Aperture And Exposure

(by Roland Lewis, photos Jorg Fedler)

The lens on a video camera, like that on a stills camera, is equipped with an iris diaphragm. This metal ring of overlapping leaves controls the aperture and thus determines exposure, or the amount of light that reaches the video camera's image sensor. It works just like the iris of the human eye, reducing the size of the aperture in bright light and increasing it in dim light.

Automatic Iris

Most video cameras have an automatic iris, an auto-exposure facility, which adjusts the aperture (opening that limits the quantity of light that enter can enter into the video camera's sensor) automatically as you shoot. This is a great benefit in many video-making situations since it allows you to concentrate on the subjects, their actions and composition.

When a video camera is pointed at an evenly lit subject, the auto irises sets itself to ensure that the brightness of the scene is fully reproduced and the range of colors effectively represented.

Backlight

However, shooting someone sitting under the shade of an umbrella on a sunny beach can be difficult with the auto iris, which will adjust itself to the general brightness of the scene and hardly pick up the person in the shade.

The auto iris responds directly to the brightness of the subject you are recording and will continually open and close the aperture when brightness changes. This can be particularly noticeable with camera movements and changes of subjects within the frame. Iris changes will be visible as momentary brightening or darkening of a scene when tape is played back.

The tendency of the auto iris to 'misread' certain situations is the inevitable consequence of an automatic system. However, you can compensate for some of the effects by altering the composition: avoid too much sky in the landscape shots that will result in the underexposure of the foreground and avoid panning across scenes with too much contrast - from sunlight to shade, for instance.

Backlight (left)

The automatic system also has problems when the light is coming from behind the subject ('backlit' subject). A person sitting in front of a window, or outdoors with the sun behind them, could appear to be lost in murky shadow or be seen only as a silhouette. The human eye would adjust to compensate for this type of lightning, but the video camcorder's less sophisticated auto iris is mislead by the surrounding brightness.

To cope with this situation most video camcorders have some sort of backlight compensator control. This control gadget will enable you to adjust the iris and correct the exposure, thus improving the image quality of the main subject, though the background may be too bright or 'burnt out'.

Manual Control

If you are lucky enough and your camcorder has a manual iris control, you can solve these kind of problems by switching over to 'manual', which gives you control of the iris setting. With an electronic viewfinder, you can assess the exposure and then choose the appropriate aperture. But for creative effect you can deliberately under- or overexpose certain shots - for instance overexposing to give a bright, dreamlike-like effect.

When setting the exposure manually for a pan shot that moves from a light to a dark area, adjust the iris to suit either the beginning or the end of the shot. If there is too much contrast, it is better to treat the scene as two separate shots and set the exposure appropriate to each.

Gain Control

Many video cams are equipped with a 'gain' control. This amplifies the video signal electronically to increase image brightness and improve contrast in low light light conditions - such as dim interiors or fading daylight. However, the image becomes slightly degraded with grainy appearance.

High Speed Shutters

In video cameras with high-speed shutters, the adjustment of shutter speed is linked to the aperture setting. As the shutter speed is increased, the aperture is opened up proportionally to ensure correct exposure.

Overexposure: With the iris fully
fully open, the scene is too bright,
details are bleached out
(entire background here lost), contrast
is lacking and colors will be weak.