Monday, 11 August 2014

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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