I got this from another forum when looking for a method that gets around the pre-flash and allows the use of a light meter. I have not tried it as yet.
1. Set your YN622N-TX to manual for the flashes you need to control.
Group A for example can be on 1/4 and Group B can be on 1/16, it doesn't matter what the settings are but we are trying to manually control the speedlights.
unfortunately it only works for two groups (it doesn't matter how many flashes you have in group A or B) . The third group, ie Group C NEEDS to be set to TTL on the 622N-TX with no flashes in Group C.
2. Make sure that all your speedlights are set to iTTL, otherwise they will NOT synch wirelessly. This is essential.
3. Again at least one channel MUST be on TTL on the YN622N-TX. This is normally ch. C .
4. On your Nikon camera you MUST set up this configuration.
a. assign the FUNCTION button on your DSLR to activate FV LOCK. on the D800 its f4 on the custom setting menu.
b. In the custom setting menu go to C- Timer/AE LOCK menu and select C2 auto meter off delay and set to at least 30sec ( i've set mine to infinity during a shoot so i can use my flash meter with no need to rush .) Make sure to put it back to 4-10 seconds after a big shoot otherwise the batteries will be exhausted vey quickly.
5. The YN622N-TX comes with a little cable which you plug into the transmitter on the left hand side of the unit and then to your cameras cable release port ( on the front of the D800 under the rubber seals).
6. You will need a spare YN622N receiver which you will use as a hand held trigger .
7. We are now ready to shoot!
8. do not use the YN622N-TX "TEST" button to shoot the test shot. You MUST take a test photo by pushing TEST on the YN622N, it triggers of a Photo from your camera. This works because of that little cable we discussed in point 5.
9. on your Nikon DSLR, push the Fn button you assigned earlier, as mine is set to infinity ive got all the time in the world to walk over to my subject and take a flash reading.
Make sure that FV Lock is visible inside the viewfinder!
a small pre-flash goes off from your speedlights.. EXCELLENT the pre-flash issue is no longer, now all your test shots are PRE-FLASH FREE!
10. go to your subject, put your flash meter where you want to take a reading, push TEST on the YN622N you have in you hand and you get an accurate reading. Put those readings in your camera and you will take a beautifully exposed photo.
I've thoroughly tested this configuration and it does work, as long as you follow these instructions to a tee..