I use Olympus 21/3.5, and Zeiss 28/2.8, 35/2.8, 50/1.7 and 85/2.8. I enjoy them all, the image quality is excellent in my opinion, they are of excellent construction, are small, and lightweight despite being made of metal. Focus and aperture mechanisms are well-damped and smooth. All are very contrasty and have colours that I enjoy.
The Zeiss 28/2.8 and 35/2.8 are clearly best at f/8 and f/11, and have only the smallest bit of barrel distortion. The Zeiss 50/1.7 may be my sharpest lens, in particular between f/2.8 and f/5.6. The 85/2.8 is very good at f/2.8 where I use it for concerts, and is excellent at f/4 through f/8. The Olympus is best at f/8, where it achieves a very large depth of field and very good contrast and resolution with almost no barrel distortion, for wide landscapes.
I use Cameraquest adapters on all the Zeiss lenses. The CQ adapters are excellent, high-quality and precise, and I get infinity focus on all of the lenses on my 5D. The Oly 21/3.5 has a no-name adapter than I'm going to replace because it is of poor quality.
I also use a Brightscreen Proscreen #2 (p232) with large microprism spot centre for manual focusing.
I should say, though, that had I not switched to the 5D, I likely would have kept and continued to use the Canon 17-40/4L that I thoroughly enjoyed on my Digital Rebel. It did not, however, perform as well at the edges and in the corners as I'd hoped it would on my 5D.
This is all just one man's opinion, your mileage may vary, and there are people who will disagree with my subjective findings. I don't really care, I use what works best for me!
Cheers,
-Rob
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http://www.pinciuc.com/photos/
You see, the thing is, I'm an absolutist. Well, kind of... in a way...