A-99 and understanding the auto focus ?

Hopefully this works as you mention. But on the Nikon it is different; which is where Trevor used it. On older Sony cams, this button (AF/MF) toggled(or locked; depending on which mode you selected) the auto-focus. In the Nikon system this button focuses as does a half press of the shutter and releases when you release the button. Even in continuous mode, on the Nikon ,this button only focuses when pressed. This feature is used in Sports shooting by many Nikon and Canon shooters, as they prefer to activate focus on the intended subject when they press that button; then press the shutter. The focus tracking is only activated when that button is pressed. It can also be programmed(D-300) where it tracks with half press of the shutter button as with the Sony, also. It's just that the continuous focusing on the Nikon is activated with this action whereas the Sony is locked; so to speak. As I mentioned earlier, I don't have the A-99, so I'm not saying you are wrong! And if I didn't get it right, I apologize!

Note:  Tracking is the key word here!

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Glenn
I'm kinda partial to video, but I'm hangin!