First one must contrast titles with captions.
Titles are brief one liners that need not have any grammatical structure.
Captions can be a short sentence of one or more lines, often also called a sub-title..
The terms are often used interchangeably but a photo can have just a title, just a caption or both.
The purpose, in both cases is
Firstly to guide the viewer to see the photo in a particular way.
Secondly to supply some context.
Thirdly to provide additional, explanatory, helpful information. Though this is usually done in a caption/sub-title.
Additionally we use titles to evoke a suitable response to the photo.
Examples could be:
- contrast, contradiction, cognitive dissonance
- pique curiosity
So, in creating title/captions we need to answer the following questions:
- How do we want the viewer to see the photo?
- Does it need context for understanding?
- Will additional information be helpful?
- What reaction are we trying to create in the viewer?
Finally the title gives us a small insight into the mind of the photographer.
Cato the Elder said it best - "Rem tene, verba sequentur" (grasp the subject and the words will follow).
Manual of Style (captions)
The Caption Machine