Posted: Sep 12, 2012 12:59 pm
by DavidMcC
This is the continuation of a discussion in the "Mary and the black and white room" thread
that started a little while ago, in the Philosophy forum, but which I feel should belongs in the evolution forum, because it seems to involve NS.
A likely selection scenario to explain what I think is the Himba's lack of SW cones is that blue light focusses at a different depth in the retina from red and green, due to wavelength dispersion as the wavelength approaches the absorption edge of the lens material, which is in the near UV. Thus a bright blue sky would blur distance vision (even though there are few SW cones in the fovea centralis), by swamping the image of dark speck on the horizon if it is even slightly off the fovea.

The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration).