focus isses: DOF vs shutter speed vs AF settings

[KmH]

You lens has an f/1.8 option making it a "fast lens", why not try that setting? .

The first issue is that many f/1.8 prime lenses deliver soft focus when used wide open. A common recommendation is to stop the lens down at least 2 stops - which would be f/3.5 - just 1/3 stop from what the OP used..

The second issue is that using a wider aperture at the same point of focus (PoF) distance would make the DoF even shallower. So much so that neither persons face could be completely in focus. as you point out getting closer would make the DoF even shallower still.

Mk III - 85 mm lens - f/4 - 8 foot PoF: Total DoF is 0.63 feet (7.5 inches)
Mk III - 85 mm lens - f/1.8 - 8 foot PoF: Total DoF is 0.28 feet (3.36 inches)

 

[unpopular]

I agree. f/1.8 is going to be way too shallow and too soft. when I saw this image, I was thinking f/8 myself, though I am not sure how that would render the background.