AF Problems when using Speedlight AF Assist Light
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?
#2. "RE: AF Problems when using Speedlight AF Assist Light" | In response to Reply # 1KnightPhoto Nikonian since 18th Dec 2006Fri 26-Sep-08 10:32 PM
Page 382 of D700 manual 'notes on optional speedlights', there are nine AF points eligible at 85mm for the SB-800 AF-assist (again assuming that is your problem .
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#3. "RE: AF Problems when using Speedlight AF Assist Light" | In response to Reply # 2khollister Registered since 29th Jun 2004Mon 29-Sep-08 11:23 PM
I was using the center focus point, so that isn't the problem. I accidentally duplicated the problem again last night - used one of my sb800's with the focus assist enabled, front focused about 2" at a distance of about 10', 85/f1.4. Turned off the assist lamp, perfect.
#4. "RE: AF Problems when using Speedlight AF Assist Light" | In response to Reply # 3Philnblanks Nikonian since 28th Feb 2009Wed 27-Jul-11 12:11 PM
I know this is an old post, but I just encountered this identical problem with the combination of a D700 + SB-900 + 85 1.4-G. Shots taken with this set up were either mildly or even wildly out of focus. I then took the same shot three ways: 1)No flash, 2)Flash with AF Assist turned on and 3)Flash with AF Assist turned off. Both #1 and #3 were perfectly in focus (all at maximum F/1.4), but #2 using AF Assist fromt he SB-900 was OOF. Did you ever resolve this? Has anyone else ever run into it? I have only seen this on the 85/1.4, so could that be a factor? My other 2.8 zooms don't seem to do this.
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