Weird purple glare on black metal

[pittendrigh]

I've been away from photomacography for a year and a half. My synapses are gradually coming back into synchrony after a year and half bout with long Covid. I set up a shot this morning I've done a thousand times in the past but suddenly I'm not getting satisfactory results.

This is a fishing fly in a light tent with a semi-transparent plastic background.
I'm using two main strobes with one small speed light in slave mode, flashing from behind the Plexiglass background, at low power.

Nikon D7000 with 105mm macro lens on tripod with Cognisys rail and ZereneStacker in control. 15 exposures at 2mm increments.
The individual exposures look fine but the stacks, in both Pmax and Dmap show an unwanted purple glare on the hook vise.

Additionally the Dmap version shows odd shadows around the contours of the foam fishing fly.
I can edit the odd shadows away with Darktable and Gimp but I'm curious to know what the cause is, so I can perhaps adjust my lighting setup accordingly. The purple glare on the hook vise is a mystery to me. I have not seen this before. I made no attempt to post process the images below. I converted directly from Nikon raw nef to jpg so the mysterious starting points would not be obscured.

I also made two stacks without the light tent, with the main strobes flashing directly onto the subject--without much difference. F8 ISO 160, strobes set so the narrow histogram bulges upward slightly to the right of center.

[pittendrigh]

Ok. I tried F5 instead of F8, made the stepping increment 1mm instead of 2 (so 32 exposures instead of 16) and moved the slave speedlight that was illuminating the semi-transparent background from behind so it illuminated the plastic background from the front.

I still get some of the halo static around a few edges and still get a little purple haze, but much less. The overall image quality is far better too. This is without ANY post processing at all. I can probably fix this image in post but I'd still like to achieve a better initial image.

I'll keep fighting with it.

[pittendrigh]

On a hunch.....the halo-like background shadows above all appeared on the right side of the subject, as if a left side strobe was too bright. So I turned the left strobe down a bit and the right one up to a bit brighter. The halos were completely eliminated.

The black jaws of the hook vise still had a slight purple sheen but I was able to remove it in post processing. Small adjustments to lighting make a big difference. Perhaps the biggest difference of all.

[rjlittlefield]

I am saddened to hear about your covid ordeal. Glad to hear that it's gotten enough better to start photographing again!

About the purple, my best guess based on limited info is that Zerene Stacker's Brightness "correction" is turning some values that are near black but with a nominally purple hue, into some brighter shade where the purple becomes evident.

Try this: go to Options > Preferences > Alignment, remove the checkmark on Brightness, then reprocess the stack.

--Rik

[MarkSturtevant]

Halo artifacts are not unusual. They can be removed or at least greatly diminished with the touch up tools within ZS. You can see a lot more halos if your "flies" have downy feathers.