Nikon makes external flash units that fit into the accessory shoe (“hot shoe”) on top of the D90. Three flash units to consider are the SB-400, SB-600 and the SB-900. The SB-400 is the most compact of the three. The head of the SB-400 can be rotated upward 90 degrees to point toward the ceiling. This enables the use of bounce-flash. If you want more power and flexibility look at the SB-600 and SB-800. You can control both of these speedlights wirelessly with your D90. That makes for a versatile and powerful system. ...

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