Trevi Fountain, Piazza di Trevi, Rome

Photograph Copyright ©2011 Loo Yen Yeo. All Rights Reserved.

Concept: Eye-lines (looking at another object); Diagonal lines (ordering); Cartesian lines (vertical);

Description: In Nicola Salvi's sumptuous baroque creation, the central figure of Oceanus stands in an alcove on a shell reminiscent of Botticelli's 'Birth of Venus', his eye-line directing ours to the rearing mount of a triton in front of him. Eye-lines from the allegories of Abundance (left) and Salubrity (right) reinforce the vector.

Salvi's design utilises a framework of vertical elements: Corinthian columns varying in scale to create a bulge in the middle of the façade with a semi-domed recess; from which spill irregular shapes of statuary, lent dynamism by the also-issuing water.

This image was cropped to emphasise the diagonal of morning light from top-right to bottom left; a line which imposes order on the composition: from the Corinthian capital of the right-most column, to the frieze atop the right-most column of the alcove, then Oceanus and his eye-line, ending finally at the tension in the triton-horse sculptural group.

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03.05 Diagonal lines

Diagonals have more freedom of direction than the Cartesian axes - verticals and horizontals - because they are released from alignment with the image frame.

1. Diagonals are dynamic
Where Cartesians have a sense of stability or being supported against the effects of gravity, diagonals have the opposite - of falling under gravity. Diagonals thus have greater dynamism associated with a feeling of speed.

2. Diagonals convey depth
Diagonals predominate the man-made environments as a result of linear perspective: non-centred straight lines or edges receding into the distance appear as diagonal lines.

Diagonal lines are thus important elements for their ability to direct a viewer's eye: we are programmed to follow the line of a diagonal because we use it for depth determination, and for gauging the speed of movement of an edge falling under the effects of gravity.

As seen in frame dynamics, diagonals interact with the image frame to create visual tension. For single diagonals, visual tension increases proportionately with the angle between the diagonal and the long side of the frame up to a maximum of 45 degrees. For multiple diagonals, visual tension:

• has more 'weight' in the case parallel diagonals because the tension created by one diagonal with the image frame, is reiterated by identical tension from the other diagonals.
• is strongest when there are multiple diagonals at different angles, where there is tension created between the diagonals as well as the image frame.

In practice, parallel diagonals can be achieved from high vantage points using the compressing effect of a telephoto lens. Multiple convergent diagonals can be achieved in low or close positions with a wide-angle lens.