A friend of mine asked out of honest curiosity what the difference was between different pieces of high-end equipment. He asked from the point of view of a photography outsider. More precisely, his real question is what does equipement worth $11K do that equipement worth $3.5K doesn't do?
Here is a very simple comparison, and let me tell you right off the bat that it's completely fair. The premise makes it fair to compare lenses with an order of magnitude difference in price. You'll find the more expensive equipment is both handily superior but at the same time the gain is far from proportional to cost.
The cheap lens in the test is a Nikkor AF-D 18-35mm ED zoom, and it's mounted on a D700. As far as wide zooms go this one is extremely ordinary, it's just like the other zooms I've seen covering this focal range and at this price point. This lens formerly occupied the market segment where the 16-35mm f/4G VR currently is.
The expensive lens in the test is a Leica 28mm Summicron-M ASPH, on the M9. This lens is not ordinary at all, though it has solid competition in the 2/28mm Distagon ZF. Compared to the Nikkor this lens is about six times brigther, a quarter of the size and yet feels heavier, oh and it costs about eight times more to buy.
I will be using this scene shot earlier today with both lenses at f/5.6:
Near the right edge:
Near the left edge:
In the outer regions is where all the money goes, so it's obvious this is where the Summicron beats the Nikkor. Incidentally that's where all the pixels are too, if you're counting. We haven't looked deep into the corners here, but the further out you go the worse it gets for Nikkor.
In the center the differences are much more subtle, and basically boil down to sensor resolution differences. Here are crops form the center of the same image.
Conclusion... no conclusion.