Nikon offers a variety of flashes that vary in size, power, and capability. Nikon calls them “speedlights”. They can be attached to a DSLR camera to expand the flash capabilities beyond what the on camera flash provides. The most inexpensive SB-400 speedlight offers more power and the ability to bounce flash off ceilings to create a softer light. More advanced models offer multi-directional bounce, zoom, color filters, off-camera remote capability, and built in diffusers.
Using the basic features of Nikon’s speedlights is fairly intuitive. Nikon does a great job at flash exposure in its AUTO TTL mode. However, to make full use of Nikon’s flashes and get ultimate lighting control, you must master some key settings. These key settings include: flash exposure compensation, flash sync speeds, flash modes like TTL-BL, color gels or filters, and knowing the right camera mode for any given lighting condition. First we must understand the capabilities of each flash.
NIKON SB-400 FLASH
This is Nikon’s most inexpensive flash. It offers more power than the on camera flash. It also offers the ability to change the direction, upward, to bounce it off ceilings for softer light. The advantage of this flash is its size and portability. It is a great accessory to take on the go, even on the more expensive cameras. For general photography, I like to carry as little as possible, and this flash fulfills that purpose.
NIKON SB-700 FLASH
The SB-700 is Nikon’s midrange flash. It greatly expands the capability of the SB-400 at an increase in cost and size. The SB-700 takes four AA batteries to power it and is substantially brighter than the SB-400. It features three modes: “ON” for use when mounted on the camera, “REMOTE”, for off camera remote trigger, and "COMMANDER" for use when mounted on the camera to control other remote flashes.
Additionally it has an autofocus assist feature that helps the autofocus in dim lighting without being as obnoxious as the on camera bright white autofocus light. Unlike the built in autofocus assist light which only allows center focus point to be selected, the SB-700’s autofocus assist light projects a broader light pattern that allows the camera to focus on a variety of focus points. Read more on autofocus assist.
Finally the SB-700 offers several exposure modes that determine how much flash power to output. These modes are: manual, TTL, TTL-BL, GRN, and A. These modes are described in more detail below.
NIKON SB-900 FLASH
The SB-900 is Nikon’s top end flash. It provides more power than any other Nikon speedlight, allowing you to shoot at distances beyond 150ft. It has a zoom head that can cover a wide focal range up to 200mm. It expands on the SB-700 flash features by providing a broader autofocus assist pattern, more control with a total of 3 wireless groups in "COMMANDER" mode, and an input for a flash pc-sync cord for studio use.
The SB-900 is a very capable flash, but its size limits its portability. This is not an everyday flash and should be reserved for professional circumstances. The newer, smaller SB-700 is sufficiently capable for most needs and has many similar capabilities of the SB-900.