Interesting question, Alex. I'm wrestling with the same thing.
First, to answer your question, the 90 Summicron will bring up the 85 frame.
Here are my candidates for the portrait lens:
Rollei 80/2.8 - It's a Zeiss design, so it keeps you in the family. It's a Planar, which is unusual to find in a Zeiss portrait length lens. I've spoken either in person or on the internet with several people who own this lens & no onwe has a single bad thing to say about it. In fact, just the opposite: they rave about it, some calling it the best lens they've ever used, including their Leicas. Its only drawbacks that I know of is the minimum focus distance (1.2 m), which is longer than most, so you need to know if this fits your shooting style, & the size 77mm & 470g. For me, this focal length is more likely to get used in every day shooting, but it's a little short for a true portrait lens. I don't know what frame it brings up, but since it's LTM, changing the adapter to a 90 will bring up the correct frame if the 80 adapter doesn't.
Leica 90/2 ASPH - Measured by Pop Photo at 88 mm, it's the best match for the 85 frame except for the Zeiss Sonnar. A superb lens from everything I've seen. The only criticism I've read is that it may be a little too sharp - a factor in portraiture. About the same size & weight (78mm, 500g) as the Rollei Planar despite its longer focal length.
Konica M-Hexanon 90/2.8 - Measured by Pop Photo at 92 mm, it's on the longish side as a match for the 85 frame on the ZI, so it will take some learning to use it properly. OTOH, it's more compact (69mm, 330g) than any of the other options. Although not designed by Zeiss, it is a Sonnar-type (Ernostar) design, so it is a Zeiss derivative & keeps you in their family more or less. It has received numerous excellent reviews by users. It can be purchased new at
www.photostop.net for $900 USD if it can't be found used.
CV 75/2.5 - Measured by Pop Photo at 77 mm, it is a longish 75 or could be considered a short 80, which makes its use with the 85 frame a possibility - no different than trying to work with the longish 90 M-Hexanon, but coming at the 85 frame from the other direction. It is LTM, so a 90 adapter will bring up the correct frame even if the 75 doesn't. It is the most compact choice (230g & 64.5mm with hood), so it's a good walking around lens, but probably not the best for true portraiture. It has excellent optics & an economical price tag. I'll be working with this for a while since I already have it.
There are many other choices, of course, but these are the ones at the top of my list.
Cheers,
Huck
