Depends on the type of birds you are shooting. For smaller birds such as common kingfisher, bee eaters, it is hard to acquire initial focus lock and by the time it does, you prolly have missed the action. For larger birds such as Grey Herons, WBSE I have no issues in acquiring focus. It could be error on my part as I haven't been practicing BIF shots. I also do not use a red dot sight for BIF.