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I know that it stands for Exposure Value, so my 6D focuses in 2.5EV worse light than a 600D.

But it's relative to... what?

What makes it meaningful? What's the "base unit" for AF sensitivity?

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Exposure Value is actually an absolute scale, where EV 0 corresponds to an nominally-exposed photo having an exposure time of 1 second at aperture f/1.0 (or of course equivalents, like 2 seconds at f/1.4).

You can see by that baseline that -3EV is pretty amazing — it corresponds to a minute-long exposure at f/2.8.

We're used to seeing EV in the context of EV compensation, where the camera is told to expose differently from what its meter tells it to, and that's expressed in relative terms (plus or minus some number EV), but the basic scale is actually fixed. More on this a What is the EV scale? and What is exposure compensation?

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    \$\begingroup\$ I'm reading this to mean that the camera has a "standard exposure value" by which it calculates normal exposure. And, that the two cameras can autofocus in situations with less light by 0.5 and 3 EV (respectively) to that nominal exposure. \$\endgroup\$
    – David M
    Jan 1, 2014 at 22:01
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    \$\begingroup\$ @David That is true, and the nominal exposure is as I said: one second at f/1.0 (ISO 100) \$\endgroup\$
    – mattdm
    Jan 1, 2014 at 22:17
  • \$\begingroup\$ Got it, now! Thanks! Good post! One more question, I would assume that every camera has a different ability to autofocus at even the nominal especially determined by lens type. Can this range be used to quantify the camera's abilities within it's nominal range, or is it merely a function of sensor sensitivity. (Not sure if that question made sense . . .) \$\endgroup\$
    – David M
    Jan 1, 2014 at 22:19
  • \$\begingroup\$ Thanks. I did notice that the 6D is brilliant to focus even in rubbish indoor lighting. Coupled with a bounce flash delivers excellent results in worst conditions! \$\endgroup\$
    – Marcin B
    Jan 2, 2014 at 14:38

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