I'm using a 3x5 grey card to set a custom white balance for a Nikon D800 camera, however, for my 50mm lens, the card does not completely fill the camera's image frame. It appears this is throwing off the custom white balance when I look at it later in Photoshop.
Is there a way to limit the custom white balance to the portion of the frame that includes the gray card? Or do I just need to get a larger card to completely fill the frame? When I get in close with the autofocus turned off, I find that I start casting a shadow on the card.