Some time ago when the Samyang 85/1.4 was first released, there was some talk on that is was 'darker' than other 85/1.4 offerings, in that case ascribed to less clear glass elements. I'm not saying that this is the case here, but not all lenses transmit the same amount of light with the same size aperture. The technical term here is 'transmission stop' I believe, this is the way to compare exposure. (But if it had to do with T-stops, I'd guess the DA70 would perform better than the DA50-135, on account of the fewer lens elements.) T-stops do not influence the DOF, but the DOF effect could be ascribed to the different focal length of the FA77. I'd say DOF is very difficult to measure absolutely anyway.
I hope this helps