A flash which reacts to the output from another flashgun, usually one mounted on a DSLR or similar camera. By firing simultaneously, the scene can be illuminated by a number of sources.
On sale: 14th Feb
We introduce the new and improved look of What Digital Camera with 102 camera reviews, while in a high-resolution battle, the Sony A7R and Nikon D800E go head to head, plus we've put together an essential guide to Wi-fi.
Which of the new cameras announced so far this year are you most excited about?
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