With the Micro 4/3 system, the sensor is half the size of 35mm film.
Any lens from a 35mm SLR camera will give the same result as a
lens of double its focal length when mounted onto the Micro 4/3 camera.

These adaptors incorporate a manual adjustment lever for the aperture.
This allows any setting between minimum and maximum aperture
but it does not allow setting to a specific f stop. 

The Nikon adaptor is also suitable for  F lenses which do have manual aperture rings.