As suggested by 位酷哥，I got this ISO12233 QS chart from aliexpress 
and took several pictures with Sigma 50-500 mm lens at 500 mm, Nikon D300s and ISO 200.
Pictures were enlarged by 100%, chopped and not sharpped by software.
Here are the results, it seems that f/14 is the sharppest (for some reasons I missed f/13). Do you agree?
#1357, f/6.3, 1/10 s, ISO 200
#1358, f/7.1, 1/10 s, ISO 200
#1360, f/8, 1/6 s, ISO 200
#1361, f/9, 1/5 s, ISO 200
#1362, f/10, 1/4 s, ISO 200
#1363, f/11, 1/3 s, ISO 200
#1364, f/12, 1/2.5 s, ISO 200
#1365, f/14, 1/2 s, ISO 200
#1366, f/16, 1/1.6 s, ISO 200
#1367, f/18, 1/1.3 s, ISO 200
#1368, f/20, 1 s, ISO 200
#1369, f/22, 1/0.8 s, ISO 200
#1370, f/25, 1/0.6 s, ISO 200
#1371, f/29, 1/0.5 s, ISO 200
#1372, f/32, 1/0.4 s, ISO 200
#1380, f/13, 1/8 s, ISO 800
