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Speedlight causing AF issue

(13 posts) (5 voices)

CyberSammy

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Hello everybody.

I noticed my shots are backfocused when I use SB-700 or SB-900 flash in dim light where AF assist lights UP. I'm using Speedlights mounted direct on camera hot shoe.

I have also discovered my shots are focused spot on when I turn off the AF assist light in flash menu. I have D700 with MB-D10 and Nikkor 24-120/4 VR, Nikkor 50G/1.4 AF-S, Sigma 85/1.4 HSM, Sigma 50/1.4 HSM lenses. This issue is present to some extent on every lens.

Lately I have found it's more visible on the non-cross type AF points, but on the cross type ones it's OK with SB-900. With SB-700 is backfocus present on every AF point, but on the cross type ones to much lesser amount. Camera is still able to achieve focus when I turn AF assist light off in flash menu, but it's slower so I would be glad if I can use this feature.

Is anyone experiencing same problem with Speedlight AF assist light and is there any solution to this? I was told in service they never heard about such problem :-(

Thanks for answers :-)

Posted 1 year ago #
Drab

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Do you have, perhaps, comparison photos of the same subject (ideally shot back-to-back) of AF assist on vs AF assist off? I'm finding it hard to imagine a possible cause for such behavior, much less a cause which persists across multiple lenses and (more importantly) multiple AF assist projectors.

Would be quite curious to see if photos reveal any clues.

Posted 1 year ago #
CyberSammy

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You can look at images here:

1) without AF assist centre point:

2) with AF assist centre point:

3) without AF assist outer point:

4) with AF assist outer point:

5) AF target without AF assist centre point:

6) AF target with AF assist centre point:

7) Another AF target without AF assist centre point:

8) Another AF target with AF assist centre point:

I can post bigger if you need, but on these issue is evident.

Posted 1 year ago #
Drab

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Nice set, appreciate it. Nicely organized too, makes it easy.

Unless I'm nuts I'm seeing exposure variation between the some photos which is bugging me but I don't think is causal.

The one thing all those photos appear to have in common is you are focusing on a planar surface angled away from you. Does the same thing happen when shooting a flat surface which is perpendicular to your line of sight?

Not sure if it makes a difference, I'm still thinking, don't have any good ideas ATM.

Posted 1 year ago #
CyberSammy

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I will try, but I think it will be the same. Without AF assist OK, with AF assist back focus. I think exposure varation is because the shifted AF. I'm deciding to send my gear in Nikon service. May be something is out of alignment. I started same thread in our local country photo page and it seems more people have this problem. I received messages from two photographers one with D90 and second with D7000 posting pictures with same AF shift with or without AF assist enabled and on Dpreview I've seen someone posting about this issue with his D3.

Posted 1 year ago #
sevencrossing

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Sorry Cyber I am not clear by your reference to "center point"

The SB-900’s AF-Assist Illuminator sometimes called the WIDE ANGEL AF Assist Illuminator-supports the dynamic-area AF system.

So if your are only using a single point, it might not work

The camera’s focus mode should be set to S (Single Servo AF with focus priority), AF-A, or AF.

Posted 1 year ago #
TaoTeJared

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I see the difference but I do not see it as an issue but just fairly normal and nothing wrong with the camera. You are shooing wide open so the DOF is shallow and to move/miss by 1/4" (6mm) is really easy to even on a tripod. You are not shooing perpendicular to the subject (need to for testing) so for focus to move 1/4" as the plane of focus is at an angle, is really normal. You could get your lenses re-aligned and checked. Sigma is notorious to be off and at f/1.4 that can be a huge problem.

What results do you specifically believe should happen? I don't think we are clear what your expectation is.

Posted 1 year ago #
CyberSammy

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I really don't get why there have to be difference between focus without AF assist and with AF assist. This in real shooting means to have eye in focus or not in focus at all. It's same with Nikkor's and Sigma's and above photos are taken with Nikkor. I can also see difference at f/4 shot too. For example with 24-120/4 VR at 120mm f/4 without AF assist is focus where I want and with AF assist is focused behind. It's clearly backfocusing. With this behaviour is AF assist on flash for me not useful. Without AF assist are 100% of my shots spot on and with AF assist are 100% of my shots backfocused.

Posted 1 year ago #
TaoTeJared

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I guess my point was that it easily could be coincidence as missing focus is easy. I have not seen or even heard miss focus issues with the AF assist before. You are using what sevencrossing said above and not the dynamic AF with AFC?

Hearing that it is happening more on the SB-700 than the SB-900 is interesting. The SB700 is more marketed to DX users. I'm sure there is some sort of compensation built in for DX vs FX that the flash is auto configuring - maybe it is off in some calculation that is pushing the focus back since you are shooting FX? A decimal rounding in the wrong spot could make that difference you are seeing.

I would just call Nikon directly on this one.

Posted 1 year ago #
Drab

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If he was on a tripod (and a quick overlay in PS says he was, or damn good handholding) then "coincidence" should split 50/50, not 100/0 as in the presented dataset.

Again, with the presented data the only common element is the body. We have reports of it happening on different lenses, different lens manufacturers, and different flashes.

Lex parsimoniae says it's a characteristic of the body. Note I didn't say "flaw" or "error".

While I'm curious about the result when shooting perp. to your line of sight (as I mentioned before) I disagree that it is "necessary". The reason it is standard in optical testing is because it makes it easy to quantify error. This is a qualitative discussion and knowing if the error is 1 mm or 1 m doesn't much matter.

I'm still w/o a theory I can't dismiss easily, so this last bit is grasping at straws:
In all your presented focus tests there are multiple high contrast lines for the camera to pick from. How about a test with only 1? A completely artificial test, planar target, single thin black line against white, assist vs none.

Posted 1 year ago #
TaoTeJared

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Drab said:
Lex parsimoniae says it's a characteristic of the body. Note I didn't say "flaw" or "error".

I'm still w/o a theory I can't dismiss easily, so this last bit is grasping at straws:
In all your presented focus tests there are multiple high contrast lines for the camera to pick from. How about a test with only 1? A completely artificial test, planar target, single thin black line against white, assist vs none.

Instead of the camera (since it get's it right) I'm thinking it is the flash in the communication somehow. I'm not sure how the flash-body-lens-af point communication path goes.

I like Drab's idea. I also think trying all the AF option combinations with the AF could be the next testing if you wanted.

Either way it seems you have tested it thoroughly enough and a call to Nikon is warranted as we could test all day and not be able to create solution but just show more examples on what doesn't work.

Posted 1 year ago #
momentcapture

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I have the exact same issue with my D700 and SB-700.
I have pinged an email off to Nikon, will update with their response.

Also if anyone finds a solution that still allows you to use AF assist I would be very grateful.

Posted 1 year ago #
CyberSammy

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Joined: Aug '11
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momentcapture said:
I have the exact same issue with my D700 and SB-700.
I have pinged an email off to Nikon, will update with their response.

Also if anyone finds a solution that still allows you to use AF assist I would be very grateful.

I know at least 4 other people experiencing the same problem with their D700. Did you get any reply from Nikon? Have you fixed it somehow or just turned AF assit beam to off?
Thanks for answer :)

Posted 1 year ago #