...I've heard conflicting stories on how the filter and shade fit together. Does the filter screw into the internal 43mm thread and the Lens Shade screw into the filter thread? Or, does the shade mount on an external thread on the Heliar, with the shade sort of going over the filter with the filter going inside of the shade.
Hi Brad -- The hood screws onto external threads on the lens body.
I have not used this lens on my M8, as the resulting effective focal length is longer than I like. But it was the first lens I fitted to my M9. Performed well, consistent with Sean's test. I cannot compare it to another 75mm.
One thing that bugs me, though, is that the 75 Heliar is not a Heliar optically, more like a Planar.
You asked about the collapsible Heliars... Of course those are 50mm and come in several flavors, all limited production specials. They are true Heliars optically, and very well made. The 50mm f/3.5 Heliar originally came in chrome finish, but as of a year ago there's a new nickel-plated one, otherwise identical. Stocks are running low I gather.
The other collapsible Heliar is a 50mm f/2.0 in M-mount more commonly seen in black finish but some were made in chrome too. Both a limited production run. And again as of a year ago there's a new nickel-plated one that has an old-style infinity lock and focus tab, thread mount, looks collapsible but is actually rigid.
I just ordered a pair of nickel 2009 Heliar 50mm lenses, one f/3.5 and one f/2.0.
Anyway, I've had my black M-mount 50/2.0 Heliar Classic for some years, and used it on the M8. It's been to DAG for 6-bit coding and focus adjustment. I've been a little surprised at how much I like this lens; it has a nice "look." It too has been tested by Sean Reid, who detected a bit of focus shift. It doesn't collapse very much, only about 1cm, and in the collapsed position the lens still doesn't protrude inward from the mount flange.