Solar spectrum & Canon 5D MkII response
The H-alpha line lies where the camera's response is falling to about half its full value, due to the infra-red blocking filter
So 1 more stop of exposure is required
Profile drawn by my own GRIP software
Astrophotographers generally believe that in order to photograph H-alpha emissions it is necessary to have a DSLR modified to remove the infrared blocking filter that is in front of the detector. I have demonstrated that the modification is not necessary, at least for Canon 5D MkIII cameras. Probably all Canon DSLRs use the same IR filter.