just wondering...

The system backfocus (the rear lens) aperture setting is interactive with the front SLR lens aperture. You have effectively three aperture controls to play with.

My personal preference is to use the camcorder itself for all initial light settings with the rear lens aperture kept as much as possible in the f3.5 - f5.6 range. Wide-open on this lens will make the groundglass artifacts diminish but your sharpness will also go off.

In your UK outdoors lighting environment, (50 degrees north aren't you?), you may well find you have to set up a lot more with sunlight from behind camera ( like the ancient days of slow films speeds ).