A lens with a focal length of 40mm to 60mm on a 35mm film camera is known as a “standard lens” because it delivers natural perspective, close to what the human eye perceives. The usual definition of a standard lens is one that has a focal length close to the diagonal length of the image format. The focal length of the DP2x's lens is 24.2mm, and the diagonal length of the image sensor is 24.86mm. So, the lens used in the DP2x really do deserve to be called a standard lens.
In the past, standard lenses have traditionally been of either the Tessar or the Gauss type. The Gauss type is basically used for larger apertures with high performance. Its disadvantages include susceptibility to saggital coma flare when used with a point light source, and a tendency for the light volume to decrease towards the periphery. The Tessar type, on the other hand, has a simple structure, making it easy to miniaturize. However, its drawback is that it tends not to deliver high performance at larger apertures.