tmhk, I'm sorry that I was in a rush this morning when I posted, but I would like to elaborate on my comment a little bit. As I mentioned above, the 40/2.8 Sonnar was developed as a candid lens, a people lens. Although it is generally very sharp, there is a little softness in the resolution of very fine detail - something which can be quite flattering to your human subjects who might not want a blemish rendered quite so perfectly. This contrasts with a lens like the Leica 50 Summicron, which is sometimes described as "painfully sharp" because it is so good in resolving even very fine details. For some purposes, of course, such resolution is preferable.
One of the advantages of a 40mm lens is that, like other wide angles, it has great depth of field - even as compared to a 50. Again, this makes it very useful for shooting candids because it is easy to estimate or pre-set focus & to then be able to shoot quickly in situations involving people. To use the lens in this manner means a preference often for aperture settings of f/5.6 or smaller to maximize depth of field for quick shooting & accurate focus. When you need them, f/4 & f/2.8 are there. It is for this type of shooting that this lens shines. I find that for low light shooting or for situations when I want shallower depth of field, I'm more likely to use my 50 Summicron.
The Rollei 40 Sonnar will work well with the R3A because the 1x magnification allows for use with both eyes open & therefore a more spontaneous kind of shooting to capture "the moment." However, if the R3A is the camera of choice in order to take advantage of greater precision in focusing for shallow depth of field or in low light, a lens with a wider maximum aperture would be preferable.
Cheers
