At the Cannes Film Festival, sunglasses are indispensable, whether they're worn by actresses hiding bad eye surgery, action stars trying to convey mystique or film directors wanting to act nonchalant after a negative review.
They are also a semiotic shortcut: a way of declaring the wearer has the ice-cool glamour of Grace Kelly or the roguish attitude of Jack Nicholson. Men in particular are becoming more knowledgeable about the icons – or, as current fashion-speak has it, "guy-cons" – they want to evoke.
A perennial favourite, Steve McQueen is particularly popular at the moment as fashion moves in a more obviously manly, rugged direction. When the eyewear group Luxottica, which produces sunglasses for many of the major brands, introduced a limited-edition version of McQueen's trademark Persol 714 shades earlier this year, they attracted a waiting list then sold out within two days.
It's not only celebrities who pay this kind of homage. Gareth Scourfield, senior fashion editor at Esquire magazine, says he bought his Ray-Ban Clubmaster classics "after I shot a fashion story inspired by the late great JFK, a man who is unnervingly cool". Scourfield says that he would like to branch out into aviators but, "they don't suit my face shape. I can't help but think I look more like Tweety Pie or Mr Potato Head – not a look I would particularly like to endorse this summer".
In more conservative careers, classic styles are preferable. London barrister Peter Stevenson of Stone Chambers, Gray's Inn, says: "On a sunny day you could probably get away with a pair of Ray-Bans but mirrored lenses are out of the question. Barristers need at least to give the impression of being trustworthy; concealing your eyes is not the best way to achieve that."
Sunglasses aren't a subtle prop; they demand bravado, because it's pretty obvious what the wearer is trying to do with them. Victoria Beckham wears oversized frames to remind her public that her days of heavy lipliner and unnatural-looking hair extensions are behind her, and that her new soignée look puts her in the same category as Jackie O and Audrey Hepburn. Unfortunately, as a reference it's about as nuanced as watching Sideways and declaring yourself a wine buff.
Sunglasses shorthand is by no means limited to celebrities. When Vladimir Putin, Russia's prime minister, embarked on his shirtless showcase of horseriding and submarining in 2009, a conspicuous pair of mirrored wraparounds were an essential part of a macho display that recalled – bizarrely – the American action heroes of the cold war era.
Musicians famously use sunglasses as a prop to telegraph that they are at the cutting edge of popular culture. The stylist Nova Dando, who works with singer La Roux, says: "Sunglasses are really important to a singer's image and when they are surrounded by paparazzi flashing their cameras at them it helps to have a shield." Dando also thinks that "it helps make every photo look good because singers don't have to engage with the photographer's lens to get the perfect image. The face looks good from most angles in sunglasses." However, singer Mr Hudson, who has collaborated with Jay-Z and Kanye West, cautions against wearing sunglasses indoors: "I don't wear shades that often on stage as I think they form a barrier. People pay to watch you perform and I think eyes are an important way to connect."
Folk singer Devendra Banhart is the face of eyewear brand Oliver Peoples' summer campaign, and Ray-Ban's marketing strategy invokes the musicians who wore its designs in the past – Iggy Pop, Jeff Beck, Eric Clapton – and those who wear them now. Music festivals are the perfect forum for promoting the right image, according to Ray-Ban brand director Sara Beneventi. "We selected Isle of Wight Festival for the UK, Coachella Festival and South by Southwest for the US, and Primavera Sound for Spain. The common element between them is that they are fancy, niche, indy-rock-music-focused; more in line with the core values of the brand."
Fortunately for people who don't suit Ray-Ban Wayfarers, which were practically the only style you'd see at any music festival for about three years, there is more of a variety of styles this season; and that means more easily classified tribes. For women, the most directional shapes are cat's eye frames, small round glasses and kooky, quirky glasses in the Dame Edna, Peggy Guggenheim mould. Prints, from florals to checks, are another key theme. In the offices of Grazia magazine, an incubator for trends, fashion news and features editor Kay Barron says: "It's all about round, ideally with a coloured frame. Asos is doing great pairs that are cheap and in a wide range of colours." Her own frames are also round, "a black mock-croc style by Illesteva, the It brand of the summer".