As we discussed in last month's Sharpshooters Tips, the iris is primarily a light control valve but it has the secondary function of affecting the apparent sharpness of a scene. Understanding how varying the f/-stop gives you creative control is only the first step toward managing the depth of field in your shots. The rest of the challenge is learning how to override your camera's automatic exposure system while maintaining the proper video level. This Sharpshooters Tips will focus on ways to adjust the iris without compromising exposure.
Creating the shallowest possible depth of field requires that the iris be wide open. Since reducing the intensity of the light passing through the lens will cause the camera to compensate by opening the iris further, one way to force the iris open is to introduce a neutral density (ND) filter into the light path.
Mounting a polarizer or ND filter in front of the lens or dialing up additional ND using the camera's internal filter wheel is a simple way of accomplishing this. On high-end lenses, flipping in the internal tele-extender will also soak up one or two stops.
With the camera on auto-exposure, it is helpful to monitor the iris setting to determine how much filtration is required to cause the iris to open fully. On most cameras, the f/-stop setting can be read from the engraved markings on the external iris ring or via a viewfinder indicator.
The ability to adjust your camera's ISO setting—the way you can on a still camera—would offer a perfect way to manipulate the iris; increase the ISO and the iris will stop down to compensate. While this feature is not yet a standard option on most video cameras, two simple electronic adjustments—db gain and shutter—will also force the iris to bend to your will.
Increasing the gain by 6 db will cause the iris to close down one full stop, offering the run-and-gun shooter greater depth of field with only a minimal decrease in image quality. Iris change can also be created by switching on the camera's electronic shutter. Setting the shutter at 1/250th instead of the usual 1/60th will force the iris to open two additional stops. This technique should be used with caution on action-filled scenes; high shutter speeds can cause motion to be distorted on playback.
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