Though this objection persists in textbooks and on the internet it was actually disposed of years ago when it was shown that the cells which sit over the retina actually act like fiber-optic cables channeling light directly onto the photoreceptor cells. As Casey Luskin writes at Evolutionary News and Views:
These cells ensure that there is no loss of visual acuity due to the presence of the optic nerve, as the paper found, revealing the retina "as an optimal structure designed for improving the sharpness of images." As New Scientist put it at the time, these funnel-shaped cells "act as optical fibres, and rather than being just a workaround to make up for the eye's peculiarities, they help filter and focus light, making images clearer and keeping colours sharp."In other words, the eye functions better because these cells are located where they are, but that's not all. Now, new research shows that these "fiber optic" cells act as funnels which separate light by wavelength to enhance day vision without limiting night vision and minimizing light distortion. Luskin comments:
The implications of these findings have not been lost on expert optics commentators. A striking article at Phys.org about this new paper, Fiber Optic Light Pipes in the Retina Do Much More Than Simple Image Transfer," reflects a keen awareness of the debate over whether the vertebrate eye is sub-optimally designed. It concludes that the retinal architecture, as it now stands revealed, settles the debate. In the words of Phys.org, the notion that the vertebrate eye is sub-optimally wired "is folly." Why? Because "Having the photo-receptors at the back of the retina (i.e. behind the cells of the optic nerve) is not a design constraint, it is a design feature."Of course, none of this is proof that the eye is intelligently designed. It's still logically possible, after all, that given enough time and enough mutations and enough incredible blind luck, such a system might have evolved by chance, just as given enough time a troop of blind-folded monkeys pecking away at keyboards could produce War and Peace. I mean, it's possible. It's just that the probability of the chance development of the human retina is vanishingly small while the probability of it being designed, given our experience of designs by intelligent human beings, seems much more likely, unless one rules out the existence of a non-natural designer apriori. But why do that other than one just doesn't want to accept the existence of such a being?
It seems that every time critics of intelligent design come up with a feature of living things that confutes the design hypothesis researchers get busy studying the feature and show that, in fact, it's much more "optimal" than the critics had believed. This has happened with the human appendix, junk DNA, and a number of other features.
Maybe the best strategy for opponents of ID is, when they think there's a poorly designed system in living things, to just keep quiet so as not to draw attention to it.