The EF 24-70mm f/2.8L IS lens is rumored for a long time. Previous rumors said that this lens will be announced in late 2015 to early 2016. Now according to CanonRumors, Canon is working on this lens, and the release date will be in next 18-24 months.
There are two or three prototypes exist in testing. Back in 2012, Canon didn’t release this lens because the size is too big when with image stabilization feature, so Canon released a smaller EF 24-70mm f/2.8L II instead. According to CR, the new EF 24-70mm f/2.8L IS lens will featured with new BR element as on EF 35mm f/1.4L II USM Lens (Amazon, B&H Photo Video, Adorama), and the IS being worked on for the lens will provide better performance than anything currently in the Canon lineup.
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