One of the more mysterious settings in the E-620 "Custom Menus" is the one for "AF Sensitivity." There are two options for AF Sensitivity: "Normal" and "Small."
The actual AF area used by the camera is quite a bit larger than the small square you see in the viewfinder. If you want to be safe about it, you should probably assume that anything in the much larger central circle could be picked up by the center AF point. If there are contrast areas at different distances within the AF area, the Olympus focusing system tends to favor the contrast areas closer to the camera. Thus there are many types of focusing sistuations where the Normal sized AF point will give you front-focus.
So why not always use Small AF Sensitivity? That way the camera would focus on what the AF point is actually pointed at, right?
Well the problem with Small AF Sensitivity is that you have to be much more careful about pointing the AF point at an area of contrast, otherwise the camera won't focus at all, even in broad daylight. Also, the Normal sized focus area seems to have greater focusing precision over the Small sized focus area because with the larger area there is just more data for the AF system to work with.
Because of the problems introduced by using Small AF Sensitivity, you should stick with Normal. There's a reason why Normal is the default setting and Small is hidden deep in the custom menus. The camera works better with AF Sensitivity set to Normal.
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