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"EV" means exposure value, and is an indication of the amount of light. The system was developed by a German photographer and shutter maker named Friedrich Deckel in the 1950s. At any given exposure value there are a number of ISO, aperture, and shutter speed combinations, known as equivalents, that will give the same exposure.
EV -3 is pretty dim, and many cameras' autofocus systems will not be able to lock onto a subject with so little light; so the 6D's AF capability in low light is pretty astounding. It would be great for wedding and event photographers, and anyone doing a lot of existing light photography.
For an idea of how dim EV -3 is, if you were using an f/1.4 lens at ISO 100 you'd need a shutter speed of 15 seconds. It's about as much light as a moonlit night.
The regular AF sensors work by locking onto areas with distinct patterns and comparing the phase shift as they rack focus. (Live View Focus uses a different method, Contrast Detection.) "Cross-type" sensors can work with either vertical or horizontal lines on subjects, making them more accurate with a variety of subjects.
Thanks for the clear explanation!
For comp arising, do u also know the EV's of other camera's autofocus? For example the 5D3 and 30D?
5D IIIs AF went down to -2 EV (like 1D Xs
5D IIs AF offers - 0.5 EV, EOS 30, 40, 50, 60D the same