There she stands on the posters: a teasing front-of-shop sylph wearing nothing but a wisp of diaphanous gauze, a necklace of gold and a lascivious come-hither glance. Lucas Cranach's Venus is as boldly coquettish as any high-class courtesan. She is meant to be. And so it's hardly surprising that a couple of weeks ago she caused a kerfuffle on the London Underground.

Advertising standards, apparently, dictate that hoardings may not “display nude or semi-nude figures in an overtly sexual context”. And, of course Venus flouts them. To suggest otherwise would be rude to her professional skills. Officials may have capitulated after a high-profile spat and, bowing to outside opinion, allowed the goddess to feature. But there's no getting away from it. It's like putting a squirming pole-dancer on the poster.

Cranach certainly meant her to look alluringly erotic. His composition caresses the sinuous curves of her (nowadays, admittedly, rather unfashionable) figure. His colours emphasise the foxy smile that hovers about her pursed lips. A hair-fine brush picks out the rosebud points of her nipples, strokes each fluttering lash of her sly feline gaze. Her pale flesh glows in the darkness, as tempting as a pearl in a black velvet box. And being small - about the size of a Playboy pull-out - she is perfect for private consumption.

Little wonder that so many people wanted her. And Cranach, as head of a vast and astonishingly prolific workshop, could oblige. His studio was like a production line. “Wherever one turns in every nook and cranny there is a picture,” as one contemporary put it. This was the artist who produced at least 50 painted versions of the Adam and Eve story, who received an order for more than 60 copies of a double portrait in a year. As the image of Venus rolled off the production line, it became a bit like the 16th-century equivalent of the hooker's card posted in public phone boxes.

This was great for the punters. They could all have their peek. But art historians tend to be a little more snooty. Cranach may have been extremely successful (a tax return of 1528 states that he was one of the two richest men in the city of Wittenberg), but that ability to paint speedily, to turn out a multiplicity of copies, which helped to put him financially so far ahead of his fellows, has prejudiced the scholars of more recent periods against him. They have often dismissed him as the maker of slickly fashionable pieces that lack the soul of authenticity or the spark of experimental discovery. And, indeed, when his facility is compared with the superlative talents of such northern contemporaries as Hans Holbein and Albrecht Dürer, he is often found to be lacking.

But Cranach is far more subtle and distinctive than some mere “also-ran”, and he cannot simply be dismissed - least of all in this day and age, when popular appeal and mass productivity can put a contemporary at the top of the bill. The winged serpent stamp of his studio was a quality-tested brand. Cranach deserves a proper re-assessment in this country.

In the autumn the Courtauld Institute whet appetites with a wonderful small-scale and typically scholarly show based around its own lovely panel painting of the temptation in Eden. Now the Royal Academy follows up with the first major exhibition in Britain devoted to this artist who, born in the early 1470s in the town of Kronach in Northern Franconia (now Bavaria) and from which he took his name, found success as a court painter to several successive electors of Saxony - a powerful, but often politically precarious, position, which he kept almost without interruption until his death.

With some 70 pictures on display, this show represents - albeit in a rather tokenistic manner - the full range of Cranach's skills. It shows him as the accomplished designer and painter of portraits, religious subjects and devotional pieces (including a colourful triptych of the Holy Family carefully displayed so that its contrasting grisaille flipside can also be examined). It shows him as a master of biblical, mythological and classical scenes as well as a printmaker in woodcutting and engraving whose works included illustrations to Luther's Bible.

As you wander through the chronologically hung display, you watch him finding his artistic feet - both technically and socially. You can see him experimenting with the gory conventions of modish Gothic dramas, with the peaceful sculptural compositions of Renaissance pieces, with the linear precision of Dürer's scrolling graphics, with the intellectual penetration of Holbein-style portraits.

You can watch him perfecting what were to become his incredibly popular renditions of the pinnacled landscapes of rocky northern regions and the wild animals that might have haunted their shadowy woods. You can see him building up the stock of emblems and symbols, patterns poses and motifs that his workshop would later draw on, slotting different variants into their pictures. Venus, for instance, is far from unique. Eve, Judith and Diana, as well as a host of mythological lovelies and contemporary flirts, all have her sly face.

Cranach soon established himself in the circles of rulers, academics and clerics. In Wittenberg, Martin Luther, who became a close friend and a godfather to one of his children, used his presses. And a penetrating series of portraits of this theologian, taking him from Augustinian monk to mastermind of the Reformation (though sadly no farther, since a haunting deathbed sketch is not included) is one of the highlights of this show. Cranach explores the physiognomy of his subject with a physical rigour and psychological depth to compete with those of any contemporary rival. It is a pity that more of his lively preliminary sketches are not included. But what marks Cranach out as a portraitist is his acute consideration not simply of individual appearance but of the role that his sitter plays in society. His paintings double up as public relations pieces. They are about presentation.

Cranach, undoubtedly, was a canny diplomatist. A pioneering portrait of Albrecht Brandenburg hung adjacent to the Luther series makes this very clear. At the same time as this artist was handling Luther's visual PR, he was also keeping on working for his powerful Roman Catholic patrons, including this cardinal, one of Luther's bitterest enemies.

How could he reconcile the decorative adornment of papist beliefs with the high moral purpose demanded by a Lutheran ethos that was becoming dangerously hostile to Catholic idolatry? The entrepreneurial Cranach had an astonishing ability to adapt and invent that left him few rivals in his often precarious era. He created a flattened picture plane that, harking back to instructive medieval images, could combine fleshly pleasures with complex moral instruction.

His pictures play teasingly - sometimes dangerously - with the boundaries of taboo. Spiritual subjects are painted with a sensual glee. Flattened surfaces are embroidered with glittering colour. The formulaic is enlivened by delightfully sharp observations. The reverend and the visceral, the instructive and the voluptuous, all merge.

Heaven knows what effect his divine coquette Venus will have on hapless Underground commuters. With luck, most will be tempted into the Royal Academy. And after that: there are plenty more pictures from this most productive of artists in British collections. For the rest: the brothels of Shepherd's Market are just round the corner.

Cranach is at the Royal Academy, W1, from Saturday (020-7300 5839)

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