of year, and changes and development in narrative, the temperature of the light changes as the story progresses: light differs from place to place and in relation to time.
Copyright: Eugene Doyen 2019