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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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.

www.candyjarfilms.co.uk

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

CANDY JAR FILMS