Which lenses have apertures at f/1 or better, and work on cameras like the 6D or A7?
In Sony's case, an adapter is acceptable.
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For Canon EF mount you have pretty much only one option, the EF 50mm f/1.0L, which went out of production in 2000.
For the e-mount you could adapt a huge number of lenses, good choices would be
Someone has this Rayxar 50mm f/0.75 with a custom E-mount adapter for sale on eBay at the moment: http://www.ebay.com/itm/191743971223
Sounds like a quirky lens though. Fixed-focus and fixed-aperture, it was originally used for x-rays. Has a dreamlike effect to the photos it produces.