Canon 7D: Problem Focusing With Spot AF

I use the Single Point Focus for around 60% of the photos. I use the Zone Focus for around 30% of the photos. The rest of my photos are made with the other modes: expansion or spot. I don’t think that I’ve ever used the 19 points AF.

The other day, I was taking photos of a very “deep” beach. I wanted to get the focus right, time to switch to Spot-AF. I won’t get it anymore precise. But Nooo, it doesn’t focus, it hunts. I can’t get the “darn” AF to work properly. What’s wrong? I recently did the micro-adjustment, what gives?

• First, I ranted and raved.
• Second, I fired a couple of brain cells at the same time and found the source of my problem.

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1. The spot AF only works on the center focus point.
2. The Auto Focus system works with contrast. No auto focus in the world can focus on fog, on a clear blue sky or on a smooth white wall.
3. It’s much more difficult to find a contrasty spot for the spot af to focus on.

The point, I was focusing on, was flat and grey. No contrast! I moved to a more contrasty spot and the auto focus worked properly. The problem is that it wasn’t where I wanted. I had to switch back to the single point AF.