One thing I’ve noticed in photographing Suki using flash: She’s a quick blinker! She has her eyes closed in countless photos I’ve taken of her using my speedlights. On one recent evening I was trying to get a shot of her face using an off-camera strobe, and her eyes were closed up in every single shot. What to do?
When using Nikon’s speedlights in i-TTL mode, whether hot-shoe’d to the camera or fired off-camera through Nikon CLS, the flash will fire a monitor pre-flash when the shutter button is pressed. The camera uses this pre-flash to analyze things like color and reflectance on the subject, and based on what it sees through the lens, it will then send instructions wirelessly to the off-camera speedlight(s) on how it should fire during the exposure.
If your subject is a quick blinker, however, these pre-flashes can pose a problem for you. If he/she blinks in reaction to the pre-exposure flash, their eyes could still be closed during the actual exposure! This is where FV lock can be handy.
FV lock, or Flash Value lock, allows you to initiate the monitor pre-flash yourself, separate from the entire exposure sequence. I have this feature assigned to one of the programmable buttons on my D300s. I simply frame the shot, hit the button, and the flash fires while the camera’s meter reads the scene. Suki blinks, but it doesn’t matter, because I haven’t taken the picture yet. I’ve just let the camera decide what it’s going to do with the remote flash. Now when I hit the shutter button, the camera skips firing the monitor pre-flash and uses the data it already collected from using FV lock to command the remote flash to fire. I can now get light on Suki before she has a chance to blink!
Problem Solved! – Nikon D300s + Tokina 50-135 f/2.8 –
Once activated, FV lock will lock in the determined flash exposure value in the camera until you either power-off the camera, activate FV lock again, or allow the camera’s meter to shut off.