There are a number of companies that produce flashes for Nikon's i-TTL system.
Digital camera companies like Nikon use a sophisticated metering system for their flashes that reads the light that passes through the lens and onto the sensor. The system then works with its automated exposure programs and the flash unit attached to produce the best possible exposure. Nikon calls their system i-TTL (Intelligent Through The Lens) metering. In addition to Nikon Speedlights, there are a number of third-party flash units that will work with i-TTL-compatible cameras.
Metz
Metz is a third -party manufacturer that produces a line of external flashes known as "Mecablitz" flash units. They have several units designed to work with Nikon's i-TTL system, including 24AF-1, 36AF-5, 44AF-1, 48AF-1, 50AF-1, 58AF-1 and the 58AF-2.
Sigma
Sigma produces lenses and flash units designed to work with "Original Equipment Manufacturer" (OEM) camera companies, such as Nikon. Their flash units compatible with Nikon's i-TTL system include EF530DG-Super Flash, EF530DGST, EM-140DG Macro Flash, EF610DGST and the EF610DG Super.
Sunpak
Sunpak is another third-party flash manufacturer who has designed a number of flash units to work with Nikon's system. The Sunpak flashes that work with Nikon are RD2000, PZ42X, PF30X, PZ40X,
Nikon
In addition to camera's and lenses, Nikon produces external flash units known as Speedlights. Nikon produces seven separate Speedlights and out of those seven, four are compatible with their i-TTL system. The SB-900, SB-700, SB-600 and SB-400 are all i-TTL-compatible flash units.
Tags: flash units, work with, i-TTL system, with Nikon, Nikon i-TTL, Nikon i-TTL system