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AF Problems when using Speedlight AF Assist Light


Oviedo, US
146 posts

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khollister Registered since 29th Jun 2004
Mon 15-Sep-08 03:16 PM

I have recently discovered some really puzzling behavior on my new D700 this weekend. I recently got a 85/1.4 AF-D and was doing some focus tests to see if I needed to set the AF fine tuning (something I have never fooled with previously on my D300). What I discovered is that if I attach a SB-800 and turn the focus assist light on (the red one on the flash - I have always turned off the white one on the body), a majority of AF shots are clearly out of focus even though the green dot comes on in the VF and the subject appears reasonably in focus on the VF screen. If I turn the focus assist lamp off (or remove the flash), the lens focuses perfectly even on subjects of marginal contrast & size. I also duplicated this with my 50/1.8 AF lens. I have no tried my AFS zooms or the D300 yet, although I think I tried the 85 with the speedlight on the D300 and got a much lower rate of focus failures. Manually focusing with a number of lenses on the D700 using the electronic rangefinder works fine. The problem is only with the assist light enabled.

I am stumped as to what is going on here since all of the lenses work great on the D700 without flash. I am going to try other lenses and the D300 this evening.

I didn't find anything directly related in searching yet - does anyone have any ideas or info on this?