>Could you explain a bit more? Why will the camera underexpose? >Surely it will just expose for the overall scene and then add >flash to brighten the subjects? ... I can just about >understand that the overall scene will be underexposed as when >I put the Flash on the camera exposure automatically drops by >a stop or so but I can't see why the subjects would be >underexposed.
This is a very complex issue.
The camera and flash are two separate metering systems. Both meter based on reflective principles, which means they try to expose for an overall color equivalent to 18% gray. This means that when you meter a scene that is very bright, like snow, the camera and flash will both think it is too bright, so they reduce it back to 18% gray. This will tend to turn the white snow to gray and the subjects will become very dark and the whole scene will be underexposed by a stop or two. Therefore you have to increase both the flash and the camera compensation to brighten the image to where the white objects (snow) is white again.
>Also, does TTL-BL not work in manual mode?
TTL-BL will work in any camera mode, but it assumes the camera meter is zeroed when it calculates the required flash power for fill. I recommend P mode, because it always zeros the meter instantly and prevents the overexposure problems of camera A mode due to the shutter speed restriction of flash sync speed.
Professional wedding photographers (the Nikon shooters) I worked with all used P mode whenever shooting fill flash. We joked that P mode must stand for Professional mode, since it seems that only professional photographers use it. P mode does exactly what TTL-BL needs.
Also, be careful not to use Auto ISO when shooting flash. It doesn't work well. Use fixed ISO 100 or 200 for outdoor fill flash.