Trigonometric calculation used for obtaining a theoretical value of the visual acuity. Visual acuity was calculated in cycles per degree (cpd) using the following formula: 1/(2(arctan(s/f)). f: focal length; s: distance between the center of two red-green double cones; ΔΦ: inter-receptor angle at the nodal point of the lens.
Haug et al. Frontiers in Zoology 2010 7:8 doi:10.1186/1742-9994-7-8