That is the kind of uncertainty I want to avoid. I need reproducable and stable results under constantly the same conditions in order to have the possibility to compare.
The easiest and cheapest way to achieve that is a scanner
I only had to find a setup that I like once. I now use the same setup for all coin images and therefore have reproducible and comparable results. How do you cope with differences in toning? Some can be nearly black, whereas others have none.
My only objective is to capture accurately how a coin looks, so a darkly toned coin should look darker than one that is not. But, having said that, the camera is still mostly in charge of exposure, so a darker coin will need a longer shutter time to gather the necessary amount of light and this will vary from coin to coin.
I should also say, that there is no post-processing in any of my pictures, other than to remove the background.