Introduction
Arguably the most important aspect of drawing is line, after all, everything is composed of lines, even a heavily shaded area is made up of lines. Now, the same rules of composition apply to photography as with all visual art so line is equally important in a photograph as a drawing. We just don't tend to think of photographic composition in terms of line so much.
Crossposted on my blog:Composition – Line | Minimalist Photography
One possibility is that photography is subtractive, we need to take a complex visual world and simplify it whereas the painter or drawer is building their visual world from the ground up, i.e., it is an additive process. As shapes and tones are made up of lines the drawer has no choice but to think in these terms. The photographer though doesn't. The line is the start when working with pencil or charcoal but this is not the case for the photographer.
I often wonder if many photographers, who have a much more developed eye than I do, miss out because they don't do the final reductive step, which is to reduce the image to line, to compose a two dimensional image from one dimensional elements. I need to state a bais at this stage, I tend to reduce everything, it is the way I deal with the world and my photography is no different. Art is the extension of the artist, never a bolted on extra.
This reductionist approach has advantages and disadvantages. The advantage being that it gives me a handle on very complex ideas and subject matter. The disadvantage being that it leads to a tendency to over simplification - some things are complicated! The only reason I mention that is in case any photographers are reading this and bristling at my conclusions.
What do lines do ?
This may seem like a weird question, one answer being that they create shapes and enclosing space. This is completely valid and leads to discussions of geometry, positive and negative space and other compositional points but that is jumping ahead a little. Lines have a huge impact on an image in their own right.
Orientation
Put very simply:
Horizontal lines = tranquility, peace.
Vertical lines = rhythm, liveliness
Diagonal lines = drama, impact
The more emphasis on the line the more it will affect the overall emotional impact of an image. The effect can be further strengthened by echoing the line - having one or more lines run parallel. The weight of the line also affects the emotional impact - this is where tone is really important - it determines just how much attention the line draws to itself. Large contrast in the areas on each side of the line give the line emphasis, subtle contrast reduces the emphasis.
Discussion
It is the combination of lines and the use of tone that determines the overall emotion of the piece, a lot of diagonals with very hard contrast will give a very dynamic and probably quite disturbing image. The next time you are watching a movie and the camera angle goes to 45 degrees and the scene looks like it was lit with a single hard spotlight w the director will not be aiming for a romantic Hallmark moment - more an in your face Pyscho moment I suspect. There is one danger with a very strong diagonal line. Said line is perfectly capable of shooting the viewers eye straight out of the image (possibly never to return) and this, by and large isn't a good thing and the reason why strong diagonals are rarely allowed to extened to the corner of an image.
In reality the whole subject is a lot more complicated than I've just laid out. I haven't looked at lines that mitigate other lines e.g., the idea that a line at right angles to a diagonal at an end will stop the eye from leaving the picture or the effect caused by the angle that two lines meet at e.g. the tension. caused by two lines meeting at a sharp angle - technically known as a point. Also the differences between a curved and a straight line and then a whole book could be devoted to implied lines where, for example, there may be three similar objects in an image. The brain wants to draw lines connecting them. Think of a simple image of three birds in flight against a featureless sky - the mind will see the triangle before it sees the detail on the birds.
I also have my doubts about much of the theory and suspect that like a lot of art theory it is more conventional wisdom than hard fact. The best and by far the most gifted visual artist I have known in my lifetime eschews the whole idea that it is possible to have rhythm in a still image, that the term is meaningless yet art theorists the world over will give whole lectures on the rhythm created by the implied lines from one figure to another in the work of El Greco.
I am sure that some reading this will be referring back to the famous Manet quote that states that there are no lines in nature. While this may be technically true we need a model for discussing these ideas so the line model is as good as any. And that is as esoteric as I want to get on this subject.