Our eyes clearly pick out the shape of a human face painted by artist Chuck Close. But zoom in and you will see that the picture is actually a clever construction of small, square shapes.

Just what you see would see in an art gallery would depend on how close you stood to the picture - you would need to be several metres away from the portrait to get the "big picture".

This sort of image has been used to great effect by artists down the years. You may recall the picture of the infamous child killer Myra Hindley that was vandalised in London's Royal Academy. Her face was made up of hundreds of individually-painted, little hands.

But how do these images work? How do we recognise the bigger picture in the smaller shapes, and what does all this tell us about the way we see things?

Tape measure

Many scientists have assumed that we perceive things the same way at all sizes. You would recognise your friend in the same way, for example, whether he was off in the distance or looming large right in front of you.

Marcus Harvey's portrait of Myra Hindley was composed of tiny hand prints

But after taking a tape measure to a Chuck Close Retrospective, vision scientist Denis Pelli challenges this explanation.

The New York University researcher says the dramatic effect of viewing distance on the appearance of Close's paintings shows that perception of shape depends on visual size.

Pelli conducted "nose tests" on 33 Close paintings to measure the transition from the grid of small shapes to the big picture of the face.

Critical distance

Observers were asked to move forward and backward in front of paintings and say when they the nose emerged from the canvas.

These "critical distances" were recorded for each observer. Pelli also measured the size of the blocks making up the painting, and calculated the angle from the observer's eye at the critical distance.

To his surprise, this critical angle was always about 0.3 degree, for all the paintings, despite their great variety in face size, block size, and number of blocks per face.

Pelli says it is clear from Close's work that a shape's size can dramatically affect how we see it.

A Chuck Close retrospective is now at the Hayward Gallery in London, 22 July to 19 September