For highly specular surfaces with scratches on, setting up a scene where the direct reflection off the specular surface is as dark as possible, then lighting the surface such that the scratches are highlighted by the light source should be key here.
The trick will be finding a light source direction, and size of light source that highlights the scratches ...
Your 6D has a flash sync speed of 1/180. You cannot use faster shutter speeds than that, or else the flash will light only part of your picture.
I would say what is the "best" depends on what you are trying to achieve. If you want to light up the background in a manner that the flash cannot do, you could use a slower shutter speed than 1/180. But do note ...
You have a couple of choices. The terms are "Dark line" and "Light/or White line" lighting for glassware. Against a dark background scratches and dust will look light and vice-versa against a white background. The lighting set-up to use would duplicate the same set-up you use to see the defects.
In other words, if you can see the defect under directional ...
Lighting that is almost parallel to the surface of the object will emphasize texture (scratches, whatever). Shooting straight on - i.e. mostly perpendicular to the light source - with a large enough depth of field (shouldn't be too difficult if the surface is mostly flat) will show that texture the best.
It's hard to tell from the image you have posted, but if that's not a crop this is quite likely reticulation. This is something that happens when your temperature control during processing is poor, and in particular it happens:
if there are large, steep changes in temperature between stages, where 'large' is somewhere above 5°C C (5 is pretty safe, 10 ...
If I had to guess, I would say that this image was underexposed and then corrected for (perhaps unwittingly) when scanning.
Two reasons for this guess:
You mention you metered using the camera's built in reflective meter. I believe the FM2 uses centre weighing for its metering. That means that it determines the exposure mainly on the light in the centre of ...
Given the average luminance value of each frame L1 and L2, you can calculate the ratio of the exposure values EV1 and EV2 as
EV1/EV2 = log2(L1)/log2(L2)
And see Wikipedia for more details on this formulation.
IMO, it works best with the camera at a low angle up close to the disc, and try a single light source in different directions. Nothing fancy: a normal lamp usually works for whatever I'm doing. The trick is finding the right angle for the specific situation, especially for highly reflective surfaces. Macro/close up mode also helps.
I am not sure about the reliability of the brand. I do have some Neewer stuff, but none electronics.
Sayed that, you need to make some basic checklist.
Do you actually see the flash firing?
Are the batteries ok?
Are the images underexposed or not illuminated by the flash at all? Did the flash actually illuminate the scene to some extent?
Is there a big fat ...