For doing tighter portrait shots, on a FF body you'd need a focal length of about 60-120mm to get good perspective. With APS-C, about 40-80mm. A fast aperture is important for controlling degree of background blur. The FA 77mm f/1.8 Limited is a great lens for either FF or for APS-C, for numerous applications, and has unsurpassed bokeh quality- really wonderful. Your 18-50mm will also be usable on APS-C, however I would not put trust in it even at 50mm on a FF body. But it is no FA 77mm Limited! On the less expensive side, the FA 50mm f/1.4 has a very good reputation.
From what I've seen, your Sigma lens does not seem to be an internal focus design, which is good if true. IF lenses tend to be tricky regarding focal length accuracy at closer shooting distances. Where you think you have the rated 50mm, you might actually be down to well under 40mm. Another thing about many zooms is they often have a short focus rotation, making fine manual focus more difficult.
Even so, you might try your Tamron 70-200mm at 80-100mm using APS-C, since I think it is an IF lens, shoot wide open or f/4, and see how that goes.
Last edited by mikesbike; 05-02-2017 at 08:14 PM.