Chastened by my inability to achieve consistently accurate autofocus touchup with the Zeiss Sonnar f/1.8 on the Sony NEX-7, I made a series of 50 photographs using manual focus and analyzed the results. The only important difference between the focusing experience using autofocus override and that using manual focus on the NEX-7 with Sony lenses is that using manual focus allows you to focus using 12x magnification. I found it easier to focus the Sony/Zeiss lens with the increased magnification, but it was still difficult to get the focus point exactly where I wanted it.
As expected, the greater magnification increased my probability of success with manual focus, raising it from about a one third chance of getting a sharp image using 6x magnification to a 92% chance of getting a sharp image with 12x magnification.
Then I took the Sony/Zeiss lens off the camera, put on the Leica 24 mm f/3.8 Elmar, and made 50 more pictures. Every one was sharp. Not only that, I was able to achieve precise focus much more quickly than with the Sony/Zeiss lens, and the focusing operation was not at all stressful.
As I said in the previous post, I think the source of the less accurate, less pleasurable focusing experience on the Sony/Zeiss lens is the focusing algorithms that are built into the NEX-7. Having made so many exposures back-to-back with a lens with a normal focusing mount and the Sony/Zeiss 85 mm, I think I have some insight into what the focusing algorithm in the NEX-7 is doing. It seems that when you rotate the focusing ring rapidly, the ratio of lens movement to ring movement is higher than when you rotate the ring slowly. This may be a deliberate move on the part of the Sony engineers, who may be thinking that when you’re moving the ring slowly, you want the lens to go and some fine-tuning mode where it takes a lot of movement of the ring to get the lens to move a little bit. Maybe that would be a good thing for somebody who hasn’t spent much time using a real focusing helicoid, but it drives me nuts. I’ve spent fifty years learning how to focus really well, and I don’t want anybody to change the rules on me.
Now, let’s take a look at the flip side of NEX-7 manual focusing. With the Elmar, a wide angle lens, I got 50 successive perfectly focused pictures. I couldn’t do that with a loupe and a large-format camera. I couldn’t do that with a split-image or microprism insert in the middle of a SLR ground glass. I couldn’t come close to it with manual focusing on a modern DSLR. With the right cam in the right rangefinder camera (the odds of that aren’t good) I might be able to do it, but if the camera and the lens aren’t perfectly matched, I’d get no sharp pictures. This is spectacular focusing accuracy that NEX-7 users can rely on. It’s a major step forward.