Durst 606 adjustable mask usage

I've been using my durst 606 for about a year or so now, with both 35mm and medium format. I find that the built in adjustable mask leaves rather large blurry borders. The condenser & mask are in great shape, near mint (I have two of them, the other one works just the same). I'm assuming that it works as intended? As in I'm just supposed to use them to cut out the area up to the sprockets or so.

They are to decrease flare. Use a baseboard easel if you want sharp borders on the image area.