COMPARED @ 40mm: Canon EF 24-70mm f/2.8L II, EF 24-70mm f/4L IS, 40/2.8 STM
Added to my review of current Canon 24-70mm L zooms is a first comparison between the 24-70mm f/2.8L II, 24-70mm f/4L IS, 40/2.8 STM.
For any doubts about the optical merits of the EF 40/2.8 STM, this comparison should put them to rest. At about $149, the Canon EF 40mm f/2.8 STM offers true value.
B&H Photo has the 24-70mm f/2.8L II and 24-70mm f/4L IS and 40/2.8 STM in stock.