In optics, Matthiessen's ratio is the ratio between the distance from the centre of the lens to the retina, versus the lens radius.[1]
This is of particular importance in fish, where the value may decrease from as high as 3.6 to 2.3, decreasing the focal ratio of the lens. A higher focal ratio is thought to compensate for the relatively high Matthiessen's ratio brought about by constraints of small eye size during early development. This provides a means for larval fish to focus images from different distances, before the ability to accommodate is gained.