Photo shows House of Windsor's future secure

Official 90th birthday portrait of Queen Elizabeth II with Prince Charles, The Prince of Wales Image copyright 2016 Nick Knight
Image caption In the photo, Prince Charles appears to smile at the Queen, who looks directly at the camera

The most arresting images can be the ones that leave you wanting to know more.

In this instance, what was the wisecrack uttered by the son that made the mother want to laugh as the camera captured them together?

On a day in May devoted to horses this photographic assignment was over in minutes.

The significance of the image is obvious. This is a 90-year-old monarch with her heir by her side. As things stand, the future of the House of Windsor is secure.

One of the most recognisable women on the planet has sat, countless times, for photographs and portraits.

She never comments on the finished results. Others aren't so reserved.

An oil painting by Lucien Freud was said by one art critic to have given the Queen six o'clock shadow and made her look more like a rugby player than a monarch.

This latest addition to the genre leaves us wondering about the nature of the joke cracked by Prince Charles; a joke the four people in the room when the shutter clicked may well now have forgotten.