Putting Your Trust in the Iris
Most professional videographers prefer to shoot with the automatic iris switched off. This allows them to maintain continuity of exposure when shooting a sequence by preventing overly bright or dark backgrounds from influencing the iris setting as they switch from close-up to wide shot. Setting the iris on manual also prevents annoying and distracting iris-bounce when transient highlights, such as the flash of sun on glass or chrome or a person wearing a white shirt walking through the scene, cause the iris to momentarily stop down.
But while these occasional unwanted variations in video level are sometimes the mark of a shooter who trusts the camera's judgment more than his own, there are also times when you just don't have enough hands to pan, zoom, pull focus and ride the iris. If only you could trust the iris not to bounce and instead count on its automatic function to maintain proper level as your subject moves from deep shadows to bright sun.
Turns out you can. The secret solution lies under a little rubber plug on the bottom of the motor housing of most professional lenses. Usually marked with a "G", the iris gain control lets you adjust the speed with which the iris adjusts to changing conditions. Use one of those little green-handled screwdrivers that live in the shirt pocket of every video engineer you've ever met to set the iris gain pot to its lowest setting and watch that heretofore nervous automatic iris settle down to an absolutely snail-like rate of change.