>So then you just shot Tiff all the time
Shooting TIFF does not make sense. Larger files at lower quality. Can the D800 even still shoot TIFF? I guess Guy ment the converted TIFF.
>instead of the "small" DNG files at about 42Mb?
Nikon shoots NEF and not(!) DNG. You could convert to DNG but this only would make sense if you never use different raw converters than ACR/Lightroom.