HC 80mm becomes less sharp with apertures above 8.0... Any ideas why?

[baudolino]

Hi,

I gave my H1D-22 camera to a fellow photographer yesterday for a studio work. He almost immediately discovered a strange behaviour: when stopping down above f/8.0 (e.g. f/22) and/or making shots with exposure faster than 1/250 (e.g. 1/500, 1/800), the picture becomes visibly less sharp, even in the point of focus. I cannot understand why..

Did anyone faced similar behaviour before?

Thanks for your replies,
Roman

[KeithL]

The optimum aperture on the 80mm is likely to be around f5.6 to f8. At smaller apertures the image quality will start to suffer due to diffraction.

Having said that I can see very little if any difference between f8 and f11 when using the 80mm on my H3D11-22.