Taking pictures of coins in PCGS slabs is different than coins in NGC slabs. For me it comes down to PCGS slabs being translucent and NGC slabs being opaque white. The different backgrounds influence the images. I want only the light reflected from the coin to affect the image. I use a non-reflective black/dark gray material under the slabs. For PCGS coins that works well and the coins come out close to true with regard to toning. But the white insert in an NGC slab somewhat overexposes the picture making the coin darker than in real life. I adjust the NGC problem by using a black "mask" with a hole a bit larger than the coin. That brings me a lot closer to the correct toning. I hesitate to change camera settings since I try to have consistent results. Anyone use a different (and maybe better) technique?
The older NGC slabs were tough for me. I had some success doing a custom white balance, just for this type of white gasket. Some folks are good at fixing it with post editing. I need to practice more with post editing.
The bigger issue is dealing with the glare from the slab. For exposure, I always set the exposure manually and check the histogram to make sure I'm not truncating the dynamic range. I use a white piece of paper under the slabs because the full-slab shots of PCGS coins look better this way. In post-processing, whether I use a black background, a white background, or something else is up to the person I'm doing it for.
As with messydesk, I would suggest setting your exposure manually. By allowing an automatic exposure, you are telling the camera to include the bright white slab, and that's exactly what it's doing. Your other option is to allow automatic exposure but apply adequate exposure compensation to get the coin back close to the correct brightness. That's doable, but at a point why not just set it manually. Especially if you have control over the lighting, that manual setting (along with a manual white balance setting) can get your first shot very close to what you want as the final exposure.
I do the same thing - use some kind of mask for the ngc slabs, and playing with the back ground I put the slab on. I will usually try white, black and grey back grounds just to see what it looks like. With PCGS slabs I usually just play with background colors.
I have no idea what camera you are using, but my camera has 3 setting for light adjustment - whole picture, center, and center point. So long as I use center point and center the coin, the "influence" is nearly gone.
I've got a Nikon D90 with a Nikkor 60mm macro lens. So it's capable of doing "center point" concentration. I used to use it but everything may have changed on the last vacation. I need to check what the current settings are.
