Sharpness at 40mm

For these tests, the Olympus M.ZUIKO Digital 40-150mm f/2.8 ED Pro lens was attached to an Olympus E-M5 body, which in turn was mounted on a sturdy tripod. The self-timer was activated. The 100% crops represent about 1/400th of the total frame area.

The full frame at 40mm

Wide open at f/2.8, the image centre is incredibly sharp, so much so that stopping down does not bring any extra improvement – but then again, excellence is hard to improve upon. The effect of diffraction starts to become noticeable in the f/5.6-f/8 range, but does not impact image quality significantly until you hit f16.

The borders have very decent edge sharpness even wide open, but resolution is lower than in the centre. Stopping down a notch helps a lot here, and the sweet spot is reached 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