I've written in previous articles about the fact that the flash metering system measures only the center of the frame. This means that if the subject is not in the center of the frame, the brightness of the subject will likely be wrong.
Above is an example of a subject that is not in the center. I used FV Lock to make sure the subject was exposed properly by the flash. I was outdoors at a reception on a pitch dark night, so there was zero ambient light, and I wanted the very dimly lit waterfall to come out in the background, so I used my DSLR handheld in manual mode, ISO 400, 1/8 sec, f/2.8 and my flash in TTL.
I first pointed my flash at the subjects and pushed the FV Lock button to fire the pre-flashes and lock in the correct flash power. Then, I recomposed the shot to place the subject on the left 1/3 line (rule of thirds), and released the shutter. Notice that the subject is sharp despite the very slow 1/8th second shutter speed, because the flash stopped any motion. But, look at the waterfall. There is definite motion blur there, but I used f/2.8 to make sure it was out of focus so the motion blur from handholding the camera wouldn't show.
If I had just framed it as above to start with and taken the shot without using FV Lock, the flash would have metered the center of the frame, which was the waterfall about 70 feet away, and it would have set a very high power. The subjects would have been totally blown out.
One very important thing to know when using FV Lock is that once it is pushed the flash value remains locked at that power setting until one of three things takes place: 1) FV Lock is pushed a second time, 2) the camera is turned off, or 3) the light meter in the camera times out. Since the default light meter reset time is very short (only 6 seconds on my D200), you need to use your camera menus to extend the light meter timeout to as long as possible when you are using FV Lock.
Don't forget to return the light meter timeout to its default time after you are finished with FV Lock, or it will run your camera batteries down faster than normal.
FV Lock is very useful for shooting a series of shots of a group of people, so that all the images will look exactly the same from one shot to the next.
It is also useful for those rare people who are 'fast blinkers'. A fast blinker can blink at the pre-flash and their eyes will be closed by the time the main flash happens. Lots of animals are fast blinkers, so FV Lock is also good for them.
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