I only scanned this thread quickly so apologies if this has already been answered.
The AF 105/2.8 is a good macro lens, and is reasonably sharp at medium-far distances so is a good all-round lens. However the background rendition tends to be harsh at typical portrait distances, I wouldn't recommend it for this purpose. It's also relatively big and heavy for the focal length/aperture because it is a macro lens. The focus throw near infinity is very short, very difficult to focus accurately by hand, so you would have to rely on AF. Also, it is an old "screw-drive" AF lens so auto-focus is relatively noisy, with no instant manual over-ride.
The AI 105/2.5 is an excellent portrait lens, but is manual focus only. Results are excellent IF you can focus it accurately, it could be suitable for seated portraits where you have time to focus accurately, but it will be difficult to get good results for moving subjects. The D850 is very unforgiving of the slightest focus errors unless you don't enlarge greatly (in which case you may as well use a D750 instead).
Given your limited budget, I would take another look at the AF-S 85/1.8 G. It is a well regarded portrait lens, the faster aperture will give you more DoF control compared to the 105/2.5 and it has AF so you should get a much higher rate of well-focused pictures.
Good arguments against the Micro and for the AF-S 85/1.8 G.
Thus possible candidates (150-350,-) would be the :
- 105mm f/2.5 Ai (150-200 EUR used)
- 105mm f/1.8 Ai-s (with little scatches for 230 EUR)
- AF-S 85/1.8 G (330,- EUR NEW 390,- ./. VAT)
- AF 85/1.8 D (230,- EUR USED) ("Bokeh not so good; too harsh for elder faces")
----Other
- Samyang 85mm f/1.4 Aspherique IF Nikon ? 240,- EUR NEW
- Samyang 85mm T1.5 VDSLR AS IF UMC II ? 260,- EUR NEW
- Zenitar-1N MC 85mm f1.4 ? 260,- EUR NEW
Some important comments from you summarized for my decision making:
"nailing the 105 2.5 is very difficult imho at full opening"
"All mf lenses like the ais lenses are very difficult to nail focus, to shoot wide open portrets the model will need to be at complete standtill"
"The G is very smooth / balances perfectly with a D3 or gripped D500 or D850 / heavily underrated lens / it is better to use the G for women ... much finer, smoother skin rendering here"
"I second Roland, the 105 mm Micro Nikkors (including the MF versions) are not for portraits as good bokeh is not their strongest side."
"Older Lenses:
105mm f/2.5 Sonnar-version
Nikkor 105mm f/2.5 P.C."
"I would strongly second the recommendation for a 105/2.5 lens if your ultimate goal is to get the 105/1.4. I have the Nikkor P.C. with scalloped focus ring. This lens is simply amazing. There is a thread somewhere by Olivier Anthony who took an extensive series of excellent portraits with this lens"
"I have not used the older AF 85/1.8 D but from comments I have read, the background bokeh wasn't so smooth unless you get relatively close to the subject with a distant background. It also flares when shooting into light and shows purple fringing in some situations (correct me if I am wrong). It is a "screw-drive" AF lens so focusing is noisier than the AF-S version and no instant manual over-ride. I think in most respects, the AF-S version is a better option."
Conclusion for me:
A. I will look for a really cheap 105mm f/2.5 Ai (130-150,-)
B. I will test the AF-S 85/1.8 G in a store...
Thanks a lot for your help!
PS:
I have just found a nice DOF-Tool:
https://dofsimulator.net
