The most advanced flash mode on the Nikon flash is TTL-BL. Originally, the BL meant BackLit, but Nikon marketing gurus changed it to mean BaLanced Fill. (Why didn't they change the initials to TTL-BF?). The word BackLit suggests the situation for which this mode is most useful.
I have discussed in the previous article that in the TTL mode the Camera Metering System is entirely separate from the Flash Metering System, and that they work independently. Well, in TTL-BL, the same two metering systems do communicate with each other.
First, let's discuss what Fill Flash is and when it is needed.
Fill Flash is needed any time the background (also called ambient) is brighter than the subject and this often occurs outdoors on a bright day. In this situation, if you look closely at the subject's face, you will see shadows in the eye sockets, under the nose, and under the chin. When you shoot this type of shot, it will look much much better if the flash is used to 'fill' the shadows. Another situation would be if the background behind the subject was the bright sky. Again, flash should be used to 'fill' the subjects face and make it balance with the bright sky. One more situation would be when indoors during the daytime, and you want to shoot the subject in front of a window with a beautiful brightly lit scene behind (like the picture above). Again, the flash should be used to brighten the subject's face to balance with the bright scene behind.
These are the situations where Nikon's TTL-BL mode works best.
In the above example image the subjects are in front of a window and I wanted the outside to be exposed properly by bright ambient light, while I needed lots of fill to bring their faces up to be slightly less bright than the outdoors. I put my DSLR in Manual mode, ISO 200, and used the built-in light meter to set f/3.5, 1/160th, and TTL-BL with -1.7 EV worked great in this situation to just bring up their faces to what seems a natural brightness.
Here are the steps that the camera and flash do for you as you press the shutter:
Given: Flash in TTL-BL, Camera in P mode, Matrix Metering, and AF-S focusing.
1. As the shutter is half-pressed, the focus system becomes active, the focus is achieved, and the overall scene is metered by the camera.
2. The data from the camera metering system and, the focal distance from the D or G lens are sent to the flash metering system. This is the only communication that takes place between the camera metering system and the flash metering system.
3. As the shutter is pressed down the rest of the way, the flash fires the pre-flashes, and the flash metering system measures the reflected light. This reflected energy is a secondary factor to the distance reported from the D lens in the TTL-BL equations. If a D lens is not being used, the pre-flash reflected power assumes a more important role in the TTL-BL equations. When using a D lens, the main purpose of the pre-flash is to set the white balance.
4. The flash metering system (which actually resides in the camera) then uses the distance information from the D lens and the data from the camera metering system, to determine the amount of added power required to make the subject brightness equal to the overall scene brightness. In other words, it adds light from the flash to balance the brightness of the subject with the ambient.
5. The shutter opens, the flash fires at the power level determined above, and the shutter closes.
Now, for this all to work well, a couple of key things must be considered. First, the subject (obviously) can only be brightened by the flash. There have been several advancements to TTL-BL since I originally wrote this, and now TTL-BL handles the situation fairly well when the subject starts out brighter than the background in low ambient. However, it is still best not to use TTL-BL if the subject is already brighter than the overall scene. Regular TTL usually works better.
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