Size and Scale
Just as with depth and distance, the physical size of things can be difficult to tell from a photograph alone. For this reason, it can be helpful to provide some sort of scale object that gives the viewer an idea of just how big, or how small, the subject really is.
Of course, this is not necessary with all, or even most, subjects. Many of the things that we photograph are sufficiently familiar to every viewer to ensure that they don’t need any guide to their size. For example, if you see a picture of a still life, you make automatic, and fairly accurate, assumptions about the size of the bottle of wine and the apples that have been arranged.
When photographing less familiar objects, it can pay to provide some way for the viewer to gauge the proportions of what they are seeing. In scientific photography, such as might be used on an archaeological dig, the solution is simply to lay a ruler beside the subject that is being documented, and include this within the frame. In artistic and everyday photography, a more subtle approach is called for.
In order not to ruin the composition, a subject of a known size is used to act as a visual comparison. People are frequently used, for example, to provide a sense of scale to a rock formation or a building. This is particularly useful if the subject is unusually large—you can reveal the true height of a tower
more easily if you can see ant like people on top of it.
However, it is important to realise that this trick only gives a general idea of scale—and can be used to completely mislead. You can only judge the size of one subject compared to another if they are the same distance from the camera—otherwise the farther one will look smaller because of perspective. If you put the person significantly closer to the camera than the main subject, then it becomes much harder for them to be used as a scale. However, you may not be able to pick out a passer-by who is close enough to the building you are shooting, or it may not be feasible to get a model close enough to a mountain—by necessity, the scale will be less than accurate. Nevertheless, this technique is still often useful to give an approximation of size.
The huge size of the green hedge in the gardens of the palace of Versailles is evident from the fact that it includes the human element. Otherwise, one may get a somewhat similar perspective of a smaller hedge using a wide angle lens from a very low view point.