Closing down the lens by one F-stop has what effect on exposure?

• A. Increase the exposure by 50%
• B. Decrease the exposure by 50%
• C. Double the exposure
• D. No change

Answer: (B)

Aperture controls how much light hits the sensor, and each full stop change doubles or halves the light. Closing the lens by one stop makes the opening smaller, so half as much light reaches the sensor. That means exposure drops by 50%. If you needed the same exposure after closing by a stop, you’d compensate with a longer shutter, a higher ISO, or a combination of both. Options that say exposure increases, doubles, or stays the same don’t fit because they presume more or the same light, which isn’t the case when you close the aperture by one stop.