The concept of informational sensitivity is derived intuitively as the gradient g = dD/dE divided by the product of Selwyn granularity $\mathcal{G}$ and the square root of the area s of the spread function. When the diameter of the camera lens and the size of the final picture are held constant, the relative focal length of the camera lens for equal detail rendition varies directly with the product $\mathcal{G}\sqrt{s}$. The required exposure time then varies inversely with informational sensitivity $g/\mathcal{G}\sqrt{s}$. For a stationary camera, detail rendition improves as the focal length of the camera lens increases. But the exposure time also increases, and when the camera moves relatively to the scene, detail rendition is thus diminished from this cause and is optimum for a certain focal length. Examples are given to confirm these conclusions.