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Archive 2010 · Flash causing autofocus issue?

Jawnathin
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p.1 #1 · Flash causing autofocus issue?

Hey there,

I recently picked up a Sigma 24mm f/1.8 a few weeks ago as a reasonably priced wide angle fast prime. Main purpose is a reasonably sized/weighted lens for low-light indoor party/event photos. I'd prefer the Nikkor 24/1.4, but at 5x the price, it was hard for me to justify. I was worried about focusing problems, but throughout my usage of the lens, it seemed to hit AF without problem, and for the most part, the lens has met my expectations for intended use.

Recently, I did some testing with my SB900 and this lens, and I noticed an issue. This combo seems to miss focus a distances beyond 10 feet or so with the external flash, but seems ok without the flash firing.

Nikon D700
SB-900 Speedlight
Sigma 24mm f/1.8

Here are a couple of test shots... hardly scientific, but what I see 'in the real world'. The pics are 100% crops near the center portion of the frame.

The setup is on a tripod with the same autofocus point (center). The DVD case is about 15 feet away from the camera. The D3 bandana was about 10 feet away from the camera.

For each shot, I reset autofocus to infinity then back to the point.

24mm, f1.8, 1/200, ISO1600, no flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1882/web.jpg

24mm, f1.8, 1/60, ISO200, with flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1881/web.jpg

24mm, f1.8, 1/200, ISO1100, no flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1892/web.jpg

24mm, f1.8, 1/60, ISO200, with flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1891/web.jpg

Then for kicks, I did this with my 85mm f/1.4D. Obviously sharper since this isn't nearly as much of a crop and the distance to the subject was much more suited for a 85mm lens than 24mm lens.

This was meant to show it isn't an issue with the body/flash, but something with the other combination.

85mm, f1.8, 1/200, ISO3600, no flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1888/web.jpg

85mm, f1.8, 1/60, ISO 200, with flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1889/web.jpg

85mm, f1.8, 1/200, ISO1250, no flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1893/web.jpg

85mm, f1.8, 1/60, ISO 200, with flash


	http://gallery.me.com/jonathan.stroud/100739/DSC_1894/web.jpg

Normally, I'd say the lens/camera missed autofocus on the shots with the flash. But this is fairly reproducible using the flash and not using the flash. Normally I'd also say its a problem with the lens, but it hits AF points just fine without the flash. So I'm a little confused.

Any ideas? Anyone else seen this before?

Oct 11, 2010 at 01:19 AM

Jammy Straub
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p.1 #2 · Flash causing autofocus issue?

It could be the lens distance information is interacting weirdly with the flash. Does it exhibit the same behavior in brighter light, or only at this low EV?

Oct 11, 2010 at 01:39 AM

Bruce Sawle
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p.1 #3 · Flash causing autofocus issue?

Is the af assist on your flash on? If so this could be the culprit.

Oct 11, 2010 at 01:53 AM

Jawnathin
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p.1 #4 · Flash causing autofocus issue?

Excellent question. I never tried using the flash in brighter light, just for indoor stuff. I'll see if I can do some tests tomorrow when its bright outside.

That said, I have used the lens without the flash in brighter light (outdoors), and it seemed to be ok, plenty sharp. No complaints there...

Oct 11, 2010 at 01:56 AM

Jawnathin
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p.1 #5 · Flash causing autofocus issue?

Yes, the AF Assist is enabled. Without the flash, it uses the built in AF Lamp, with the flash, it uses the red lights/matrix from the flash.

I will disable the illuminator and try again.

Oct 11, 2010 at 02:01 AM

Jawnathin
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p.1 #6 · Flash causing autofocus issue?

Hmmmm, very interesting, I think you figured it out Bruce.

If I disable the autofocus illuminator on the flash, I notice the autofocus point was hit. Once I turn it back on, I am able to reproduce the problem where the autofocus misses.

Using the built in illuminator on the body, theres no problem.

Any explanation for this? Any suggestions to get it fixed? Why does it only happen on the 24mm and not the 85mm?

Oct 11, 2010 at 02:37 AM

Kittyk
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p.1 #7 · Flash causing autofocus issue?

interesting, i would love to see how nikon 24 would react to this. if we should add it to (list of) sigma issues or is more body issue.
The fact that 85 have less problems can be many, for one, it sees the frame much bigger and less distorted.

Oct 11, 2010 at 04:22 AM

Ivo Heshusius
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p.1 #8 · Flash causing autofocus issue?

Kittyk wrote:
if we should add it to (list of) sigma issues or is more body issue.

IMHO this is likely not a body issue but a sigma issue: The D700 is optimized for use with original Nikkor lenses and flashes (and reverse). Any other non-Nikkor lens uses other, non-identical and sub-optimal electronics with their own algorithms.

I suggest you do a test with another lens (in a shop) and see what the result is like. If the problem persists Sigma has to update the firmware of the lens in order to get of this flaw.

Oct 11, 2010 at 04:31 AM

Jammy Straub
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p.1 #9 · Flash causing autofocus issue?

Jawnathin wrote:
Any explanation for this? Any suggestions to get it fixed?

Gremlins dude, gremlins.

As Ivo says, there probably is no fix.

Oct 11, 2010 at 04:45 AM

Kittyk
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p.1 #10 · Flash causing autofocus issue?

Ivo Heshusius wrote:
IMHO this is likely not a body issue but a sigma issue: The D700 is optimized for use with original Nikkor lenses and flashes (and reverse). Any other non-Nikkor lens uses other, non-identical and sub-optimal electronics with their own algorithms.

I suggest you do a test with another lens (in a shop) and see what the result is like. If the problem persists Sigma has to update the firmware of the lens in order to get of this flaw.

afaik there is no firmware in the lens. It is barely a electronic meter and chip ID. There is nothing to fix.
I would not be surprised it is sigma issue, their inconsistent metering and focusing is reason why i had to return all i ever tried.
Coincidentally i tested this sigma some weeks ago, but didnt seen the point. It is big expensive lens which needs to be stopped down seriously to get some useful results. Afaik 24 f2.8 would be better investment.

Oct 11, 2010 at 05:57 AM

Ivo Heshusius
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p.1 #11 · Flash causing autofocus issue?

Kittyk wrote:
afaik there is no firmware in the lens. It is barely a electronic meter and chip ID. There is nothing to fix.
I would not be surprised it is sigma issue, their inconsistent metering and focusing is reason why i had to return all i ever tried.
Coincidentally i tested this sigma some weeks ago, but didnt seen the point. It is big expensive lens which needs to be stopped down seriously to get some useful results. Afaik 24 f2.8 would be better investment.

All lenses contain chips and small circuitboards - these electronics contain data and instructions...

Oct 11, 2010 at 06:15 AM

Jawnathin
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p.1 #12 · Flash causing autofocus issue?

This isn't a huge deal. I've found that the built in AF illuminators annoy people, so I don't use it often, I usually have the one on the body disabled anyway. Though, if possible, it'd be great to understand this a bit more and if theres a way to resolve this, get it done. I'm not making any excuses for the lens, it shouldn't behave this way, its just not a huge deal, or a big enough of an issue for me to send back. The lens seems to autofocus just fine without the flash's illuminator, so if thats the only issue, I can deal with it.

I guess the thing thats most interesting is that doesn't the lens & body combination determine the focus point based on light and contrast of the AF point in question?

If so, why would it be any different because a flash is using its illuminated LEDs instead of the built in camera one? The lens/camera is still using available light...

Oct 11, 2010 at 06:48 AM

Kittyk
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p.1 #13 · Flash causing autofocus issue?

Ivo Heshusius wrote:
All lenses contain chips and small circuitboards - these electronics contain data and instructions...

yes but it is not doing anything. It is just info for camera saying aperture, distance metering and type of lens. Lens is passive device. You can hack it some ways (distort the distance it reports for example) but there is no firmware to update which would fix how lens behaves (dynamicaly) given fairly simple and consistent commands from (various) camera(s).

Oct 11, 2010 at 07:29 AM

Jawnathin
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p.1 #14 · Flash causing autofocus issue?

Regarding the performance of the lens, I actually like it quite a bit. Sharpness seems just fine even at wider apertures, love the psuedo-macro ability, and the ability to go to f1.8 is very helpful in the low light situations I find myself using it in. The lens has exceeded my expectations and since I got it and I have used it quite a bit.

I considered the 24/f2.8, but I determined it was too slow for what I needed, and based on what I read, seemed to have its own set of kinks. The price difference was about $70 ($450 vs $380). The Sigma isn't heavy, but it isn't light either. The lighter weight of the Nikkor would be nice.

The Sigma is not perfect, it has its flaws, but its performance and usefulness for the price makes its a solid value. With its extra speed, I think I'm able to capture shots with the lens, that I might not have been able to with a slower one.

Edited on Oct 11, 2010 at 02:24 PM · View previous versions

Oct 11, 2010 at 07:34 AM

theSuede
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p.1 #15 · Flash causing autofocus issue?

Kittyk wrote:
yes but it is not doing anything. It is just info for camera saying aperture, distance metering and type of lens. Lens is passive device. You can hack it some ways (distort the distance it reports for example) but there is no firmware to update which would fix how lens behaves (dynamicaly) given fairly simple and consistent commands from (various) camera(s).

Not exactly. The lens also contains a grid database over "corrections" that are to be applied to the AF-module measurement before the focusing action is calculated. If it's a zoom lens you even get multiple look-up-charts for different zoom positions. It also gives information about AF-actuation mechanical "play" for different FL-positions and focusing distance positions to predict errors from over- or under-carry of the actuation movement...

The lens is in a very clear way an "active" part of the AF system.

You CAN actually program a lens to give inaccurate focus - front-focus at minimum zoom, back-focus at maximum zoom. This at close to MFD. If you WANT to, you can program it to be the other way around at infinity focus.

Oct 11, 2010 at 12:39 PM

Kittyk
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p.1 #16 · Flash causing autofocus issue?

theSuede wrote:
Not exactly. The lens also contains a grid database over "corrections" that are to be applied to the AF-module measurement before the focusing action is calculated. If it's a zoom lens you even get multiple look-up-charts for different zoom positions. It also gives information about AF-actuation mechanical "play" for different FL-positions and focusing distance positions to predict errors from over- or under-carry of the actuation movement...

The lens is in a very clear way an "active" part of the AF system.

You CAN actually program a lens to give inaccurate focus - front-focus at minimum zoom, back-focus at maximum zoom. This at...Show more →

interesting, is there anywhere available more info about it? Some opensource directory sort of? I am fluent with programming of eeproms though

Oct 11, 2010 at 01:54 PM

Jawnathin
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p.1 #17 · Flash causing autofocus issue?

theSuede wrote:
Not exactly. The lens also contains a grid database over "corrections" that are to be applied to the AF-module measurement before the focusing action is calculated. If it's a zoom lens you even get multiple look-up-charts for different zoom positions. It also gives information about AF-actuation mechanical "play" for different FL-positions and focusing distance positions to predict errors from over- or under-carry of the actuation movement...

The lens is in a very clear way an "active" part of the AF system.

You CAN actually program a lens to give inaccurate focus - front-focus at minimum zoom, back-focus at maximum zoom. This at...Show more →

Thanks for the info, very interesting, but it doesn't explain why it does it only with the Speedlight's AF Illumination and not any other light sources.

Any theories or information about that?

I'm also most interested to hear from Bruce, who asked that I try without the AF lamp. How did you initially assume this is the issue? Have you seen this problem before?

Anyone with the Nikkor 24mm 1.4 wanna give this a shot too?

Oct 11, 2010 at 02:20 PM

theSuede
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p.1 #18 · Flash causing autofocus issue?

To complicate the question even further.... The camera also receives a WB-table to correct for (from an EPROM in the lens) - this will adjust the focusing depending on the WB-temperature you take the measurement in.

This is probably what confuses the Sigma, it overcompensates for the difference between the ~2800K indoor light and the 5700K flash light. I'm not deep enough into the lens-communication protocols to know how this is done, but both patents and Nikons own technical literature on this matter suggests that WB temp AF correction has been active since the first version of the new type of AE-measurements - which would be sometime around the introduction of AF-D lenses.

Oct 11, 2010 at 06:30 PM

mcaswell
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p.1 #19 · Flash causing autofocus issue?

Just came across this thread. I'm battling this problem... my first copy of the Sigma 50 1.4 MASSIVELY backfocused when the SB900 AF assist light was used. The second copy was a lot better, but there was still a definite shift when the AF assist was on.

However, it's not limited to Sigma lenses... it also happens on a variety of Nikon primes I've tried (50 1.4G, 24 1.4G, 85 1.4G). It happens with the 85 1.4D as well, though to a lesser extent. With the 85 1.8D, it's barely perceptible (even when comparing to the 85 1.4G at f1.8).

It does not happen with the SB600... just the SB900 (I don't have an SB800 to try). It happens with both of my D3s bodies, both of my D700 bodies, and with all 4 of my SB900s.

I sent in a D3s with an SB900 and 50 1.4G to NPS, several months ago, but the problem still existed when I received it back.

Here's the 24 1.4G...

50 1.4G...

85 1.4G...

My workaround is to dial in the appropriate level of AF micro adjust depending on whether I'm using AF assist or not, but that method is prone to mistakes as you might imagine.

--Michael

edit: forgot to mention, this happens even if I physically cover up the SB900 AF assist light so that it's not actually visible on the target.

Nov 21, 2010 at 09:51 PM

sethny
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p.1 #20 · Flash causing autofocus issue?

I had a similar problem in low light , with the AF assist of the SB-800 until I realized that the red pattern was not in line with the center of the AF sensor. It was because the flash sits with a little play on the shoe. moving the entire flash makes the beam fall in the center spot again. So basically every time a use the flash I need to double check if the beam of the red assist is aligned . if it's not then the sensor (which is bigger than what it looks like in the VF) will try to find the contrast where the beam hits.

at first I thought it was the shoe but then a metz 54 hits the center all the time . The difference is that the metz uses the old fashion ring (to lock on the shoe) while the SB800 uses that fancy locking mechanism, which is cool but NOT precise (and since we deal with precision optical tools it makes a very little sense (if you ask me)

but if you cover the beam and the lens mistakes the target then it makes no sense. The only explanation (that I can come up with) is that the gear is heavier with the flash and even the movement of the finger pressing the shutter can change the distance shooting @ 1.4 handholding

what else it could be? ...

Nov 27, 2010 at 06:27 AM

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