Exposure Bracketing wth Nikon SB910 Speedlights

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dblarch
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Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

Hi Stas,

I am attempting to create a stack with exposure bracketing. I'm using a Nikon D800e and two SB910 Speedlights. The flashes are fired by my on-camera flash in commander mode. I have a third SB910 that could be mounted on the camera and set as commander. That would give me more options.
Wishing to use a constant aperture, I have tried changing the shutter speed of the camera using Helicon remote but, the resulting images all appear to have the same exposure. The bracketing is set in Helicon Remote to three exposures per focusing step. It does appear that I am successfully taking three exposures at each focus position as the list of images in Helicon focus shows three images per focus step number.

Thanks,
Bob
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Stas Yatsenko
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by Stas Yatsenko »

Hello,

So what is the problem, exactly?
Changing the shutter speed in the "Camera settings" panel will reflect on the exposure bracketing settings as well. You can simply press the "Info" button to verify that by inspecting the individual exposure steps. If something doesn't work as described, please explain the nature of the error.
dblarch
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

The problem is that all three exposures in a three exposure bracket appear the same despite setting a different shutter speed in exposure. I suspect that Nikon's Speedlights are compensating for the shutter speed change. I will set the remote flashes to manual instead of automatic (iTTL) and report back.
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Stas Yatsenko
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by Stas Yatsenko »

As long as the actual exposure settings in the camera body do change appropriately, it's not a software problem and we can't help with that, sorry.
vwsweeden
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by vwsweeden »

The problem here is a misunderstanding of how flash photography works. The brief flash of a strobe, usually well less than 1/1000th of a sec., is much shorter than most shutter speeds. As long as the shutter speed is within the flash synch speed range, adjusting the shutter speed has zero effect on flash because the entirety of the flash occurs within the exposure time at all synched speeds. When shutter speeds exceed the maximum flash synch speed, a portion of the image does not receive flash exposure because the shutter never opens completely over the entire image.

In flash photography, the shutter speed is adjusted to include or exclude ambient light and control the ratio between flash and ambient.

Adjusting the aperture affects both flash and ambient exposure. And in your case you wish to use the same aperture throughout.

Adjust your flash output to bracket flash. You may also bracket with ISO as long as it doesn't introduce ambient light effects.

Test for ambient light tainting by making an exposure with the flashes off to confirm that ambient records dead black.
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Stas Yatsenko
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by Stas Yatsenko »

Thanks for the great description, hopefully it will be useful to others who might have the same question.
dblarch
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

I have solved the constant exposure issue with the Nikon SB 910 Speedlights. I placed the remote Speedlights on manual mode thus allowing the shutter speed controls in Helicon Remote to actually change the exposures for bracketing. This is possible because the SB 910 Speedlights allow shutter speeds over the entire range of my D800e. I can make exposures up 1/8000 seconds.

Having solved that I tried a simple 16 focal position focus stack with 2 exposures at each position. In Helicon Focus I had 32 exposures with every other frame being darker, as expected. I selected the odd numbered, lighter frames (1,3,5,...) and stacked them. I did the same for the even numbered, darker frames (2,4,6,...). Here is where I noticed that Helicon Focus seemed to be adjusting the exposure of each darker frame making each as bright as the previous lighter (normal) frame.

I had wanted, and expected, the darker stack to remain dark so that I could use it for HDR editing in Photoshop CC 2015. Is there a way to disable the exposure compensation of Helicon Focus? Is this already addressed with the RAW-in-DNG-out workflow?

Thanks,
Bob Farrell
dblarch
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

May I offer some clarification of the Nikon Speedlight issue. My SB 910 and several other Nikon models offer a high shutter speed mode in which the unit fires repeatedly during the exposure; it apparently strobes thousands of times during the exposure. This creates a series of light pulses that somewhat mimic a continuous light source. By this method the shutter speed can exceed the normal flash sync speed.

This simulation of a continuous light source therefore allows the exposure bracketing controls of Helicon Remote to function as expected.
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Stas Yatsenko
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by Stas Yatsenko »

Apparently, one or more images in your selected set were of different exposure.
I recommend using batch processing mode in Helicon Focus (File -> Batch processing) - load all your images and split stacks by exposure.
dblarch
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

I batched the exposures in two stacks. The normal exposures produced a normal stack,i.e., the stack appeared to be exposed as each of the component thumbnails. However, the stack from the underexposed images appeared brighter than the component thumbnails and as bright as the normally exposed stack. I then brought the underexposed images into Photoshop and each showed dark as expected and combined into a dark stack as was my original intent.

It appears that Helicon Focus is trying to correct the underexposed images during the stacking process. You mentioned a new RAW to DNG workflow that leaves all of the corrections to the external editing software. Is this the method I should be using?
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Stas Yatsenko
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by Stas Yatsenko »

Helicon Focus doesn't do that. Apparently, the raw converter does. At the very least switch to the DNG converter and see if that solves the problem, and if not - save the result to DNG as well.
dblarch
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Re: Exposure Bracketing wth Nikon SB910 Speedlights

Post by dblarch »

Setting the RAW development to 'RAW-in-DNG-out loader' solved the brightness shift problem. Now the images on the main screen appear to be at the same bightness as the thumbnails on the source image panel. This remains so even after stacking as I'd hoped.

Thanks Stas,
Bob Farrell (dblArch)
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