Angular diameter of defocus blur discs

When the eye is defocused for any reason, the diameter of the blur disc may be used as a first approximation to estimate effects on visual acuity. More exact treatment must use physical optics which take diffraction effects into account. Traditionally, defocus calculations of blur disc diameters have started with a particular schematic eye and rays traced to find the retinal image diameter of the blur disc. The angular or perceived diameter of this disc is then found by calculation of the angular subtense of the retinal blur disc measured at the back nodal point. A formula is presented here which bypasses the need for a specific schematic eye and is valid for any level of accommodation. The formula, although it is approximate, is very simple and only contains a defocus or refractive error quantity and the pupil diameter.