Chapter 35, Problem 1CQ

To determine

Whether the exposure of a camera will become overexposed, underexposed or same if the focal length is changed to 16 mm without changing the diameter of the lens aperture. The correct exposure is when the focal length is 8 mm.

Answer to Problem 1CQ

Solution:

Exposure of camera becomes underexposed.

Explanation of Solution

We have Exposure = $I\Delta t_{shutter}$, where I is the intensity and $\Delta t_{shutter}$ is the shutter time.

And Intensity, I $\propto \frac{D^2}{f^2}$, where D is the diameter of the lens aperture and f is focal length.

Here D is same and f is changed from 8 mm to 16 mm

➩ Intensity reduces, since Intensity inversely proportional to square of focal length

If intensity reduces exposure also reduces, since exposure is directly proportional to Intensity.

➩ So, the exposure of camera becomes underexposed.

Conclusion:

How the camera exposure is changed when the focal length is increased is found out.