Metering is the technique, which is used to determine the optimal exposure for the subject and the overall scene based on the information camera gets from available light sources. Nikon uses the flash metering system to determine the flash output when the flash is set to auto (also called TTL or i-TTL) mode, and the camera metering system to determine the overall exposure of the image if the camera is set to A, S or P exposure mode. In manual (M) mode, you would take the reading from the camera metering system as a reference whether it is negative (underexposed) or positive (overexposed) and set the exposure values (Aperture, Shutter speed and ISO) manually to adjust the exposure. Nikon D90 and most of the other Nikon DSLRs support three types of Metering Modes, and they are Matrix Metering, Center-Weighted Metering and Spot Metering, which we discussed in previous blog.
In TTL mode, the built-in flash, as well as the external flash unit attached to the camera, adjusts the flash power to properly expose the scene by using the information from one of these three available metering modes. When the flash is set to TTL mode, choice of the camera metering mode determines whether the flash metering system, which shares the same metering sensor with the camera metering system, should consider the ambient light and balance the overall exposure or ignore the ambient light all together and only count the light from the main subject to determine the flash output power.
What is TTL mode?
Nikon’s latest DSLRs and the flash units use the TTL technique to determine the amount of lights reflecting from the scene and adjust the flash power accordingly. TTL stands for Through The Lens and it refers to the process of gathering information through the lens and passing that information to the metering system in order for the camera to deploy the appropriate flash power. Older camera systems either used manual calculations to determine the flash exposure or used a sensor in the flash itself rather than the metering sensor in the camera. And, i-TTL (intelligent-TTL) refers to the latest TTL flash system from Nikon. In latest Nikon cameras, TTL refers to i-TTL by default and I am using TTL and i-TTL interchangeably in this blog.
There are two types of i-TTL methods used in Nikon’s lighting system and the DSLRs, which are iTTL Balanced Fill-flash and Standard iTTL Fill-flash. Now, let’s talk about these two different TTL methods and discuss how they work differently.
1. iTTL Balanced Fill-flash (i-TTL/BL or TTL/BL)
Nikon's i-TTL (intelligent through-the-lens) Balanced Fill-Flash automatically balances the output of the Nikon Speedlight to match with the scene's ambient light. This flash mode is selected by the camera automatically when the flash is in TTL mode and the camera metering is in either Matrix Metering mode or Center-Weighted Metering mode. If the camera is set to A, S or P exposure mode and the flash is in i-TTL/BL mode, Nikon Speedlight fires the series of pre-flashes to determine the exposure value for the subject by taking the ambient light into account and send that information back to the flash metering system in the camera. The flash metering system then combines that information with the metering information from the camera metering system and the focal length information from the D or G lens to analyze and calculate the final flash output in order to balance the overall exposure. All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance. This is a very powerful and easy to use technology from Nikon and probably one of the best in the industry.
In short, using this technology, Nikon DSLR measures the available light and then adjusts the flash output to produce a natural balance between the main subject and the background. It will reduce the harsh shadow as well as the highlights in the subject caused by the over powered flash.
2. Standard i-TTL Fill-flash
This mode is activated when the flash is in TTL mode and the camera metering is in Spot Metering Mode or if you are using the external flash and the flash is set to standard mode. In this mode, the flash output is adjusted only for the main subject in the frame, and the brightness of the background is not factored in while calculating the flash power. If the standard i-TTL Fill-flash is chosen by the camera by dialing the camera metering into the spot metering mode in A, S or P exposure mode, the background light is ignored and the camera will fire the flash to give you the correct exposure for the main subject only. If the overall exposure for the scene and the main subject is properly balanced without using the flash (shooing outside in bright daylight for example), using the standard i-TTL Fill-flash may overexpose the subject by adding the extra light to already properly exposed main subject. You should use this mode only when the flash is main source of the light for the subject (shooting in dark room for example) and you want to emphasize the main subject at the expense of the proper exposure for the background.
In this picture above, the exposure on the main subject was metered by the flash metering system by firing the pre-flashes on the main subject and then analyzed the light reflected from the main subject along with other information that camera provided, including the metering mode and the focal length. The flash metering system then adjusted the flash power and fired the flash just to illuminate the main subject. If you notice the background, which is relatively darker, was metered by the camera metering system and exposed by the ambient light only. The flash power was calculated by the flash metering system to expose the main subject only which was weakened by the time it reached to the background, and hence flash didn’t contribute for the background exposure at all.