02-12-2012, 11:21 AM
If I may revive this thread, I had a thought the other day about how a usable (distance) tracking contrast AF could work. Probably not the first ever, but anyway...
Consider single shot contrast AF, which moves quickly through the focus range, and looks for maximum contrast to decide when focus is achieved. The difficulty with a subject moving towards/away from you is the focus will keep changing so it isn't so easy to adjust focus exactly to the subject. The answer is not to try! At some point during the AF seek, it will pass through the optimal plane of focus. If you can buffer the previous images for a short time, you can recall the frame of optimal focus and save that one.
Of course, this assumes a few things that need to be in place. You would have pure electronic shutters anyway for the continuous feed. It assumes the shutter speed is fast enough so you don't get "focus blur" from the changing focus. Shouldn't be a major problem since if you're using tracking AF, the chances are whatever you're shooting is moving and you need a fast-ish shutter speed anyway. Still it might require adaptive focus seek speed for maximum quality. The biggest unknown is if you can buffer an entire sensor's output for long enough. Getting data off the sensor would be very bandwidth intensive, so logically I think it would need to be built into the sensor itself. I know some cameras already offer you the chance to go back in time, but they don't store the full image for every cycle of the AF.
Consider single shot contrast AF, which moves quickly through the focus range, and looks for maximum contrast to decide when focus is achieved. The difficulty with a subject moving towards/away from you is the focus will keep changing so it isn't so easy to adjust focus exactly to the subject. The answer is not to try! At some point during the AF seek, it will pass through the optimal plane of focus. If you can buffer the previous images for a short time, you can recall the frame of optimal focus and save that one.
Of course, this assumes a few things that need to be in place. You would have pure electronic shutters anyway for the continuous feed. It assumes the shutter speed is fast enough so you don't get "focus blur" from the changing focus. Shouldn't be a major problem since if you're using tracking AF, the chances are whatever you're shooting is moving and you need a fast-ish shutter speed anyway. Still it might require adaptive focus seek speed for maximum quality. The biggest unknown is if you can buffer an entire sensor's output for long enough. Getting data off the sensor would be very bandwidth intensive, so logically I think it would need to be built into the sensor itself. I know some cameras already offer you the chance to go back in time, but they don't store the full image for every cycle of the AF.
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