Optical aberrations are deviations from a perfect, mathematical model. It is important to note that they are not caused by any physical, optical, or mechanical flaws. Rather, they can be caused by the lens shape itself, or placement of optical elements within a system, due to the wave nature of light. Optical systems are typically designed using first order or paraxial optics in order to calculate image size and location. Paraxial optics does not take into account aberrations; it treats light as a ray, and therefore omits the wave phenomena that cause aberrations. For an introduction on optical aberrations, view Chromatic and Monochromatic Optical Aberrations.