Do photographs depict the truth? This thorny question has dogged the medium from its earliest days, and now, living in the age of fake news, most of us likely think we are savvy enough not to be easily fooled by photos, well aware that the medium is capable of manipulation or deceit. And yet, there remains an instinct to believe. Photography’s power can be subtle, and its emotional heft makes it easily confusing.

This question lies at the heart of Thomas Ruff’s art. His works roam across every field of photography, from portraiture to reportage, architectural images to porn. What unifies them is that in every piece he forces us to raise questions, about what photography makes us feel and how it can beguile and hoodwink us.

The Whitechapel Gallery in London is currently showing over four decades of Ruff’s work, and the exhibition serves as a masterclass in both the challenges and charms presented by photography.

Early in the show is a selection of Ruff’s famous colour portraits taken of his peers at the Düsseldorf Kunstakademie in the 1980s (where Andreas Gursky and Thomas Struth were also students, taught by the conceptual artists Bernd and Hilla Becher). These are passport-style photographs writ large, each blown up to over two metres in height.

The series brought Ruff immediate attention, and still now they prove captivating, offering as they do, the opportunity to examine every detail of another’s face. Unlike Bruce Gilden’s series of similarly sized close-ups of men and women who have been ravaged by life, Ruff’s subjects appear impossibly youthful and untouched and we are thus invited to project our own fantasies and presumptions upon them. And, looking at them 30 years on, to wonder where they are now, what has become of these unlined faces, frozen in time.

Other works directly ask us to examine how photography can affect our understanding of a situation. Nacht (Nights) is a series of works created in the mid-90s, in the wake of the First Gulf War, and shows the suburban streets of Düsseldorf shot with a low-light vision camera that bathes the scenes in a eerie green glow, reminiscent of the war scenes that were endlessly being played on TV at that time. The innocent, average streets immediately become both ‘other’ and a target.

In the series ‘jpeg’, Ruff tackles how the internet has affected our relationship with photography. Here he takes images of both natural and manmade disasters and blows them up to a monumental scale, highlighting the pixels that form digital shots. In this blurred and low-res form, the horrors are lessened, and Ruff comments on how our constant exposure to images of disaster online distances us from their reality.

The internet is examined further in the series ‘nudes’, where Ruff looks at the proliferation of porn on the web. By taking shots from online, blowing them up and placing them in a new context, these photographs are by turns comical and appealing. In the titling of his works, Ruff often appears to be blandly descriptive yet here he is also aiming to connect the shots to the art historical understanding of ‘The Nude’, a genre, of course, with far more respectability than internet porn.

A number of recent series sees Ruff no longer creating new works, but working with archive photography. Early in the show is a series of old black-and-white photographs of an early Jackson Pollock show in New York, which Ruff has manipulated by adding splashes of colour, giving the images a surreal, unsettling quality.

In another series, ‘negative’, Ruff takes a number of 19th century portraits of members of upper-class Indian society, scans them and turns the sepia tones into blue and white, utterly changing the impact of the scenes. And in some of the most striking images on show here, Ruff reprints negatives from a catalogue of machines and engineering parts from the 1930s, reproducing the objects large and emphasising their metallic beauty with dark hues.

Ruff’s work is simultaneously a love letter to photography, and its ability to capture our world and fix it in time, as well as a ruthless examination of the medium’s ability to manipulate and confuse us, and to offer up multiple truths. It is a timely reminder, needed in our current media climate as much as ever, that we must never forget to question what we see.

Thomas Ruff: Photographs 1979-2017 is on show at the Whitechapel Gallery in London until January 21; whitechapelgallery.org