Understanding the Iris, Part 1

Lenses used in videography contain a valve that regulates the amount of light which reaches the image sensor.

The iris (or diaphragm) is a series of movable metal blades that form a circle that reduces the diameter of the path light follows as it passes through a lens. Just as the iris of the human eye is controlled by the brain, the iris in a video lens is automatically controlled by a servo-motor. It closes and opens in response to instructions from the camera to ensure that the light focused upon the imager is neither too bright nor too dark.

But if acting as a light valve was the diaphragm's only function, it would be of no more interest to the creative videographer than the valve inside a water fountain which controls the pressure so a person taking a drink doesn't also get his face washed. Due to the fundamental laws of optics, the iris has another important function. It determines the portion of a scene that appears to be in sharp focus.

When the iris is reduced to its smallest diameter, the distance at which objects will appear sharp (in front of and beyond the point at which the lens is focused) will be much greater than when the iris is wide open. In extreme cases, this phenomenon, called depth-of-field, can cause spots or motes of dust on the front element of the lens to appear as part of the image.

While the run-and-gun news shooter quickly comes to appreciate that a small aperture—around ƒ/11 or ƒ/16—means that fewer shots will appear out of focus, videographers wishing to accentuate their subject often prefer to use larger iris settings so the foreground and background do not appear in sharp focus.

How to shoot at any desired ƒ/-stop without under or over-exposing the image will be the topic of next month's Sharpshooter's column.