autofocus screw drive question

Started May 28, 2012 | Discussions thread
tarnish
Senior MemberPosts: 1,470
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It's definitely not JUST the motor that's different
In reply to Burnie, May 31, 2012

When I bought a 16-35 zoom from the local camera store I didn't have my D800 with me. I asked to do a basic check of lens functions at the sales counter using whatever body they had handy. It was a D7000. The lens focused surprisingly slow. When I got the lens home I found that it focuses nice and quickly on the D800.

So let's sort through the details:

1. The lens has a built in AF motor so the body's motor is not a factor.

2.The D7000 and D800 use the same model of battery so there are no upstream differences in the electric current capabilities. (There could be differences in the camera's power drive circuits though).

My conclusion is that the AF control systems account for a huge amount of the focus speed differences, it may even be the dominant factor. The possibilities for the root causes include the data acquisition rates (you don't move the lens until you know how far and which direction to go) and the rates of the motor step pulses going out to the motor, wherever it may happen to reside.

It's also possible that there is a lowest common denominator effect going on: if the built in motor is less powerful in lower cost bodies, maybe all of the AF control functions are slowed down for their sake, and AF-S lenses are merely innocent victims of the derating.

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