A photo is like a sentenceThat's to say, it should speak on a single subject. Well, sentences are punchier if you do. If it goes rambling on about this and that, it won't be to the point.
If your photo is about a baby's expression or a cat looking superior, get right in and fill the frame with that subject. Don't let anything else into frame that is extraneous, that says 'look at me', even if only quietly.
I was showing one of my photos—(reaching for the skies) — to an abstract painter friend of mine (Martin Bush) and he suggested cutting off the lowest section entirely. He asked me, what's it about?
I said that I was taken by the parallels and angles of the barn and I also liked the way the weeds looked all gritty and grungy against the wall. See what I did there? Two subjects.
The reason that I liked the weeds in the shot (and hence the title, "Reaching For The Skies") was that they were an organic contrast to the regularity of the corrugated roofing and I could picture the weeds being guided by them in their reaching. Sort of... you know what I mean.
Now I'm not saying it's always clear-cut; even this one. Martin is kinda right and I should probably have made two shots, the first "sentence" about the geometry and the second about the lighting on those plants against the wall.
So, to summarise, one photo : one subject.
Comments on exceptions to the rule are particularly welcomed ;)