The problem with buying an L lens, IMO, is that you're paying a big premium for an image circle that you're not fully utilizing. So, unless you plan on upgrading to a 5D-series or 1D-series body at some point in the next few years, you're spending money on glass that would probably be better spent on that body upgrade.


Yes, many L lenses are very sharp compared to their non-L counterparts, and that's a legitimate reason for buying them regardless of what you're putting behind it. And if you buy a cheaper, non-L EF lens, you're still paying for that bigger image circle. But the L glass is most often distinguished from the non-L lenses not by how well they perform in the center, but how they do in the periphery--precisely the area that an APS-C sensor doesn't see.


So why not buy EF-S? Because there are virtually no options for fast aperture EF-S lenses. To date, Canon does not make an EF-S lens faster than f/2.8.


As a result, I think you have essentially three choices:
  1. Third-party crop lenses (e.g., Sigma 30/1.4)
  2. Canon EF non-L lenses (e.g., EF 28/1.8)
  3. Canon L primes (e.g., EF 24/1.4L, 35/1.4L)



Of the three above, I'd say the 28/1.8 has the worst image quality--and it is also obviously the slowest. The Sigma won't work on a full-frame body; and the L glass will make you poor but happy.


Actually, there is one more choice: The EF-S 17-55/2.8 IS. It's around 2 stops slower, but you get IS as partial compensation. For still or slow-moving subjects, it would work just as well as having an f/1.4 lens, but it will not be adequate for moderate action in low light. But its distinct advantage is that it gives you an ideal focal length range at an aperture that is still relatively fast. You can't get 17mm @ f/2.8 otherwise, without spending a LOT more money.