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Author Topic: x-sync head scratcher  (Read 427 times)

joeclarkx

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x-sync head scratcher
« on: January 04, 2021, 05:38:34 am »

I've been going round in circles trying to troubleshoot issues I have observed with shading, presumably caused by the shutter, during flash photography.

I occasionally do repro work for artists alongside my architecture and exhibition documentation work and I have found that for repro tasks where a small amount of shading would still be significant, that I am only able to use about half of the stated x-sync speed.

5DMKII
Stated: 1/125
Reality: 1/80

A7R
Stated: 1/160
Reality: 1/60

A7Rii
Stated: 1/250
Reality: 1/80

To a certain extent I can just work around this by using the slower sync speed, or simply use an LCC frame in Capture One to correct for it, but it troubles me that I don't feel totally in control of my equipment. I'd like to be sure that this slow sync really is the cameras and not another aspect of my setup. Plus I really could use the ability to block out more ambient sometimes.

To describe the exact problem in more detail: It isn't so easy to spot with the naked eye but once I go above the speeds above then I see the RGB values start to drop - first at the top of the frame and then progressively further down. I have tested this by shooting tethered into C1, lighting a white wall with flash and placing colour readouts around the frame.

I have tried this with two different flash systems - elinchrom and godox - and also although I was originally using the canon godox trigger, also got the sony trigger in to test. I also thought perhaps that the metabones adapters that I am using on the sony might be the issue- but the fact that I am also struggling with the old 5D suggests this might not be the case.

I recently bought the a7Rii to try and finally deal with this issue. Otherwise I am totally satisfied with the image quality and usability of my old systems for my applications. Unfortunately, despite the much higher stated x-sync (1/250) the a7rii exhibits the same slight shading above 1/80

So. Are my expectations too high? Does anyone have some sage advice for me?

Thanks
Joe
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BobShaw

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Re: x-sync head scratcher
« Reply #1 on: January 04, 2021, 06:03:20 am »

I would say that the first thing to do is eliminate the triggers with a cable. All triggers have a delay. Elinchrom triggers are very good. I have used them with slight vignetting at 1/2000 with the X1D.

I assume you are using a tripod but the shutter speed needs to be fast enough to eliminate problems with ambient light. Take a shot with no flash and make sure it is completely black.
The flash speed also needs to be reasonably fast and the lighting even and screen calibrated.

The colour accuracy of most 35mm cameras is not great for art reproduction. I owned a 5D Mk2 and the colour is all over the place.
« Last Edit: January 04, 2021, 04:36:46 pm by BobShaw »
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Dave Ellis

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Re: x-sync head scratcher
« Reply #2 on: January 04, 2021, 06:20:32 pm »

I've been going round in circles trying to troubleshoot issues I have observed with shading, presumably caused by the shutter, during flash photography.

I occasionally do repro work for artists alongside my architecture and exhibition documentation work and I have found that for repro tasks where a small amount of shading would still be significant, that I am only able to use about half of the stated x-sync speed.

5DMKII
Stated: 1/125
Reality: 1/80

A7R
Stated: 1/160
Reality: 1/60

A7Rii
Stated: 1/250
Reality: 1/80

To a certain extent I can just work around this by using the slower sync speed, or simply use an LCC frame in Capture One to correct for it, but it troubles me that I don't feel totally in control of my equipment. I'd like to be sure that this slow sync really is the cameras and not another aspect of my setup. Plus I really could use the ability to block out more ambient sometimes.

To describe the exact problem in more detail: It isn't so easy to spot with the naked eye but once I go above the speeds above then I see the RGB values start to drop - first at the top of the frame and then progressively further down. I have tested this by shooting tethered into C1, lighting a white wall with flash and placing colour readouts around the frame.

I have tried this with two different flash systems - elinchrom and godox - and also although I was originally using the canon godox trigger, also got the sony trigger in to test. I also thought perhaps that the metabones adapters that I am using on the sony might be the issue- but the fact that I am also struggling with the old 5D suggests this might not be the case.

I recently bought the a7Rii to try and finally deal with this issue. Otherwise I am totally satisfied with the image quality and usability of my old systems for my applications. Unfortunately, despite the much higher stated x-sync (1/250) the a7rii exhibits the same slight shading above 1/80

So. Are my expectations too high? Does anyone have some sage advice for me?

Thanks
Joe

Joe you don't say what flash power you are using. If you are using full power, I'd suggest you try it at say 1/4 power instead. This will give a much shorter flash duration. The key to flash sync is that the flash fires just after the first curtain opens and finishes it's output before the second curtain starts closing ie the flash is putting out power only when the sensor is fully exposed.  Depending on the model, flash duration can last several msec at full power and this could cause some uneven illumination if the flash is still lingering on after the second curtain starts to close.

Dave
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joeclarkx

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Re: x-sync head scratcher
« Reply #3 on: January 07, 2021, 06:04:45 am »

Thanks Dave and Bob for your responses

I got a cable in and also tested a various powers

The cable made a very small difference. a couple of points out of 255 RGB. It could mean that 1/100th was free from any shutter shading at all, but it is still far away from the 1/250th promised by the a7rii :(

Testing at various power levels made no noticeable change to the shading effect I'm testing for.

So, sadly my conclusion is that, although these shutters may have ratings that can be trusted for general purpose photography, very precise work will need to acknowledge these limitations. So I'm back where I started, using reasonably slow x-sync or correcting for the shading with an LCC.

Its not the result I was looking for, but at least I know...

So, I will be continuing to use my cameras at 1/80th - 1/100th
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