Depth of field is very critical with these longer lenses. The 100 f2.8 is equivelent to a 200 lens in 35mm terms. Try manually focusing a lens like that at f2.8. Very difficult and camera shake is also a real concern. Notice the image above, taken a f5.6. Hasn't been cropped at all. I really would have liked to have stopped down to something like f11-f13. When you do this on a m4/3 camera you are well withis the diffraction area and softness will creep into your images. Light gathering is also a concern at those apertures and a tripod is a must. F11 is 4 stops down from f2.8 and halving the light at every stop gets your shutter speed moving into the seconds. No chance of hand-holding.

With the m4/3 system, f8 should be your smallest stop. Normally lenses sweetspots tend to be at f4 -f5.6. After that diffraction sets in.

Notice how much depth-of-field I have in the image above, that is not a lot.