The word fovea refers to any small pit or depression in the surface of a structure or organ. The Fovea Centralis is a small depression in the center of the macula lutea of the retina. The average diameter is 0.3 mm and the area is rod-free area with very thin, densely packed cones, allowing for a point of maximal focus.
Measured density curves
for the rods
on the retina display an extremely high density of cones in the fovea centralis. Cones in this area are also slightly smaller in size than anywhere else on the retina (with diameters of 0.0030mm to 0.0015mm). Since both colour
vision and the highest visual acuity
are associated with the cones, an image must be focussed on the fovea centralis .
The field of vision
is typically around 200 degrees
but the acuity
over most of that range
is poor. For examination of intricate detail, the light must fall on the fovea, and that limits the acute vision
angle to about 15 degrees. The total absence of rods makes the fovea a second blind spot
since cones have very low light sensitivity.
Ref : Hecht, Eugene, Optics, 2nd Ed, Addison Wesley, 1987