When you google for reviews of the 75/1.8 Heliar, you find lots of references to "great portrait lens", "soft in the corners", etc., but the typical test shots shown are close focus portraits, with little exact detail of the lens' corner and infinity performance.
Longer term RFF members know that I love my short RF teles. So I had to try this one as well. I use teles for (1) portraits, and (2) documentary/journalism style and landscape photography. For (1), I'm interested in ease of focus (shift, among others) and bokeh , for (2) in infinity, or close to infinity performance.
The great 75/1.8 bokeh is well documented on-line. For the other measurements, please see the below tests on my M240. All images are straight from the camera (DNG -> TIF -> crop -> jpg).
Wide open, my lens front focuses by less than half inch, pretty much perfect to compensate for the cosine effect.
Until f4.0, the beginning of the DOF window stays mostly the same, and extends towards the back as you close down. At f4, I have a nice 1/3rd / 2/3rd split of the DOF window. This is very typical for most of my 50 - 105mm tele lenses. Even the 7-element 50/2.5 Color Skopar, and the VM 50/1.5 Nokton behave like that.
At f5.6 the DOF window starts to come closer to the camera.
Very nice behavior, nothing that will hinder shooting.
Shot from our deck. Forgive the tilted horizon ....
The red circles show you the points at which I will look at center and field performance.
Center crops (f1.8, 2.8, 4 and 5.6 / click on the picture and it will display a little larger):
Field crops (f1.8, 2.8, 4 and 5.6):
Please look at the electrical towers in both crops, and how their cross-beams are resolved in the pictures.
Basically the lens is sharp in the center at all f-stops.
In the field, it's not bad wide open, but becomes really sharp at f5.6.
This is at least as good as a bunch of classic lenses that I have tried, including 50 Summilux pre-asph, 75 Summilux, 90 Summicron pre-asph (v1 and v3), etc. The 75/2.5 Heliar that I use as well, is similar but a little sharper in the field. Say, the 75/1.8 at f5.6 performs like the 75/2.5 at f4.
Note also that I have to give credit to Ken Rockwell: of all the online information on the 75/1.8, his review is the one most closely matching the initial experience with my lens copy.
Thanks for looking,