None of the Nikon flashes have radios. They are all optical triggering. For example, the SB800 can be used as a commander and SB600 as a slave. In TTL, the SB800 will produce a short series of flashes before the exposure that tells the SB600 what to do. Then, the shutter trips, both flashes fire and the exposure is made correctly. A number of Nikon digital cameras can use the pop up flash as a commander, too. You do not need the SU4 or SU800 unless you do not want a flash on the camera.
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