The camera-type eye of humans (& in fact all vertebrates) is often held up as a classic example of what ‘intelligent design’ (ID) proponents call irreducible complexity. The argument goes like this: a) the camera-type eye needs all its parts to function. b) It couldn’t possibly be assembled randomly as Darwinian theory claims. c) The eye thus supports the concept of intelligent design. After all, Darwin himself commented that “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree” (1859, “On the origin of species”).
For starters, that comment about evolution occurring through random processes couldn't be further from the truth, and I'll write something about that later. But for now - if the ID hypothesis were true, then intermediate stages in eye development would be useless. Darwin recognised this possibility, and countered it by saying that, 'if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations can be inherited, which is certainly the case; and if any variation or modifcation in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable to our imagination, can hardly be considered real" (1859, "On the origin of species").
The eye is a structure that can: detect the difference between light & dark (& in many cases colour, as well); determine the direction that the light's coming from; and focus the light to get a sharp image (for people with 20:20 vision, anyway). In other words, it's a structure that helps us to gather information about our environment. And natural selection can favour an improved ability to gather this information, even in tiny increments, in comparison with other alternatives available at that point in time.
For example, a very basic eye would consist of a few light-sensitive cells, allowing the animal to distinguish light from dark. An individual with a slightly curved 'eye', rather than a flat one, could gain some selective advantage as it would be able to tell what direction the light was coming from. Such functional intermediates do exist in nature: there is a complete series in molluscs, from a flat light-sensitive surface to the complex camera eye of cephalopods. What's more, eyes have evolved independently in at least 5 other phyla. The lens proteins are the same as, or similar to, existing proteins with other functions, but have been co-opted for another function. (In other words, a key structure in the eye did not have to evolve 'from scratch'.)
And how long would this take? In a 1994 paper, Nilsson & Pelger modelled the eye's evolution through the continuous small improvements that would be expected, if possession of even the simplest light-sensing organ had a selective advantage. Their most pessimistic estimate for the time it would take to move from a light-sensitive patch to a focused lens? Less than half a million years. A camera-type eye is indeed an impressively complex structure - but its complexity is certainly not irreducible.