I have a Mamiya 65mm F6.3 lens that came with the Mamiya Universal I recently purchased. The aperture settings go from F6.3 to F22. The strange thing about this is that at maximum aperture F6.3 the aperture blades seem to be partly open (maybe about 60 to 70% of the lens area). The aperture control is very smooth and there is no apparent oil on the blades. I am wondering if this is normal, or if it's a problem. I know that there are some lenses with aperture blades that are designed not to fully open. Is this the case with this lens?