Life In Cold Blood - the name of the current series - began in 1979 with Life on Earth.
Speaking of the close-up black and white shot, Sir David said: "I haven't been shot so intensively since I was photographed for Madame Tussauds."
The final episode of the series will be broadcast on Sunday at 1600 GMT.
Photographer Richard Boll, who shot the portrait, said: "I was about 80% certain of what I was after when I started the shoot. After that it was a question of getting an expression that struck the right chord.
"When you're producing a portrait that is as close up as this one, all the details of the expression count, and consequently I took over 300 exposures to ensure I got what I wanted.
"I was grateful for his patience, which allowed me to get a portrait I am happy with," he added.
In a nationwide poll conducted last year, the natural history presenter was named Britain's top living icon, beating musicians Morrissey and Sir Paul McCartney.
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