A picture paints a thousand words... The following are screen captures of Lightroom's extremely useful crop grids (known as overlays). Until today, I thought there was only one available: the rule of thirds. Thanks to Marsha, I have now learnt that you can cycle through several, very useful guides for composition. I hope you can make them out here,
Incidentally, the image I use here was inspired by Gursky's Rhein II (the most expensive photograph in the world), which also uses one of these rules. Obviously for copyright reasons, I can't use his $4 million picture.
The first rule is about how the image is built. Most cameras offer a default 4.3 or 3:2 ratio. This means that the longest edge (before cropping) is four-thirds longer than the shortest edge. Sometimes this will be three halves or 1.5 x longer than the shortest edge.
Some cameras offer a 16:9 format (the longest edge is 16 ninths longer than the shortest one). This equates roughly to what humans see with both eyes open. Our field of vision is wider than it is taller and the proportion is... 16:9.
Then we come to crops.
Why crop or use composition rules? Because perfectly balanced images tend to look flat. As we're dealing with a two-dimensional representation of a three-dimensional world, anything that adds depth (DOF, contrast and composition rules) usually helps the image stand out.
The first one is a 1:1 crop (the image height is the same size as the image width). These are notoriously difficult to compose, perhaps because humans see in rectangles, not squares. When you get it right, however, the effect is stunning.
By far the best well known one is the famous "rule of thirds", shown here:
The image is divided into equal thirds and two horizontal and two vertical lines are drawn. It is considered standard practice to place a focal point (the most noticeable feature in a landscape, for example, or the subject's eyes in a portrait) at one of the intersections, usually (but not always) the upper ones because people tend to start looking at the top of an image and work their way down.
(BTW, you can't see the lower horizontal line in the above example because I composed the image so that the horizon would be exactly on it!)
The second is the golden mean, as shown here:
The golden mean is similar to the rule of thirds, but far more subtle. It requires a formula that no one can really apply when taking the shot, but can be applied in post-processing.
The third is the diagonal emphasis, as shown here:
This forum thread is a work in progress. It's very late and I need to get some sleep, but I will return to it tomorrow.
In the meantime, if you use Lightroom, you can apply an overlay and crop according to the grid lines in the Develop module. Use the the crop tool (the dotted rectangle at the top) and press the letter "o" to cycle through each one. Thanks again to Marsha for this invaluable tip.
I repeat my recommendation for a great how-to book on composition: Michael Freeman's The Photographer's Eye (2007), which explains all these concepts very well - but rather dryly.