The shape of human cone photoreceptors depends on the retinal location. Human foveal cones resemble rods. They are elongated and thin; whereas peripheral cones become progressively shorter, fatter and more tapered with retinal eccentricity (Schultze, 1872; von Greeff, 1900). Because the length of the outer segment decreases in going from the central fovea (average length = 35.5μm for L- and M-cones) to the parafovea (average length = 22.5μm for L- and M-cones), the pigment optical density and the spectral sensitivity of the cones will be affected. See summary table of data and references.
There is little quantitative information about the regional differences in S-cone morphology. It is clear that at the same retinal location, the lengths of the outer segments of S-cones are shorter than those of L- and M-cones. This is in keeping with their longer inner segments. Peter Ahnelt (personal communication) estimates that at retinal eccentricities > 5 mm (17.7 deg), the outer segments of the S-cones are shorter than the outer segments of the L-and M-cones by 15-20% for L- and M-cone outer segments of 15-20 um. The difference in the fovea is probably somewhat smaller, perhaps no more than 5%.
There is also little quantitative information about regional differences in rod morphology. However, it is clear that rods are not uniform in size across the retina. Polyak (1941) reported that human rod inner segment diameter increases slowly from 1 μm in the fovea to 2.5 μm in the far periphery. It is to be expected, therefore, that the length of the rod outer segments, like those of the cone outer segments, decreases somewhat with eccentricity.
von Greeff, R. (1900). Mikroskopische Anatomie des Sehnerven und der Netzhaut. In: Graefe u. Saemisch Handbuch d. Ges. Augenheilkunde, 2. Aufl., 1. Teil, bd 1, Kap. V.
Polyak, S.L. (1941). The retina. Chicago: University Press.
Schultze, M. (1866). Zur Anatomie und Physiologie der Retina. Archiv für Mikroskopische Anatomie 2, 175-286.
Schultze, M. (1872). The eye: the retina. In: Manual of human and comparative histology (ed., Stricker, S.) 3, 213. The Sydenham Society, London.