It's pointless, I think, to go at the nature of rangefinder focussing directly. We get into all kinds of matters that have no meaning but to technoids and camera makers. But is is VERY easy, not to mention instructive, to compare rangefinder focussing to an SLR. And - hopefully - get to the heart of your concerns.
Back in the mid '80s, the late Bob Schwalberg wrote about focusing fast rangefinder lenses and fast SLR lenses. Along the way, he talked about the limitations of focus accuracy with an SLR, usung the Nikon F3 as an example, compared to a Leica M4P.
At the same time, I had the misfortune of breaking my leg in a soccer game and while hobbling around the house took on an investigaton of the same problem. I was relieved to find my crude discoveries validated by Schwalberg. By the way, Schwalberg was not your 21st century pundit, being both a master photographer, passionate teacher, fine writer, and very capable technical apologist.
Anyway, here's the salient points. A first rate 35 slr, like the F3, has it's focus accuracy limited by the effective depth of field induced by the focus screen. The longer the lens, the greater the possible magnification, and the greater the accuracy.
A rangefinder, however, has a given ability to distinguish detail, with no relation to the lens it is focussing.
The rangefinder on an M4P has almost the same accuracy as a 135 lens at f/2.
Compute the effective aperture and you can compare other focal lengths. [ 135/2=67.5mm aperture ].
Focus a 50/1.4 lens on an M4P, and the rangefinder is as discerning as a 135/2... or a 50mm f/0.74 [ 50/67.5=.74 ]. An Nikon F3 is far less discerning, dependent upon the focus screen. No SLR has been better ( arguably, the Leica R8 or R9, but nothing else --- and AF accuracy is nowhere close, nor as consistent ! ).
Look at the effective focus aperture of a 21 mm lens: 21 / 67.5 = f 0.3 !
Any wonder why people still use Leicas ?
NOW, the Bessa rangefinder is not as discerning as the Leica. But it is still FAR superior to any SLR and the equal to Canon's rfdrs, or Nikon's.
With the Canon at f/1.4, at minimum focus range, you will probably find the Bessa to be able to focus perfectly sometimes, and very well, all the time. The f/1.4 performance of a Canon / 1.4 is very good, and a good match for the Bessa. If you want to record hairsplitting detail from 1 meter, I doubt you'll be able to do it. But if you want to be able to get sufficient information to make beautiful pictures in low light, no trouble at all.
Finally, in order to surpass the images you'll get from the Canon on the Bessa, in a meaningful and visible way, you'll need an M rangefinder, AND the latest M Summilux. With that combination, you'll see a difference... if you want to be hairsplitting.
SO, the Bessa and 1.4 Canon should be a fine combination on it's own merits. Compared to ANY SLR, it's markedly superior. And compared to the very best, it is close enough in performance to not worry over. You'll never lose a picture.
Finally, finally. There is a small chance the bessa rfdr will need to be adjusted for the Canon lens, or the Canon lens collimated. Not a big deal. There are a good technicians in Toronto.
have fun !
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