What effect does increasing the focal length of a lens have on an image?

Increasing the focal length of a lens has a particular impact on the way images are captured. A lens with a longer focal length magnifies the subject more than a lens with a shorter focal length. This magnification leads to a larger image size when compared to an image taken with a shorter focal length under the same conditions.

Furthermore, with an increase in focal length, the depth of field tends to decrease. This means that while the main subject may appear larger and more detailed, the background may become more blurred, allowing for finer detail on the subject itself. The long focal length effectively compresses perspective, which can add a sense of depth to the image and enhance detail.

Overall, the increase in image size, along with the potential for enhanced detail on the subject, contributes to the correct answer. A longer focal length is particularly advantageous in fields like surveillance, where clearly defined images of distant subjects are critical.