Real World Digital Photography, 2nd edition

The Lens

Camera lenses are surprisingly complex and sophisticated in their construction and design, containing a series of elements—typically made of glass—that refract and focus the light coming into the lens. This allows the lens to magnify the scene and focus it at a specific point.

We've come to the conclusion that digital photography requires higher-quality lenses than film photography. Film has a variable-grain structure, whereas pixels are all the same size; this means that digital cameras operate at a disadvantage when it comes to capturing fine detail. In addition, an imaging sensor is more sensitive to light hitting it directly as opposed to hitting it at an angle, resulting in a slight loss of sharpness particularly around the edges ...