Polyester was a bad choice for the Asian heat, but in the early days, New Zealander Derek Williams didn't know any better. He quickly learnt though and went on to forge a career as a sound man and cameraman that spanned four decades. The Vietnam war, Tiananmen Square, coups in Thailand - Derek Williams was there for all of them. Among his many friends and colleagues was the legendary Australian cameraman Neil Davis. Now, after 40 years, Derek Williams is leaving Asia and our South East Asia correspondent Liam Cochrane sat down for a chat with him about life behind the lens.