When lighting is good and doesn't change, which iris type is best?

• A. Manual iris
• B. Auto iris
• C. Fixed iris
• D. Auto-exposure iris

Answer: (C)

When lighting is steady, you want exposure to stay constant and the image to be predictable. A fixed iris provides a constant aperture, so brightness and depth of field remain the same without any automatic changes. Because there’s no iris motor trying to adjust to light, you avoid tiny fluctuations in exposure or any unintended shifts in focus characteristics that can happen with auto controls. Auto iris or auto-exposure iris are designed to react to changes in light, and even in a stable scene they can introduce small, unnecessary adjustments. A manual iris could be set to a fixed opening as well, but a fixed iris is already built to stay constant with no need for ongoing human or automatic intervention. In a scenario with constant lighting, the fixed iris gives the most stable and reliable image quality.