Can I use Nikon - SB-900 Speedlight External Flash Unit on my Nikon D5000?

I wanted to buy an external flash and i don't know if the SB900 would work on my D5000.

In your user manual, it lists all the flash units your fine D5000 can use.

Look on Page 79, 163 and 164

Yes you can - it will work.

Your D5000 doesn't have the Nikon wireless capability, but you could trigger a SB900 off camera using its dumb optical slave mode (SU-4) & your onboard flash.

Alternatively, if you want to save around £230, you could get a manual flash (YN560) and a set of radio triggers (RF602's) -£90 as opposed to the £320 a SB900 costs. Still… It's your money.

Yes you can use it Nikon SB-900 AF Speedlight Flash
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The SB-900 will definitely work on the D5000. If you are buying it new, I recommend getting the SB-600 instead. The SB-9000 has features your D5000 can't take advantage of, and is really only needed if you are creating a multiple-flash set-up for studio work. The SB-600 is a wonderful flash unit with excellent range and flexibility for the D5000, and is cheaper and more lightweight than the SB-9000.

I do NOT recommend getting a non-Nikon flash unit. The Nikons are designed to communicate with your Nikon DSLR camera's metering system, and will give you superior results.