AF-C problems solved by AF Fine Tune or am I imagining things?

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Robert Palmqvist
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AF-C problems solved by AF Fine Tune or am I imagining things?
2 months ago

I have been struggling with an AF problem for a while. I thought I should share some information if someone else is in the same position.

It all started with my first serious attempt to convert to AF-On/Back Button Focus and a 24-85mm VR I bought used (I needed a lightweight traveling lens for my D700).

This lens hunted like crazy while I kept the AF-On button pressed. I expected it to settle or give up. However focus never settled and the VF rangefinder indicators never ceased to flicker. This didn't happen all the time but frequent enough to drive me crazy. I shoot a lot of static subjects and this behavior rendered the AF-On technique more or less useless for that kind of photography. Switching to AF-S solved the problem, focus was instantaneously acquired but I really wanted to switch exclusively to Back Button Focus and not have to switch between AF-S and AF-C.

I went over the D700 in question, inspected the mirror box, made sure sensors and lens connectors was clean, made all kind of focus tests, using tripod, good lighting and optimal focus targets. Problem persisted and the really bad was how the constant micro adjustments affected focus. Continuous small small corrections, most of the time in focus, but suddenly a greater “jump” where you could actually observe moments of severe out of focus through the VF. AF-On require release priority and releasing during one of those out of focus moments generated an unacceptable rate of out of focus pictures.

I started to suspect the lens and was just about to send it to Nikon for service when I by chance observed a similar behavior from my 105mm f/2.8 Micro under certain conditions! You could suspect the body but this D700 focus perfectly with the 24-70 f/2.8 using the same Back Button Focus technique (with the very same settings). I managed to force the same behavior with the 70-200 f/2.8 VRII but only under very specific circumstances. E.g. use a light pole as focus target, lit from the side so that one side is bright and the other dark with a gradient transition in-between. Zoom until the light pole more or less fill the rectangle representing the active focus censor. Press and hold AF-On while moving the censor slightly up and down the pole. This made AF go crazy in a similar fashion even when trying it out with a D4 (but nothing else did, the D4 AF was rock solid trying the same with the 24-70 f/2.8 for example).

I started to search the Internet for references, maybe someone else experienced something similar? I didn't find much but I stumbled across this thread:

http://www.dpreview.com/forums/post/50774257

I had already tested my lenses on the D700 in question but I never managed to end up with consistent figures and I therefor decided to leave it off. The idea of using LV to acquire optimal focus and then the VF rangefinder arrows to bracket out an interval never occurred to me so I decided to give it a shot with the problematic lenses.

The 105mm ended up with an interval of 17 to -10 and the 24-85mm with an interval of 10 to -1 @ 70mm and 2 to -3 @ 24mm. What to dial in? I ended up using 4 for the 105mm and 3 for the 24-85mm. Both values quite close to zero and I therefor didn't expect any improvements enabling AF Fine Tune. However to my utter surprise, these lenses doesn't hunt like crazy anymore! I can still force AF to hesitate and flicker a bit but not at all like it was before. AF-On is now usable but I have no idea if it was the values I dialed in or just the fact that I enabled AF Fine Tune that made the difference. Everything else is however the same, no other settings changed.

I would not read too much into this but it might help someone else so I thought I better document and share my experience. I haven't had much time to verify my findings yet but I did extensive tests before and after under the same lightning conditions using multiple everyday objects as focus targets (e.g. black cross on a white wall, flowers, a computer mouse on a table, some high contrast objects at different distances outdoor) and it was like night and day. Anyone else experienced something similar?

Nikon D4 Nikon D700
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