Strange back focus when using AF assist and outer AF points.

insanefred · December 30, 2013

I am using a Nikon D700 with SB-900 and a newly acquired Nikon 28mm 1.8G lens (my awesome girlfriend got me for x-mas :clap)

When shooting normally, the lens was always on focus and fast, but I noticed something weird. When using my sb-900 on AF-S with single servo selected and using a outer focus points. I was getting severe back focus, I tried repeating with with my other lenses, but they all seem to focus just fine.
I noticed how much more accurate continuous servo was with all AF points. So, naturally I thought, it was me or the subject that was moving and just lost focus. Then I tried using a tripod and focusing on a static subject with plenty of detail, the result were consistent. Single servo with af -assist and outer points where just plain out of focus. Continuous servo, all pretty good focus. As soon as I turned off the AF-assist on my sb-900, I got perfectly accurate results. Tried switching it back on... went right back to out of focus (back focusing).

Weird, huh?

ziggy53 · December 30, 2013

insanefred wrote: »

... When using my sb-900 on AF-S with single servo selected and using a outer focus points. I was getting severe back focus, I tried repeating with with my other lenses, but they all seem to focus just fine.
I noticed how much more accurate continuous servo was with all AF points. So, naturally I thought, it was me or the subject that was moving and just lost focus. Then I tried using a tripod and focusing on a static subject with plenty of detail, the result were consistent. Single servo with af -assist and outer points where just plain out of focus. Continuous servo, all pretty good focus. As soon as I turned off the AF-assist on my sb-900, I got perfectly accurate results. Tried switching it back on... went right back to out of focus (back focusing).

Weird, huh?

Yes, that is absolutely weird and contrary behavior regarding static subject matter and static (tripod-mounted) camera/lens.

Matthew Saville · December 30, 2013

It depends on which focus points you're using, some AF points will not trigger the red "IR" beam on the SB flash that assists with focus, and others will. There is also the grid pattern that the SB flashes make using the red light, and only a vertical or horizontally oriented light pattern may display. So there are two things that could be going wrong, or three I suppose if you count mis-calibration of the camera itself. Four possible issues if you accuse the lens of simply having field curvature, but that seems to not be the case.

Either way, I'd send it in if you think it is a fix-able problem. Everything needs to be calibrated every now and then, that's just the way it is. Or you can just be careful which focus points you use. Out of curiosity, which exact AF points are you using?

=Matt=

insanefred · December 30, 2013

Matthew Saville wrote: »

It depends on which focus points you're using, some AF points will not trigger the red "IR" beam on the SB flash that assists with focus, and others will. There is also the grid pattern that the SB flashes make using the red light, and only a vertical or horizontally oriented light pattern may display. So there are two things that could be going wrong, or three I suppose if you count mis-calibration of the camera itself. Four possible issues if you accuse the lens of simply having field curvature, but that seems to not be the case.

Either way, I'd send it in if you think it is a fix-able problem. Everything needs to be calibrated every now and then, that's just the way it is. Or you can just be careful which focus points you use. Out of curiosity, which exact AF points are you using?

=Matt=

At 28mm, the sb-900 will illuminate AF-assist for all focus points. The back focus only happens when I am not using the center af points (where the sb-900 is uses it's center AF-assist lamp). So, any of the side or outer focus points. One of my thoughts was the focus point orientation didn't match up, so I rotated the camera 45 degrees, with the same results.
I am confidant that I have isolated the issue that it is 90% of the af-assist beams fault and 10% of the cameras. The af-assist lamps do not even look like that are reaching where I focused and illuminating too far above my target.
I will try to get some photos up if you are interested.
FWIW, the lens focuses flawlessly otherwise and if this is the only issue, I am okay with that.

Edit: Another weird thing, which wasn't expected, it focuses much better in low light than my 50mm 1.8G.

Matthew Saville · December 31, 2013

insanefred wrote: »

At 28mm, the sb-900 will illuminate AF-assist for all focus points. The back focus only happens when I am not using the center af points (where the sb-900 is uses it's center AF-assist lamp). So, any of the side or outer focus points. One of my thoughts was the focus point orientation didn't match up, so I rotated the camera 45 degrees, with the same results.
I am confidant that I have isolated the issue that it is 90% of the af-assist beams fault and 10% of the cameras. The af-assist lamps do not even look like that are reaching where I focused and illuminating too far above my target.
I will try to get some photos up if you are interested.
FWIW, the lens focuses flawlessly otherwise and if this is the only issue, I am okay with that.

Edit: Another weird thing, which wasn't expected, it focuses much better in low light than my 50mm 1.8G.

Hmm, I have actually had great results using most all Nikon SB flashes and the centermost vertical column of AF points, it is the left and right edges where I have the most problems which is where I thought you were saying you had issues too.

Honestly I would just pop the camera into AF-C most of the time in active low-light situations, and see how your overall accuracy is there. The 28 1.8 is a tiny bit jittery compared to say a 24-70 AFS-G, but it's not too bad.

So aside from getting the flash serviced (and sending the lens and body in with it so they can replicate the problem) ....the other thing I might suggest is trying out an SB700 instead

. You will only get certain AF illumination lamps with vertical or horizontal AF points, however I have found that the SB700 is incredibly light and small for long days such as with weddings and the power is almost the same as an SB900. I got an SB700 as a backup to my SB800 / 900, and I wound up using the SB700 100% of the time for the past two years! The only bummer is that I don't get AF assist with the top/bottom corner AF points; only with the points that are in the central columns / rows. But honestly who uses their top-right AF point in AF-S in terrible light anyways?

=Matt=

insanefred · January 2, 2014

I opened a support ticket with some sample photos with Nikon. Let's see what they have to say.
Kinda sad that this lens is having problems so soon. In reasonable light, it is fast and reasonably consistent (about the same as my 50mm 1.8G). In low light, when focusing at anything around 3- 4 feet, it just front focuses for some reason. And of course, as I mentioned earlier, it won't work with af-assist and a outer focus point.
Perhaps, I really am asking too much, I am hoping it is just me.

I never owned a owned fast aperture wide lens. Closest thing I have used is the Nikon 35mm 1.4G for a wedding; but sadly, I didn't get to use it much at all.

Update: After opening a support ticket along with some sample photos for Nikon to look at. They said my lens is in fact defective, and I should only need to send in the lens for repair. I guess, that is 2-3 weeks of downtime foe me. :cry

Matthew Saville · January 6, 2014

Hey, BTW, I stand corrected! The AF assist beam on both the SB910 and the SB700 will work with EVERY focus point on both the D800 and D700. However, it simply uses different lamps within the red beam thingie. So, the edge points may have an issue if the red beam isn't shining right.

So either way, again I'm surprised that they're having you send in just the lens and not the flash as well. I strongly suspect that it is the combination of them both, and/or the body as well, that is causing such a weird issue.

Out of curiosity, can you tell if it is more with the lower points versus the upper points, when you're at the left and right edges? How about near or far distances? Because it looks to me like the beam pattern is weakest when it is trying to shine on the lower array of AF points, at closer distances...

(I just performed a test with both cameras and both flashes I mentioned above...)

=Matt=

insanefred · January 6, 2014

Matthew Saville wrote: »

Hey, BTW, I stand corrected! The AF assist beam on both the SB910 and the SB700 will work with EVERY focus point on both the D800 and D700. However, it simply uses different lamps within the red beam thingie. So, the edge points may have an issue if the red beam isn't shining right.

So either way, again I'm surprised that they're having you send in just the lens and not the flash as well. I strongly suspect that it is the combination of them both, and/or the body as well, that is causing such a weird issue.

Out of curiosity, can you tell if it is more with the lower points versus the upper points, when you're at the left and right edges? How about near or far distances? Because it looks to me like the beam pattern is weakest when it is trying to shine on the lower array of AF points, at closer distances...

(I just performed a test with both cameras and both flashes I mentioned above...)

=Matt=

I already sent in my lens, so I cannot test much more other than my other lenses.

I too suspected it was my camera + sb-900 (using the side lamps) + lens. The issue wasn't nearly has bad with my other lenses, including my 50mm 1.8G. However, after some extensive testing with my other lenses 35 f/2 and the 50mm 1.8g, I still see this issue, just not as bad... This leads me to believe that the 28mm 1.8G just some how exaggerated this problem that I didn't notice before or something just started malfunctioning.
No, I didn't notice any difference when using the upper AF points versus the lower. It is almost like the camera completely ignores trying to auto focus properly.
Now, I still don't feel bad sending my 28mm back to Nikon. In low light it did noticeably front focus more and more often than my 50mm 1.8g and 35 2D. Even though it is a wider angle lens and should get more DoF to help cover this up. It was was noticeable and just didn't get it right.

ziggy53 · January 7, 2014

insanefred wrote: »

... severe back focus ... (back focusing). ...

insanefred wrote: »

... The back focus only happens ...

insanefred wrote: »

... it just front focuses for some reason. ...

insanefred wrote: »

... In low light it did noticeably front focus more ...

I'm confused.

Is this a back focus problem or a front focus problem? Any image examples with full EXIF?

Are you using "Focus Priority" or "Release Priority"?

insanefred · January 7, 2014

ziggy53 wrote: »

I'm confused.

Is this a back focus problem or a front focus problem? Any image examples with full EXIF?

Are you using "Focus Priority" or "Release Priority"?

It is both.
It back focuses (focuses behind the subject) pretty badly when I use outer af points that use af-assist lamp on my sb-900 (I think when it uses the side lamps).
It front focuses when I have it turned off and shooting low light. I have tried several different focusing modes, all are pretty consistent front focus.
Outside during overcast and pretty good light, it focuses fine.

Ziggy, it is even confusing to me. I never had this issue ever before, or even heard about it.

Strange back focus when using AF assist and outer AF points.

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