What does a normal lens have in relation to the diagonal measurement of the film being used?

In photography, a normal lens is designed to produce images that appear natural and close to how the human eye perceives the scene. The focal length of a normal lens is typically related to the diagonal measurement of the film or sensor used. Specifically, a normal lens has a focal length that is roughly equivalent to the diagonal dimension of the film or digital sensor.

This relationship is crucial because it allows for image perspective and field of view that are similar to what we see in real life. For instance, a 50mm lens is considered normal for a full-frame camera, where the diagonal measurement of the film/sensor is approximately 43mm. Therefore, a normal lens has a focal length that is about the same as the diagonal measurement, making it suitable for general-purpose photography without significant distortion.

Other choices suggest relationships that either do not align with standard photography practices or are not applicable to the design and function of normal lenses.