his subject, stephanie gosk, more than half a mile away. >> reporter: ambler used a 600 millimeter lenssomething called a dub letter and a pretty fancy camera. even on a muggy, rainy day, this is what he could see. thankfully i was modestly dressed. >> if you do anything out in the public eye, you always have to imagine there's a camera on you. because unfortunately, that's the state that we now live in. >> reporter: royal private moments caught on the sly have always gone for a premium. paparazzi swarmed princess diana on vacation too. in fact, in 1994, she was also snapped sunbathing topless. even with the legal risk, it may be impossible to stop. >> i would guarantee my last dollar that any photographer in that scenario would have photographed it. you always shoot and ask questions later. >> reporter: which means even for a prince and princess who will one day wear the royal crowns, privacy may always be their most valued possession. stephanie gosk, nbc news, new york. >>> that's our broadcast on a tuesday night. thank you for being here with us. i'm brian williams. and we hope to se