Hi,
I own and use both the 40 and 20 Voigtlanders on my Canon 5D2.
I and a few other members have made comments on this earlier, so you may want to search for this.
My main reason for purchasing the two lenses has do do with wanting a camera and lens combination that was a bit closer to the size of my film Leica M with 35 and 50 lenses. The 24-105 lens is more than just fine, and as I wrote before the IS and AF features are rather great.
Comparing the 40 and 20 to the 24-105 is not that useful, but all three lenses are useful.
On a tripod, using LiveView to achieve the best focus, the 40 yields sharper images than the 24-105 set at 40. Handheld, I don't know, but I make 12 by 18 inches prints from files from both and am very pleased with the results.
I use the 40 and 20 VC quite a lot when documenting exhibitions.
The 40 comes with a close-up lens, I only used it a little to try it and not enough to write an opinion on this.
The 20 works very well, and yes it does vignette a lot wide open, but so what, easy to correct in RAW or PS and saves on burning corners in the darkroom
The 20 distorts about the same way (mustache) as the 24-105 at 24, so no brickwork documentation please.
Both lenses are very well made, focusing is very smooth.
I use the standard focusing screen and do rely on the focus confirmation light, that and, especially for the 20, on the dof scale on the lens -- judging th edof to be narrower by a stop or more.
Micro AF: on my camera, the 40 does not need any and my 20 does.
CA: 40 basically none; the 20: -21 adjustment in red-cyan
So yes, I like them, and use them a lot. But if I had to choose, I would keep the 24-105 ahead of the other two. As long as you know why you need or want the 40 and are prepared to work manually you will enjoy the lens.
Oh, I forgot to mention in my earlier posts: both lenses but especially the 20 are extremely resistant to flare, including having the sun right at the edge or inside the image.
If you read French, see the following website:
http://www.questionsphoto.com/tag/227-voigtlanderJean-Michel
