Most of the obits and commentary today have been about Snowdon as Royal photographer, but he was much more than that.
This is taken from The Times obit.
....Snowdon was also a man of great drive and talent. He was one of the pre-eminent photographers of his age, introducing an informality and dynamism to portraits of stars and royalty and exposing social ills in BBC documentaries and photo essays for The Sunday Times. Stricken by polio as a teenager, he used his royal status to campaign relentlessly for greater recognition of the needs of the disabled and handicapped. He was a talented designer — the Snowdon aviary in London Zoo being one of his proudest accomplishments....
..... At the age of 16 he contracted polio while on holiday in Wales and spent six miserable months in Liverpool Royal Infirmary, emerging with a withered left leg and a limp that he never lost.
However, his uncle, the flamboyant stage designer Oliver Messel, made his convalesence more bearable by arranging visits from Noël Coward and Marlene Dietrich, who sang The Boys in the Backroom for him.
Eton also nurtured his interest in photography and design. He revived the school’s photographic society, built a rudimentary enlarger out of soup tins and made illicit crystal wireless sets that he sold to schoolmates for half a crown each......
..... He began taking pictures for The Sunday Times, particularly its new colour magazine. Many of his subjects were famous — Francis Bacon, David Hockney, Alec Guinness — but he also produced acclaimed features on social issues such as children in poverty, the loneliness of old age, the homeless and the mentally ill. He relished the freedom he was given. “I could do long, serious pieces with the great luxury of 50 pages on India or 12 pages of Rudolf [Nureyev] and Margot [Fonteyn] in Vienna,” he recalled. “It also gave me the chance to illustrate social problems that people had tended to sweep under the carpet.”.....
.... However, the cause to which he remained dedicated throughout his life was improving the lot of disabled people, not least because of his experience of polio. His maiden speech in the Lords was on the problems that the disabled suffered in everyday life. He served as a trustee of the National Fund for Research into Crippling Diseases, with the Polio Research Fund, and on commissions that examined the needs of disabled people. He was president for England of the International Year of Disabled Persons in 1981.
Snowdon used his status to force the Royal Horticultural Society to admit wheelchairs and guide dogs to the Chelsea Flower Show, British Rail to improve access to trains for disabled people, and the Hospital for Incurables in Putney to change its name to the Royal Putney Hospital (now the Royal Hospital for Neuro-disability). In 1984, using his fees from royal photographic commissions as seedcorn, he set up the Snowdon Award Scheme (now the Snowdon Trust), which helps disabled people to benefit from further education.
Earl of Snowdon | Register | The Times & The Sunday Times
makes it sound like they were a blended family. Both David and Sarah were Tony's. 
