Hi,
Regarding focusing accuracy, I would say it makes some sense to discuss the focusing accuracy at the film plane. Let's assume that we have P45+ sensor, it has a pixel pitch of 6.8 microns. My experience/opinion is that the image starts to degrade when CoC exceeds something like 1.5X the pixel pitch. So, for optimal sharpness the P45+ would require a CoC of 10.2 microns. The amount of allowable defocus at f/8 would be 8*10.2 -> 81.6 microns, say 80 microns. At f/2.8 the limit would be 2.8 * 10.2 -> 28.56 microns say 29 microns.
Factory calibration on the Hasselblad is +/- 30 microns and on the P45+ back +/- 12 microns. So worst case would not be OK for f/2.8 but very much OK for f/8.
How can you check focusing accuracy? Simply:
- Take your fastest lens set focus at infinity and shoot a quite distant subject at full aperture at infinity stop. If you get a good image, the body is well aligned.
- If you have a split image viewfinder you can check that it indicates perfect focus on the distant object.
If possible, you should use a viewfinder magnifier for this.
How do you know that you have good focus? If you can see aliasing effects you probably have decent focus.
For good sharpness, don't rely on DoF markings. They are calculated for 66 microns, or so.
It is quite OK to stop down a bit. With the best Hasselblad lenses, like the two Sonnars, you would probably need f/5.6 for best performance and you essentially get rid of aliasing at f/16. But you can sharpen an f/16 image quite a bit.
Many Hasselblad lenses have some field curvature. In general, I found that f/11 is a pretty decent compromise. Sonnars and Planar 100/3.5 achieve peak performance at around f/5.6. Some degradation of image quality is visible at f/8.
The DoF scales on the lens are based on small prints and they will mislead if you want to make large prints that hold up to close scrutiny.
One thing to consider is that most good lenses will perform about the same at f/8. So, if you are shooting small apertures, you don't need expensive lenses.
Best regards
Erik