John Ruskin, the British art critic, declared in 1851 that “a photograph is not a work of art, though it requires certain delicate manipulations of paper and acid”. Ruskin’s dismissive view of photography was typical of his era. Although many recognised that technical skill was needed to execute a photograph, few believed in its artistic potential. In the 1850s, four photographers – Oscar Rejlander, Clementina Hawarden, Julia Margaret Cameron and Lewis Carroll – quietly began to create works which would wholly rebut this view. You need only to see their intimate, expressive pictures, currently on display in “Victorian Giants: The Birth of Art Photography” at the National Portrait Gallery in London, to see just how wrong Ruskin was.

Each of these master photographers had their own style. Rejlander’s work possesses a quiet grace, Cameron’s a brooding sensuality. Carroll’s photographs display an intense curiosity about his subjects, while Hawarden’s are striking explorations of the female psyche. The development of new photographic techniques certainly helped these artists to achieve this sensitivity. Where older methods like the daguerrotype required the sitter to remain still for ten to 90 seconds, the wet-plate collodion process, invented in 1851 when Frederick Scott Archer discovered that you could apply a different cocktail of chemicals to the photographic plate, reduced the wait to a few seconds. Photographers could now capture a fleeting moment or even an expression passing across a face. But these images display more than technical acuity. These artists deftly manipulated their medium to produce photographs as capable as any painting of pinning down emotion and human experience.

“Non Angeli, Sed Angli” (1857) by Oscar Rejlander

Exchanging ideas and inspiration, sending each other negatives and collecting one another’s pictures in albums, the four photographers formed a casual group. Oscar Rejlander, a Swedish expatriate, was in some ways the leader of the pack, acting as a teacher and mentor. His skill is demonstrated by this image, which was inspired by the two cherubs that rest at the bottom of Raphael’s “Sistine Madonna”. The title playfully inverts Pope Gregory’s famous quotation, “Non Angli, sed angeli” (“Not Angles but angels”), to emphasise that these are ordinary English children. Given the considerable time it took to take a single picture, successfully photographing fidgety children was seen as a particular feat of technical brilliance. The expressions on their faces are inquisitive and lively, making Raphael’s putti look positively bored by contrast.

“Mountain Nymph, Sweet Liberty” (1866) by Julia Margaret Cameron

Cameron’s works exemplify the psychological depth that photography could now achieve. In this arresting image, the eyes of the sitter, known only as Mrs Keene, stare out directly from the centre of the page. Simmering with emotional intensity, the photograph, whose title derives from a poem by Milton, seems to change before your eyes, the subject’s expression by turns melancholy and defiant. That Cameron’s portraits have been compared to Rembrandt’s is no surprise: both powerfully capture their subjects’ inner lives.

“Charles Darwin” (1871) and “Self Portrait – Surprise” (1871) by Oscar Rejlander

Scientists admired photography’s precision. In 1871 Charles Darwin was searching for pictures to include in his book, “The Expression of the Emotions in Man and Animals”. After failing to find suitable images, Darwin met Rejlander and together they trawled through the photographer’s archive of negatives to find pictures of people expressing the gamut of human emotion. Darwin also commissioned several photographs from Rejlander, including a number of portraits. In the charming “Self Portait”, Rejlander theatrically demonstrates what it looks like to be surprised. These pictures illustrate how, even though they are often seen as opposites, art and science can come together in photography.

“Edith, Ina and Alice Liddell” (1858) by Lewis Carroll

The children in this group portrait by Carroll were the daughters of his friend Henry Liddell, the Dean of Christ Church College, Oxford. The personalities of these three sisters shine through in this striking portrait. Shy Edith (right), the youngest of the three, cuddles her sister. Ina, the eldest, appears serene and authoritative, with her arms slung protectively around her siblings’ shoulders. Meanwhile Alice, Carroll’s favourite and the inspiration for “Alice’s Adventures in Wonderland”, draws us in with her hypnotising, curious stare.

“Photographic Study, 5 Prince’s Gardens” (1863-4) by Clementina Maude

This enigmatic picture is charged with mystery. Only the sitter’s reflection looks towards the viewer, gazing out as if from another world. Hawarden was fascinated by the idea of the doppelganger and often drew on the symbolic power of the mirror in her work. In this image, the fingertips of the subject and her shadowy double seem almost to touch, perhaps to communicate the distance between our outward appearance and our unknowable inner lives.

Victorian Giants: The Birth of Art Photography National Portrait Gallery, London, until May 20th