Canon has published a list of lenses that have limited compatibility with the 70D's new Dual Pixel CMOS AF system. Here is that list:
EF 14mm f/2.8L USM | EF 35-70mm f/3.5-4.5 |
EF 24mm f/1.4L USM | EF 35-70mm f/3.5-4.5A |
EF 100mm f/2.8 Macro | EF 35-80mm f/4-5.6 |
EF 400mm f/2.8L USM | EF 35-80mm f/4-5.6 PZ |
EF 500mm f/4.5L USM | EF 35-80mm f/4-5.6 USM |
EF 600mm f/4L USM | EF 35-80mm f/4-5.6 II |
EF 1200mm f/5.6L USM | EF 35-80mm f/4-5.6 III |
EF 16-35mm f/2.8L USM | EF 35-105mm f/3.5-4.5 |
EF 17-35mm f/2.8L USM | EF 35-105mm f/4.5-5.6 |
EF 20-35mm f/2.8L | EF 35-105mm f/4.5-5.6 USM |
EF 24-85mm f/3.5-4.5 USM | EF 35-135mm f/3.5-4.5 |
EF 28-70mm f/2.8L USM | EF 35-135mm f/4-5.6 USM |
EF 28-70mm f/3.5-4.5 | EF 70-210mm f/3.5-4.5 USM |
EF 28-70mm f/3.5-4.5 II | EF 70-210mm f/4 |
EF 28-80mm f/2.8-4L USM | EF 75-300mm f/4-5.6 |
EF 28-80mm f/3.5-5.6 | EF 80-200mm f/2.8L |
EF 28-80mm f/3.5-5.6 USM | EF 80-200mm f/4.5-5.6 |
EF 28-80mm f/3.5-5.6 II | EF 80-200mm f/4.5-5.6 USM |
EF 28-80mm f/3.5-5.6 II USM | EF 80-200mm f/4.5-5.6 II |
EF 28-80mm f/3.5-5.6 III USM | EF 90-300mm f/4.5-5.6 |
EF 28-80mm f/3.5-5.6 IV USM | EF 90-300mm f/4.5-5.6 USM |
EF 28-80mm f/3.5-5.6 V USM | EF 100-200mm f/4.5A |
EF 28-105mm f/3.5-4.5 USM | EF 100-300mm f/4.5-5.6 USM |
EF 28-105mm f/3.5-4.5 II USM | EF 100-300mm f/5.6 |
EF 28-105mm f/4-5.6 | EF 100-300mm f/5.6L |
EF 28-105mm f/4-5.6 USM |
My understanding is that the "limited compatibility" lenses will be able to autofocus in Live View using the more-conventional contrast detection method, but they will not support the new Dual Pixel AF. The reason is likely due to a lack of capability in the lens' CPU.