You hit the nail on the head, Jon.


Quote Originally Posted by George Slusher
Could you explain that a bit further? The image circles will be the same size for both lenses, I expect.
Yes, the image circle size is the same between the 24-70 and 70-200, but the difference is that the 70-200 gets it for free, whereas you have to really pay for it with the 24-70. In other words, longer focal lengths result in large image circles as a necessary part of their construction. Short ones don't, and so they require expensive lens designs (retro focal) to get the image circle large enough. And if the lens needs to cover 2.56X more area, that can add up to a lot of expense (and/or lower quality).

Compare the 18-55 IS with the 17-40 f/4 L on a 50D

The $150 kit lens beats the $770 L in resolution at several focal lengths and f-numbers, despite the fact that it costs 4.5X less! The reason for this is that the L must be able to cover a much larger sensor size. The 24-70 provides relatively low value on a crop for the same reason.

Of course, if the 24-70 provides the exact focal length that the OP needs, then it doesn't matter if Canon could have built an EF-S version for half the price and twice the quality, because unlike the 17-40, there are no EF-S 24-70 f/2.8 lenses. The reason they do not exist, IMHO, is because most lens makers think the 17-50 range is more useful on APS-C.

In any case, once you get into the longer focal lengths like 70-200, the image circle will cover 35mm without any effort, so there is nothing to be gained by building an EF-S version.