Triangles

In photography triangles are the most widely use shapes, whether implied or actual. They are easy to form, because obviously only need three points, and they are also formed by the effects of perspective which occur quite readily in photography especially when using wider lenses.
   Triangles are unusual because combine dynanism with their diagonals but also can be stable if one side is grounded by a level base ( Freeman,M. 2007)

Gates

    The above image has a subject containing triangles, a lot of built structures like this contain triangles because they contain great strength. The picture also includes several implied triangles formed by the shooting angle. It may not be the most picturesque image, but as Robert Frank said " you can photograph anything now ". ( Sontag, S.1977 )

Perspective triangle pointing up

     This picture shows how easily triangles can be formed by the action of aiming your camera up with a wide angle lens attached. I like the overall clean blue tone of this image and the panel lines enforce the triangular effect and add to quite a dynamic image.
     More difficult is to form a perspective triangle facing down, obviously if you could get high enough you could look down on a building.

Hard at work

   The book in the image above is a downward facing triangle, there is also a triangle formed by the feet and an upward facing triangle formed by the arms and head.
   When making still life images you can decide where you want the apex of the triangle to form which can help you to compose the elements of your still life.

Triangular still life with apex at top

    I have tried to make the above still life as triangular as possible and it has the tightly composed feel of the early photographers such as Hippolyte Bayard (1801-1887) and Roger Fenton (1819-1869) who were influenced by the painting tradition prevalent at the time. ( Martineau, P. 2010 )

Still life with apex at bottom

    The apples and plums above have a downward facing apex and is a deceptively simple composition, but it takes considerable effort to get the feeling of balance within the image.

   Ref: Freeman, M. 2007,The Photographers Eye,ILEX,Lewes
          Sontag,S,1977,On Photography,Penguin Books,London.
          Martineau,P,2010,Still Life in Photography,Getty Publications,Los Angeles.