Most of us eventually want to shoot portraits, or maybe thats already all you do. Regardless, two all manual options out there for mirrorless cameras are the Voigtlander 75mm f1.8 M-mount and the Carl Zeiss Contax 90mm f2.8, designed and made for the luxurious Contax G1 and G2 film cameras from the '90's. The Voigtlander you can mount on any mirrorless body using a simple $12 adapter. The Contax requires a more advanced adapter, one that incorporates a focus ring in order to allow you o manually focus the lens when not mounted on a Contax G1 (or G2) film body. These go for $40-$130 depending on quality. Used the Voigtlander lens can be found for $400-$450 range with the Contax lens being sold for $180-$250 without an adapter. You figure another $80 for a nice adapter and the Contax is a $250-$330 lens package. Not bad.
Below you will find the Voigtlander test shots on the left with the Contax shots on the right. The below series are taken with a Sony A7S with WB, ISO set the same for both lenses and focused on the target (obv).
Voigtlander 75 closer to target to emulate 90mm framing. Set at f2.8 (stopped-down).
Contax 90 at f2.8 (wide-open)
Here you can see the difference a 75mm will give you versus 90mm at the same distance from the target.
Voigtlander 75 at f2.8
Contax 90 at f2.8 (wide-open)
Voigtlander 75 at f4
Contax 90 at f4
The next are both wide-open, the Voigtlander at f1.8 and the Contax at f2.8
I then compared the lenses mounted on a Fujifilm Xpro2.
Voigtlander 75mm f2.8 framed like a 90mm
Contax 90 at f2.8
What was quickly noticed was the sharpness of the wide-open Contax 90 beat-out the stopped-down performance from the Voigtlander. The bokeh was more pleasant and the highlight bokeh balls kept a round shape even when stopped down on the Contax to f4, while the Voigtlander could not retain the round shape. Conclusion; the Carl Zeiss Contax 90mm G is a better lens and value than the poor performing Voigtlander 75mm f1.8 M-Mount.