“The laws of Nature are written in the language of mathematics…the symbols are triangles and other geometrical figures, without whose help it is impossible to comprehend a single word.” Galileo Galilei
Using triangles in photography and paintings is a classical rule of composition that dates back to the time of the Renaissance’s masterpieces. This timeless arrangement gives harmony and peace to the image, while the symmetry brings a sense of clarity and balance. [source]
"All cracked up" by Fairlie Atkinson
While a triangle pointing up can give a sense of overall stability, an upside down or titled triangle will instead introduce tension and instability. Now, don’t run out looking for triangular shaped objects, as triangles can often be just implied in your pictures, for example by arranging the subjects of a portrait in a way to create a triangular composition.
A symmetric triangle will give the image a sense of balance and harmony, while an asymmetric one will result in a more dynamic and energetic composition.
"Sunstrike" by the67
Triangles can be used even in landscape photography, they can help to create stability within the picture by organising the composition around a basic shape easy to recognise and understand for the viewer.
"The Island, Karekare" by Paul Kettel
Another way you can you use triangles in your landscape shots is to use their vanishing points as leads, for the viewer’s eye, to get even more into the scene.
There’s plenty of options to play with triangles with your camera as they really are all around us. It doesn’t have to be an actual triangle, notice how often cropped squares or rectangular objects make triangles almost anywhere you look.