[mst]

Yep, that's normal. The aperture is controlled mechanically instead of electronically (except on the PC-E lenses), so unlike to what you're used to from Canon with their electronically controlled apertures, there is some variation in the movement of the aperture blades between two shots because of the mechanical link. You can actually move the aperture lever manually when the lens is not mounted, it's a pin at the rear end of the lens.



If you do series of multiple exposures with the same aperture setting, you'll most likely notice a slight variation of exposure betwen the shots. The smaller the aperture, the more a slight variation in blade movement becomes noticeable. Up to 1/3 of a stop is pretty normal and not something I would worry about.



-- Markus