The flange thickness on a Canon / Leica-R adapter is ~3mm. 3mm is two times thicker than a Canon / Contax adapter, so the Canon / Leica combination tends to be more forgiving. Most Leica R lenses fit on a Canon EF or EF-s dSLR camera without any problems (via adapter of course). However, there are some exceptions to the rule and these specific lenses require an added degree of commitment. 

1. •Leica 21-35mm F3.5 ASPH Vario-Elmarit-R
   

2. Applies to the Canon 1Ds Family, Canon 1D Family and 5D Family
   

3. The Leica 21-35mm ASPH is a simple solution. The plastic shroud at the rear of the lens interferes with the Canon mirror. Three small jewelers screws hold the plastic shroud in place - unscrew them, remove the shroud, put the screws back in place for safe keeping. The entire process takes less than a minute with the proper screw driver and is 100% reversible. The shroud is a cosmetic piece and removing it has no effect on the lens’ operation or performance. I owned the 21-35mm ASPH and it worked with the Canon 1Ds and 1Ds Mark II. The Canon 5D may still have issues in the 21 to 24mm range, so its value “I” (iffy).
   

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5. •Leica 28-90mm F2.8-4.5 ASPH Vario-Elmarit-R
   

6. Applies to the Canon 1Ds Family, Canon 1D Family and 5D Family
   

7. The 28-90mm has a similar looking shroud as the 21-35mm; however, there is a small spring held in place by the shroud. Removing the shroud releases the spring which can lead to problems. We have received conflicting reports with one person successfully removing the shroud without losing the spring. Another person said he removed the shroud and the spring popped out and the lens had to go to Leica for repair.
   

8. PebblePlace.com does NOT recommending removing the shroud, but some owners have decided to accept the risk.  If you already have the lens and an adapter, then give it a try and maybe you will be one of the lucky ones. Before 2012 the 28-90mm was pretty much a non-starter, but now Leitax offers a conversion kit - see their website for more info.
   

9. •Leica 16mm F2.8 Fisheye
   

10. Applies to the Canon 1Ds Family, Canon 1D Family and 5D Family
    

11. Originally the 16mm Fisheye was listed as “M”, which means a mirror shave is required. However, this depends on the intended uses. Unfortunately a mirror shave is required if infinity focus is required. However, if willing to use the lens focused at closer distances such as 5 feet, it can be usable. Thus 16mm fisheye’s rating has been changed to “I”. My suggestion, get the Zeiss 16mm Fisheye - it is an excellent lens.
    

12. •Leica 19mm F2.8 Elmarit-R
    

13. Applies to the Canon 1Ds Family, Canon 1D Family and 5D Family
    

14. The second version Leica 19mm F2.8 Elmarit-R requires extreme “surgery”. In terms of optical performance, the second version is the one you want - and it costs much more than the original 19mm. The original 19mm is probably a good lens for the 19mm look, but if seeking sharp corners, the second version is a better choice. Some owners modified the 19mm Elmarit’s (second version) rear housing by grinding it down approximately 2mm. Not the entire rear housing has to be ground, just lower half which catches the mirror as the mirror swings up. For this particular lens modifying the Canon’s mirror may be a better option. The earlier 19mm F2.8 Elmarit fits the Canon bodies without modification.
    

15. It is pretty easy to realize I am conservative when it comes to lens modifications. PebblePlace.com presents the information, makes the “safe” recommendation and then the reader can decide how far they are willing to go. In the case of the Leica 19mm Elmarit - the safe route is buy one which as already been modified. Let somebody else take the risk - and you enjoy the benefit. So in case you missed the warning - PebblePlace.com does NOT recommend grinding your lens. Also, Zeiss recently released the Zeiss 18mm F3.5 Distagon ZE. The new Zeiss lens costs less than a used Leica 19mm Elmarit-R, so I would buy a new ZE avoid all the hassle. One last possible option is a Nikon dSLR or Sony dSLR with the Leitax adapter kits. For more about Sony dSLRs and Leica lenses, click here, and here for Nikon.
    

16. •Leica 24mm Elmarit-R
    

17. Applies to the Canon 1Ds Family, Canon 1D Family and 5D Family
    

18. The Leica 24mm Elmarit-R does not clear the mirror and a lens modification is not possible. If you are willing to shave the mirror on your 1Ds or 5D, then maybe. Some options include the Canon 7D or other Canon EF-s dSLR. The EF-s bodies have smaller mirrors, so there are no clearance problems. An alternative to the 24mm Elmarit-R is the 28mm F2.8 Elmarit-R which receives very favorable reviews.  (Continued on the next page)