There aren't intermediate half-stop detents on Nikon MF lenses, though the meter will measure movement between full stops. It's just a bit difficult to do more than guesstimate fractional f-stops when the distance between full stop detents on the aperture ring are so small. Needle displays help, I suppose. Early Mamiya RB lenses have half-stop detents and markings on the aperture ring; later KL lenses have half-stop markings but no detents. Not sure how you're dialing in precise 1/3 stop settings.