Another change to my photography setup

[maridvnvm]

I use a DSLR and operate in full manual mode. I am in complete control of the aperture and shutter speed.

[Anaximander]

Still, backgrounds matter to your camera settings and to, potentially, the burden placed on your post-processing of coin photos.

Here are two coins shot on three backgrounds. The penny is a stand-in for a bronze coin and the nickel is a stand-in for silver. The photos are not retouched other than to crop. My point: Exposure metering in the camera will take in the brightness of the coin and its background, and you need to anticipate the impact on the coin and compensate appropriately.  

First the penny, shot on a black, gray, and white backgrounds using automatic exposure (aperture priority). Some EXIF data is provided.
Black: sensing the dark background, the exposure is taken with an aperture of f/6.3 and a shutter speed of 1/80 second, with ISO 1600. The coin appears brighter.
Gray: the gray background is read as 'neutral' and the exposure is f/6.3 at 1/160 sec. with ISO 500. The coin appears at its most natural brightness.
White: the camera dampens the light background, shooting f/6.3 at 1/160 sec. with ISO 320. The coin appears darker.  

I think most amateur coin photographers shoot on a black background, as it avoids shadows and renders a brighter coin image. 

 

Second, the nickel, shot on the three backgrounds.
Black: A bright coin on a dark background, with no discernable shadows. f/6.3, 1/160 sec., ISO 1600.
Gray: Natural brightness, mostly. f/6.3, 1/125 sec., ISO 320.
White: Darkest coin. f/6.3, 1/125 sec., ISO 200.

The camera is a Micro Four Thirds, shooting a 60mm macro lens (crop factor = 2), for an effective 35mm equivalent 120mm macro lens.

Whether shooting bronze or silver, the brightest coin is the one shot on a black background. The white background is 'too bright' for an automatic exposure, and you end up with a coin that is too dark. Stepping down the exposure -or some other form of avoiding underexposure- is essential.