Conditions: Flash in iTTL Mode - not iTTL-BL Mode.
(TTL means iTTL in this blog)
It is very important to remember that the Nikon TTL Flash metering system is not actively coupled to the Camera metering system. Even though these two systems use the same metering sensor in the camera, they act independently. This is definitely not intuitive and not explained well in any documentation that I've read. It is alluded to in the Nikon CLS pamphlet when it talks about using the FV Lock button to meter a subject that will be off-center when the background is dark.
Note: The TTL-BL flash metering system is actively coupled to the camera metering system, and I will talk about that in my next post.
The picture above was taken in nearly total darkness in camera Manual mode with the shutter at 1/8th second and the aperture at f/2.8. Notice how the flash system handled the entire exposure of the subject, while the camera system handled the the exposure of the background. You know this because of the shift in white balance from the subject to the background. Notice that the background is very yellow and the subject color is much whiter. This is because the flash was the only contributor to the exposure of the subject, and the background was lit by incandescent lights. The flash was so weakened by the time it reached the background that it did not contribute to the background at all.
The Camera metering system does not measure the amount of regular TTL flash that will be added to the exposure of the subject. On the D200 and all cameras prior to the D300/D3, the camera does nothing to help you with this. If you are shooting regular TTL flash in bright ambient light, you risk severe overexposure if you do not reduce the camera exposure when you turn on the flash.
On a D200 camera you can easily demonstrate this problem by taking a picture, with and without flash, with the shutter set at a fixed amount. Set the camera on S mode and the shutter to 1/80th and let the camera pick the f/stop. The f/ stop that the camera selects is the same whether the flash is on or off. This means that whatever ambient light is reflected from the subject will be further illuminated by the the flash, and if the ambient is already enough to properly expose the subject, the flash will cause overexposure.
However, on the D300 and D3 and newer cameras, Nikon has made a significant improvement to help with this. While the camera metering system still does not meter the amount of TTL flash that will be added to the exposure of the subject (like it does in TTL-BL mode), the newer cameras automatically reduce the camera exposure in bright ambient light when the flash is turned ON. For instance, in the camera A mode, in bright ambient light, the shutter speed is increased when the flash is turned on. The amount of the increase is dependent on how bright the ambient light is. This process is similar to the process that the photographer was required to do with flash compensation prior to the D300/D3.
The Flash metering system just fires the preflashes and looks for the reflected light in the center weighted frame regardless of which camera metering system you are using or what the camera f/ stop and shutter is set to. The flash metering area is affected most by centered subjects and the metering sensitivity decreases as the subject is placed farther and farther towards the edge of the frame. At the edge of the frame, the subject has much less effect on the flash metering. Whatever light reflects back from the preflashes in this center-weighted fashion is what determines the power of the flash. You can see that if the subject will be way off-center, your flash power will likely end up way too high as the flash attempts to light the background in the center. This is why the FV Lock function was invented, which I have written about here: http://nikonclspracticalguide.blogspot.com/2008/01/4-so-what-is-flash-value-lock.html .
So, you can reason that the ambient light reflected from the subject is essentially added to the exposure once by the Camera metering system and once by the Flash metering system and overexposure is often the result if the ambient light is strong.
Here is an example of overexposure with flash when ambient is strong.
One important operational consideration that should be drawn from this is that when the ambient light is bright, it is usually best to turn down the flash using flash compensation to about -1.7 ev to start with. This will usually avoid overexposure on especially the subject's face.
Note: This is what the newer cameras (D300/D3 and newer) do for you automatically, except they do it by reducing the camera settings, which reduces the ambient portion of the exposure. The thing that is not so nice about this is that if you use regular TTL outdoors in bright daylight for fill, this automatic reduction in camera exposure will also make the backgrounds darker. So this improvement is really only a band aid that protects the unknowing photographer from severe overexposure. The best way to shoot fill using regular TTL is to use the camera in Manual mode where you have full control of the shutter and aperture. Then, you reduce the flash power with the flash compensation as I mentioned above.
This is why Nikon invented TTL-BL, which I'll talk about in my next post.