Sharpness at 18mm

For these tests, the Olympus M.ZUIKO Digital 12-40mm f/2.8 Pro lens was attached to an Olympus OM-D E-M1 compact system camera, which in turn was mounted on a sturdy tripod. The self-timer was used to avoid any vibrations from pressing the shutter release. Slight tonal changes are due to slight changes in natural light during the session.

The full frame at 18mm

Centre sharpness is commendably high even wide open at f/2.8, with peak performance achieved at f/5.6. Diffraction kicks in at f/16 and gets worse at f/22. The corners are also sharp at f/2.8, with the best performance again at f/5.6.

Aperture	Centre Crop	edge Crop
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22