Kirk Flash X-Tender 6

Whether you use your Nikon Speedlight as main light or a fill, the Kirk FX-6 Flash X-Tender for the SB-900 is a must with lenses 300mm or longer. The Flash X-Tender increases your flash output by a minimum of 2 full stops (3 stops from a flash's 50mm setting). This unit weighs less than 90g and folds flat for storage, making it truly portable and convenient.

Price:  £37.99
available from  wex photographic Warehouseexpress.com