I'm ready for my touch-up... the secrets of Photoshop unmasked

Glossy magazines and advertisers that retouch photographs of models and celebrities should publish a score alongside each digitally enhanced picture to alert readers to the extent to which an image has been manipulated artificially, scientists have suggested.

The researchers have developed a computerised "metric" that can automatically quantify from "1" to "5" the extent to which a digital image has been enhanced so that readers can assess how close a photograph is to reality.

Health organisations are increasingly concerned about the growing trend towards the digital enhancement of photographs to make subjects look younger, slimmer or more physically perfect and alluring than they really are. They believe it promotes unrealistic expectations of body image among young people, especially girls. Some want the technique banned completely.

Professor Hany Farid and Eric Kee, computer scientists at Dartmouth College in New Hampshire in the US, believe an outright ban is unrealistic as glossy magazines have always tinkered with photographs. Instead, they want to introduce a system that identifies and scores the most dramatic changes.

Their system scores a "1" when there is little retouching and "5" when there are significant changes. The computer programme they have developed looks at the image in terms of geometric changes, which include slimming of the legs, hips and arms, the elongating of the neck or the enlarging of the eyes. It also assesses the photometric alterations, which affect skin tone and texture and are used to eradicate wrinkles, cellulite, blemishes, freckles and dark circles under the eyes.

"We start with before and after digital images from which we automatically estimate the geometric and photometric changes, effectively reverse engineering the manipulations that a photo retoucher has made," Professor Farid said.

"You need to distinguish between simple retouches to photographs that have always occurred, such as colour correction and cropping, to really extreme forms of enhancement, such as elongating a figure to make it appear slimmer, that can affect the perception of body image."

The rating system was devised through the analysis of a diverse set of 468 original and retouched photos. Each image and its enhancement were assessed for the level of retouching that went into them, and then volunteers were each asked to score from 1 to 5 how similar to one another are each original and digitally enhanced photograph.

The scientists found a remarkable similarity between their computerised assessment and the subjective judgement of the human observers, suggesting it was possible to use the technique to produce scores that could be published alongside the image when used in advertisements and fashion magazines.

"We propose that the interests of advertisers, publishers and consumers may be protected by providing a perceptually meaningful rating of the amount by which a person's appearance has been digitally altered," the scientists write in the journal Proceedings of the National Academy of Sciences.

"When published alongside a photo, such a rating can inform consumers of how much a photo has strayed from reality, and can also inform photo editors of exaggerated and perhaps unintended alterations to ... appearance," they say.

Darkroom arts: the camera obscurers

The process of retouching pictures, which has become known by the generic trademark "photoshopping", has been around a lot longer than Adobe's much-maligned computer program. Indeed, it has been around almost as long as photography itself.

It is seen, however, as a much subtler process than is often used today. Contrast and light were commonly manipulated in the darkroom in order to divert attention from imperfections in a picture, such as overexposing a subject's face to hide blemishes, or to bring the foreground out of the picture by lighting it up. The "dodging" technique (blocking light from an area of the picture during exposure) is used to lighten it. The opposite process, "burning", is used to darken the image. Technicians who want to completely alter images can paint directly on to a negative.

Masters of the art can often command six-figure salaries and Pascal Dangin, recognised as the premier re-toucher of fashion photographs, has become notorious for his work with Annie Leibovitz, among others.

At the other end of the spectrum, however, are the simplified versions of Photoshop-like programmes which allow families to remove stray thumbs and passing pedestrians from their holiday snaps.

Kevin Rawlinson

Before and after: an expert's view

1. Twiggy Photoshop lets you take years off someone's age. Here they've lost wrinkles in the skin, taken out bags under the eyes and brightened the eyes and teeth. Flesh has been cleared of freckles and blemishes. The eyebrows look airbrushed and smoothed out. They've left some lines to give a bit of shaping to the cheeks but lost the bagginess underneath the cheeks and taken out the lines in the neck.

2. George Clooney

The lines on his forehead and around his eyes are softened, as are the wrinkle on his cheek and the lines under the cheekbones. His grey hairs have been darkened and blemishes are gone from the forehead. They've colour corrected the image which adds more shadow, more range of colours. It brightens the image.

3. Plus-size model The obvious change here is the waistline. The arms and legs have also been made thinner. Her left arm almost seems as if it's been elongated. And they've shrunk her tummy, pulling in the bump.

4. Kim Cattrall Wrinkles around the eyes have been taken out. They've smoothed the ageing lines on the facial tones. Looks like it's been softened – you're losing the harshness in the lines. It's far too yellow. She's be de-aged, making her look younger.

5. Angelina Jolie

Dimples have been taken out. They've cleared her skin of impurities. A freckle has been removed from her cheek and they've softened the lines on her neck.Defocusing on the skin takes away any blemishes. As soon as you take it out of focus you lose the harshness of lines and blemishes. The eyes have been cleaned: they taken some redness and veins from the eyes. Eyes have got whiter and stand out more – they've upped the contrast, making the details sharper. The lips stay the same.

John Bowman,picture retoucher