Hi Edmund,
Set up and do a groundglass focus and take an exposure(the exposure is correct) check the focus in the 100% window. If it is not as sharp as you think it should be, do the following. Do NOT open the lens or refocus using the groundglass. Simply take your focusing knob on whatever standard you use and make a micro-adjustment, either forward or back, the standard should just barely move(I have a sinar p and do not have to lock focusing knobs ever). Advance the shutter and take a second exposure, watch the 100% ....
I use a 150mm Apo-Sironar(a film lens) on a sinar p, with a leaf valeo 17wi to photograph jewelry, so that the jewelry is reproduced at lifesize(100%) or larger on the page. I use this all the time to fine tune the focus on diamond rings, etc.
Good luck,
Brian Woolf
As suggested above, Live view helps, and in IMO it is what makes Medium Format Digital View Camera (MFDVC) photography viable with short lenses.
You will probably have to check the focus in several areas of the picture if you are using tilt, or if your tilt/yaw is not accurately set to zero.
¿Anyone using a Hasselblad back with live view on a MFDVC?
I am still waiting for live view for my system (below).
Why not get a Schneider apo-digitar macro for jewellery? (I was luck enough to pick one up on e-Bay.)