http://www.photodo.com/topic_138.html
It argues that:
To conclude, with a wide aperture and short depth of field the sharpness is better with a larger format. When the lens is stopped down to get a deeper depth of field the difference between the different formats decreases. Above about f5.6 for 35 mm (f11 for 6x6 cm, and f22 for 9x12 cm) the difference in focus between the formats is insignificant. But this is only before we consider the film's effect on sharpness.
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