Advances in high-resolution sensors with higher pixel densities and smaller pixels are challenging optics manufacturers to produce lenses of increased performance. To understand how lenses can affect the performance of imaging systems, it is necessary to grasp the physics behind diffraction, lens aperture, focal length, and the wavelength of light. Additionally, other design limitations restrict the optical capabilities of a lens, even before manufacturing tolerances are taken into account. This article discusses how to specify lenses for megapixel cameras.