« on: November 08, 2012, 11:26:45 PM »
Although the 7D AF is rated EV -0.5–18, my center point AF 'One Shot' (this setting gives the best possible AF performance) test indicate it is already struggling at EV3.
If the spec is good, the AF should work well at EV3, as it is still very much within the working range.
Do you know why is that so? Could it be my 7D's AF sensor assembly is defective?
First, the spec is for an f/2.8 lens. So if your lens is slower, that affects EV (which is a 'stops' scale like aperture, so with an f/5.6 lens it should work in 1.5 EV.
But the other thing is that to achieve focus, you need both light and contrast. I have no idea how Canon sets the spec, but I suspect (rather cynically, I know) that they aren't using a real world scene with -0.5 EV lighting, but rather a target with maximum contrast (black/white squares, a starburst, etc.). If that's the case, performance in the 'real world' would not be up to the spec.