The problem with Neopan 1600 (and Ilford and Kodak ) is that they are EI 640 - EI 800 films. You can "PUSH" [ develop to a higher than normal contrast to raise the midtones to a density that would be normal to a faster film ] but you don't really increase the shadow speed.
What you get is a set of high values, even at 3200 and faster, that can be printed: they have not built super-proportionate density.
The real virtue of Neopan 1600 is the range of highlights that can be achieved. But it is typical that if you want to shoot these films, you have to give more development than you might think, if you are making seeking normal negs.
So, you really don't lose much speed with Aculux 2, just develop longer. But Aculux wants to make higher density highlights than XTOL, so if you will need to "PUSH" Neopan to get a normal - ish neg at 1600, XTOL is a better choice. AT least that's MY reason for doing it.
There are different design philosophies at work with Neopan 1600 vs TMZ 3200: Neopan uses a gentle shoulder in the highlights to let you shoot high contrast scenes and not lose printable highlights. TMZ has an extremely long straight line that doesn't lose energy until 2.65 or so. TMZ also has a little better shadow density than Neopan.
If I'm using Neopan, I'm diluting it 1+2, and going 12 minutes or so at 68 degrees. That is the "old" ei 3200 time, but shoot at 1600. The advantage of the 1+2 dilution is lower highlight gains while squeezing the most from the shadows. But the shape of Neopan's curve gives LESS shadow and MORE midtone than a normal film, and you may find you're using a lower contrast paper than normal to print the faces.
I choose Neopan when shooting on stage when the hot lights are more of a problem than losing a little shadow.
Were I shooting portraits, I'd go straight to TMZ in Xtol 1+1 for 13 - 15 minutes, and shoot at 800.
Of course, Delta 3200 is an option, and IT gives a curve about midway between Neopan and TMZ. Xtol, or DD-X, would be excellent. Use Ilford's data as a starter.
g'luck
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