I've shot, for a number of years, in a natatorium that has a mixture of filtered sunlight, sodium-vapor, and fluorescent lighting. My back-up body, D3200, nails the preset manual white balance every time (with either the 18% gray card, a white card, or just pointing it at a white wall). But the D7100 struggles to get it correct. It's frustrating -- I just resort to entering it manually: about 3800 K (+/- 80 depending on cloudy conditions). Have others noticed this problem on the D7100 with the PRE manual WB?