http://egami.blog.so-net.ne.jp/2011-07-11
After all, the following is obtained for the Diffraction Limits, against different F-numbers (for 550nm Green light):-
F-number
[f/]Resolution [μm]
1.00.67
1.40.94
2.01.34
2.81.88
4.02.38
5.63.76
8.05.37
117.38
1610.74
2214.76
3221.47
So, just say that the Diffraction Limit at f/2.8 is in 1.88um,