Scale

A photo can be displayed at almost any size. This means you don't know how big the depicted objects are in the real world. Sometimes this is the whole point of the photo.
This morning when I woke up, I found a big spider in my bedroom. I decided to remove it, so I caught it under a glass. Then I wanted to show my European friends what interesting wildlife we have in Malawi, so I made a photo of it.

The photo does not show how big the spider is. But the only clue you have it the glass, which could also be any size. I added a matchbox, we all know the size of a matchbox, so now you know the zsize of the spider. When I saw the photo on my lcd screen I realised it had the added advantage of adding a splash of colour, which emphasised the drab grey brown of the spider.