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D700 AF in dim-light won't focus.

ssnkssnk ssnk , Apr 20, 2009; 01:42 a.m.

Last weekend, I was shooting an event for a friend at a nighttime party with very low light. I had D700 / 24-85 2.8-f, SB900 (mounted on the camera). I couldn't get the lenses to focus in auto and couldn't see well enough to manually focus. As far as I can tell, all equipment is functioning perfectly. People at the party had point-and-shoot cameras that were working fine.
Camera was able to focus properly, when I shot in the banquet hall, where light was ok. But as soon as I went outside to shoot cake cutting, could not capture many important moments. Normally, my camera is able to auto-focus in totally dark room with the AF assist light only. But don't know what happened that day.
My camera setting were - Single Shot, Manually selected center focus point, 50mm around f/3.8, Sb900 on camera hot shoe pointing straight on to the couple cutting cake, and then kissing, and then a group shot with friends. Nopes. Camera won't focus.
I missed a lot of important shots, which was very frustrating.
I am glad it wasn't a paid gig or a wedding.

Any help would be appreciated.


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Francisco Solares-Larrave , Apr 20, 2009; 02:05 a.m.

Were the batteries fully charged? What type of AF sensor had you selected? What ISO did you dial? What was your exposure mode? Did you have a lot of people moving? Did you check if the little lever in the front that selects the AF type was on M or on S or C?

Dave Lee , Apr 20, 2009; 02:07 a.m.

When I would shoot events in a dimly lit room I could see the grid my SB-28 would project for focus assist, your SB-900 should have done the same. I think you need to use AF-S to get that, it won't work in AF-C mode.

ssnkssnk ssnk , Apr 20, 2009; 04:14 a.m.

Hi Francisco,
Thanks for your response.
Flash batteries were not fully charged for this shoot. But flash didn't show any sign of weak batteries. The flash blast was full with no under exposures of any sort. I had selected single AF mode (manually selected center AF point), matrix exposure, ISO 1250 - 1600, people not moving, front lever at S.

Hi Dave,
Thanks for replying.
I need to look into AF-S vs AF-C issue. But I think it was more of a AF assist issue than anything else. I may be wrong thou.

I had some problem with my flash gun last week also, when I was shooting in Indonesia. It will underexpose most of my pics and then suddenly overexpose 1 and then go back to underexposure. I reset my SB900 to factory defaults, and it started behaving well after that.
I think this all started when I put my SB900 in SU-4 mode for one of my shoots. After which I changed it back to Remote, and it started underexposing. Then I reset it (in Indonesia).

I need to check, how AF assist works, when SB900 is mounted on the camera? My guess would be thru SB900. If my camera can AF in low light with on-camera AF assist (without mounting SB900), then I can assume that fault lies with SB900 and not camera.

Anyways, I called my Nikon service center, and will be taking my Camera+Lens+SB900 to their techs tomorrow to see, if they can find anything.

Has anyone experienced the same thing with their SB900 before??

Hans Janssen , Apr 20, 2009; 04:34 a.m.

I have the idea that my D700 is not so good in focusing in dimlight as my sold D300, same venue, better lighting now, but the focus won't work good. Lens 70-200 2.8 VR. Focus AFC, 1 point on the richt side(top in portrait), even with the cross hairs in the middle it is not so good as the D300 with the outside points. Next time I gonna borrow my old D300 to do some tests.

Arash Hazeghi , Apr 20, 2009; 04:38 a.m.

Sounds like a problem with IR AF illuminator beam in SB900. Was the camera hunting for focus? Did you try w/o SB900 and with camera's built-in AF illuminator? That said I can focus in very dim situations with D700 and no assist beam but I have f/2.8 AF-S lenses.

ssnkssnk ssnk , Apr 20, 2009; 05:33 a.m.

Hi Arash,
Camera was hunting for focus. And I didn't try without SB900 at that time. I am going to try with and without SB900 tonite to narrow down any problems with SB900. I never had any such issues with my old SB800 (which I recently sold and bought SB900).
As per the specs, D700 AF sensor is good for f/5.6 or faster, so I have ruled out my lens as the bottle neck, in this case.
I have had shot fast moving objects with my D700 (in bright daylight thou), and never had any issues. I would still like to believe that problem could be with SB900 and not with the AF system itself (fingers crossed).

I am taking it to Nikon service center tomorrow. Lets what happens ? :(

Elliot Bernstein , Apr 20, 2009; 06:40 a.m.

"Normally, my camera is able to auto-focus in totally dark room with the AF assist light only. But don't know what happened that day." You need to determine whether the flash's focus assist light was coming on. Was it? If it was not, it is probably a setting issue. Is it possible you were you shooting in AF-C mode? The focus assist light does not function in AF-C mode. [Not sure on the D700 as I do not have one but the focus assist light will not come on if you have spot metering selected with certain Nikon DSLR cameras. If it makes a difference withe the D700 (check your manual), which metering mode were you shooting in?]

Is the flash's AF assist light working now?

Arash Hazeghi , Apr 20, 2009; 04:19 p.m.

As per the specs, D700 AF sensor is good for f/5.6 or faster, so I have ruled out my lens as the bottle neck, in this case.

That is true AF sensors work at f/5.6 but the faster the lens the better because you get more light and also it has to do with the angle of light incident on the AF sensors in phase detect system. But I agree, most likely it is your flash. Try the camera with no flash and AF assist beam ON/OFF in low light, if the problem remains send it to Nikon for repair.

Ty Yang , Apr 21, 2009; 12:06 a.m.

I had a similar issue once during a sunset shot that quickly turned to a nighttime shot. The AF assist red grid was not turning on. My camera set up at the time was a D90 + SB600 mounted on. It was frustrating as the camera/lens was whirling trying to focus.

When I got home, after a bit of testing I realized that the AF assist grid from the SB600/D90 setup would only turn on while using select focus points (the main cross points only and not the outer corner points).

You mentioned that you were mainly using the center focus point so my post may not answer your questions... but it reminded me of that particular experience of mine.

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