Focus shift is much debated today online, however it is always examined from the point of view of the digital shooter. A digital camera easily solves the problem of knowing where to focus, with a “live view” of the image formed on the sensor.
However, somebody relying on the image formed on the focusing screen may have a different kind of trouble – my case with my Nikon FM2n camera. Also, film is more expensive and thus unforgiving.
It is generally accepted that in the case of a lens that generates noticeable focus shift (such as the Nikkor 50mm f/1.2 AI-S), the best thing to do is focusing at the same aperture (stopped down) the image needs to be taken. This statement however ignores that modern focusing screens do not always show, in the viewfinder, the real focus or the real depth of focus of the taking lens, especially for apertures wider than approximately f/2.8.
I estimate that FM2n’s standard screen shows a d.o.f. corresponding to roughly f/2 and that therefore if you want to shoot the 50mm f/1.2 at f/2 you can rely on what you see in the finder. On the other hand, for shooting wider or closer, a different approach is needed.
Starting from f/1.2 and closing the aperture, it can be observed that the focus moves farther away from the camera lens: at f/1.2 it is closer (than observed) and at f/5.6 it has moved further. If a screen shows approximately f/2, this is in between the two opposite trends.
This is what I recommend, as a practical workaround:
- To shoot at f/1.2, focus normally and then rotate the focusing ring towards an indicated increase of the distance (about 1mm of linear distance on the ring). In other words, push the focus away.
- Conversely, to shoot at f/4, focus normally and then rotate the focusing ring towards shorter distances, of again approximately 1mm linearly.