Cinematography-Theory-And-Practice

close down the iris to a small opening to let less of that light through.If our scene is dark, we can open up the iris to a wider opening to letin all the light we can get — but sometimes this will not be enough.There are other things that control how much light reaches theimage plane (the surface of the film or video sensor). One of these isshutter speed, which is a measure of how long the light reaches thefilm during each frame. Most of the time when we are shooting at24 frames-per-second (FPS), then our shutter speed is 1/48th of asecond. In each frame, the light is falling on the film or sensor for1/48th of a second. If we have way too much light for the iris to helpus with, then if we expose each frame for a much shorter time, thenthere will be less exposure in every frame — problem solved.Change the BucketThere is another, more basic way to change the exposure: use a differentbucket. Every type of film has a certain sensitivity to light; thesame is true of every video sensor. This means that some are moresensitive to light and some are less sensitive. It is rated in ASA orISO, which generally ranges from about ISO 50 (low sensitivity) toISO 500 (high sensitivity).A film with a low sensitivity needs lots of light to make a goodimage. Typically films that are ISO 50 or thereabouts are only goodfor outdoor shooting, where the sun provides tons of light. HighISO films (such as ISO 500) will give you a good image even withvery little light, as do HD cameras with higher ISOs.A high-speed film is like using a smaller bucket — you don’t need asmuch to fill it up. A low-speed film is like a larger bucket — it takesmore to fill it up, but on the other hand we have more water. In thecase of film and video images this means that we have more pictureinformation, which in the end results in a better image.The Four Elements of ExposureSo we have four elements to contend with in exposure:reaches the film or sensor.but it involves a penalty: faster films tend to be grainier and have lessresolution than low-speed films. This applies to digital cameras aswell — using a higher ISO will result in more image noise.Figure 10.2. (left) The aperture (iris)closed down.Figure 10.3. (right) The iris wideopen; the lowest f/stop the lens iscapable of. Lenses are described intwo ways: the focal length and themaximum f/stop. For example, thisis an f/2 50mm lens.exposure183