I suspect they are specular reflections from small elements of the source. Aliasing could exacerbate the artifacts as could overzealous sharpening. Noise reduction. However, NR would degrade the image detail. The light from the flash is rather harsh and use of a more diffuse light source might reduce the artifacts. Does anyone know what is producing these artifacts and how one could mitigate the problem?
Hi Bill,
Yes, I suspect they are specular reflections. You can try using some diffusing material to see if it helps enough.
If the subject surface is bumpy and waxy enough, you'll need very large diffusers to suppress the specularity. At certain angles, a polarizing filter on the lens may help for some angles but to more effectively suppress them, cross polarization must be used (polarizing foil outside(!) the diffuser and crossed polarizing filter on the lens).
I use a similar type of macro flash-unit (MT-24EX) on my Canon lenses, and I have fitted Stofen diffusers to the heads for softer light and more gradual fall-off with distance.
Here is a Canon oriented thread about all sorts of DIY solutions, I have to assume something similar is available or can be made for the Nikon macro flash-unit.
Cheers,
Bart
P.S. You can also try putting both (diffused) heads on one side of the subject, thus creating a (seemingly) larger illuminating surface, and a reflector on the other side to fill in the shadow side.