Here is a brief video comparing the two at various focal lengths using the optical viewfinder and the normal Phase Detection Auto Focus. (not liveview and contrast detection AF)
I think the STM is slightly faster at short focal lengths and USM is slightly faster at longer focal lengths. Overall they are both very fast, and the STM is completely silent.
What do you think?
Photo sharpness is very similar but, The 55-250 STM is slightly better with less Chromatic Abbreviation(“purple fringing”) wide open. Here is a link to a forum post I made with some comparison photos between the 70-210 USM and the 55-250 STM: [ Ссылка ]
Both lenses were used on an SL1 using the normal Phase Detection AF and videoed with a 5D3 and 100mm Macro up against the SL1 viewfinder.
