Canon-made offerings on the wide angle focal lengths suffer from softness at the edges and corners when used with a full-frame sensor camera such as the 1Ds Mark II, or 5D. The best solution I've come across, is to use a Zeiss 21mm f/2.8 Distagon, with a CameraQuest Adapter
to mount on the Canon 1Ds Mark II. If you view these crops at size large, they'll take a moment to load, as they are rather large (~550kb each). The below photos were taken on a windless day, camera tripod mounted. The difference is clear.
If you decide that you want to spend $2 - $3K on this glass, here's what it looks like. It's lightweight, and easy to handle. The focusing mechanism is smooth flowing. It feels solid in your hand and on your camera.
Pictured here with the adapter. The adapter is very thin, goes on easily, and allows the use of the Zeiss 21 on any standard Canon mount DSLR.
Many ask - what about autofocus, and metering? It's a cinch. At such wide angle, DOF is really great - so even setting focus by hyperfocal distance is a snap. With the large, bright optical viewfinders in today's full-frame DSLRs, focusing by eye is very easy. You can also get interchangeable focusing screens from Canon to make it even easier, if you like.
I shoot with this lens, for the most part, on a tripod. Metering is easy - I set the camera in Av mode, open it wide (f/2.8) and compose and focus. Then I stop the lens down by hand to my desired aperture (typically f/8 with this lens) and hit my remote shutter release. The camera will still automatically expose the scene properly - meaning, it will calculate the proper shutter speed.
Here's the test scene for the below two sets of crops. Tripod mounted Canon 1Ds Mark II, shots taken in identical lighting with identical exposures. Shot in RAW and converted with identical settings in Adobe Camera RAW.
Click on this image to see a 100% crop side-by-side comparison of the CZ21 versus the nearest comparable offering from Canon, the 16-35 f/2.8L.
Alright, alright - it's never fair to compare a prime versus a zoom - I know. But show me a prime from Canon that can compare to the 21mm focal length and be superior to the 16-35L. IMO, there isn't one. The 14L? Softer. The 24L? Softer, and not wide enough for many landscape shooters.
Look at the detail and sharpness. There's no sharpening applied to these images - you should see how they respond to standard sharpening in-post, and the details in-print are eye-popping!
Here's the nether-regions of this image, and honestly, it's where the huge premium for this glass is worth it. With this sort of detail and sharpness, I know that my large landscape prints are going to stand up to the "up close" test!
Note the lack of CA (Chromatic Abberation) in the Zeiss crop. This lens controls CA and flare extremely well.