Assuming the visual cortex (and possibly the optic nerve itself) is as computationally intensive as the retina, successive layers producing increasingly abstracted representations, we can estimate the total capability. There are a million separate fibers in a cross section of the human optic nerve. The thickness of the optical cortex is a thousand times the depth occupied by the neurons which apply a single simple operation. The eye is capable of processing images at the rate of ten per second (flicker at higher frequencies is detected by special operators). This means that the human visual system evaluates 10,000 million pixel simple operators each second.
This is the 1976 Moravec calculation:
https://frc.ri.cmu.edu/~hpm/project.archive/general.articles/1978/analog.1978.html