Abbreviation: HSM = "Hyper-Sonic Motor" (Sigma)
The abbreviations means "Hyper-Sonic Motor"
Explanation from Sigma
This lens uses a motor driven by ultrasonic waves to provide a quiet, highspeed AF.
See examples of effected products
 | Sigma EX 70-200mm f/2,8 APO HSM DG OS |
 | Sigma 50-200mm f/4-5,6 DC OS HSM |
 | Sigma EX 120-300mm f/2,8 APO DG HSM |
See links
| sigmaphoto.com |