It is wrong to state that focus shift is only theoretical or that it is irrelevant. You can see it in photos from the CV 35/1.4 under certain circumstances if you look closely enough, particularly if you print very large.
i want to buy a r2a+35mm nokton (or skopar, or zm) and upgrade from there. if i want to avoid focus shift, can i first stop down the lens and after that focus?
No, this won't work to fix the issue.
is the focus shift REALLY a problem? i mean, will your subject by out-of-focus and the shot completely ruined? i need prints as big as i can get them with a 35mm...
If you need to get big prints and retain sharpness, carefully using slow film and a good tripod will make a vastly greater difference than anything else. And nothing will make as great a difference when making big prints as going to a larger format.
it seems silly to me to spend 1000$ or more on a lens that's not 100% accurate and somehow compensate and focus forward. i need a clean and fast focus, without wasting time and thinking how to override that expensive lens...
No lens or other piece of equipment is perfect. Somewhere or another, there are always compromises. Even in a $10K Leica lens, you can be sure it would be better if it was designed with a retail cost of $20K in mind.
Think of depth of field as a theoretical band of acceptable sharpness in space that varies in width and with distance from the film or sensor plane as you stop down. This shifts in all lenses, but shifts more in lenses that display certain kinds of spherical aberration. With the CV 35/1.4 you should get sharp photos at f1.4, although designers typically make compromises about this kind of thing too. At close range and wide open the focus point of fast RF lenses is often ever-so-slightly in front or behind the point at which you focus - if the lens is poorly adjusted and this is too great, this can result in softness, but this is 'out of specification collimation', not 'focus shift'. As you stop down the focus point shifts away or towards the film or sensor plane (depending on the lens' design). You run into problems when this shift is greater than the increase in depth-of-field.
The distribution of the field of acceptable sharpness depends on the focal length of the lens. Shorter focal length lenses always have less depth of field in front of the subject than longer focal length lenses, but they have more depth of field behind the subject than longer focal length lenses. So shorter lenses have more and more asymmetric depth of field. If you spend some time with an SLR with DOF preview you can see this.
so, is focus shift internet paranoia or am i going to get just f2 lenses if i want to be on the safe side with the M mount?
It is not internet paranoia; focus shift is a fact of life in optical systems. The focus shift in the 35/1.4 CV lens shifts the plane of greatest sharpness in the film plane about the same amount as the thickness (120 microns or so) of the very thinnest (and usually sharpest) film emulsions. What we can't tell you is if this is of relevance to the way you shoot, or what you own acceptable limit for sharpness is.
If you want a 35 mm lens with a focus shift less than the thickness of a thin film emulsion, buy a Leica 35 Summicron ASPH. If you really want to make huge prints, buy a medium or large format camera. If you try the R2/CV 35-1.4, however, you might really like it. No-one here can tell you.
Marty
