A photograph always works best when it has a single clear subject that is well defined. That isn't to say that you can't have a complex scene (like this one) and still emphasize just one subject (the Christmas decorations in the tree), but you have to find a way to make that subject dominate its background. One reliable way to make a subject pop, of course, is by limiting depth of field (near-to-far sharpness) and thereby restricting focus to just one zone of the photo. Three factors go into how much depth of field any photo will have: the lens aperture you're using (the wider the aperture the less will be in sharp focus and vice versa), lens focal length (the longer the focal length, the less that will be in sharp focus and vice versa) and subject distance (the nearer you are to your subject, the less will be in sharp focus with any given lens at any given aperture). OK, that's the mini lesson in depth of field.
But even restricting depth of field with two of those factors doesn't always have enough impact. I took the shot above of a neighbor's tree after an ice storm last week. It wasn't a serious photo, I was just chatting with a few neighbors and popping off some shots. But what I wanted was for the decorations to really jump out against the background and not get lost in the tangle of ice-covered branches. And to some degree, I got that--but not quite as dramatically as I would have liked. Two depth of field factors were at work to give me shallow focus: I was using a Nikon 70-300mm f/4.5-5.6G ED IF AF-S VR Nikkor Zoom Lens for Nikon Digital SLR Cameras set at 112mm (equivalent to 168mm in 35mm terms--which is a pretty long lens) and I was shooting nearly wide-open at f/5.6. For most subjects that would have restricted depth of field to a very narrow zone--and, in reality, it did--the sharpest focus is limited to the ornaments. But because I was about 30' away from the tree, I didn't have the third factor at work: I was a little too far away from the subject to really knock the background out of focus. Unfortunately the snow was about four feet (yes four feet!) deep between me and the tree and there was no way I was wading into that mess!
So, while I had the right lens and the right f/stop, the long lens compressed the space (a trait of long lenses) between the main subject (the ornaments and foreground tree) and the background trees and branches into a bit of a visual mush. Had I gotten physically closer (probably 10' closer) and isolated just a few of the ornaments I could have easily tossed the background completely out of focus. Or I could have zoomed out more, but from where I was standing I couldn't find a good composition using more lens.
The photo is OK, it's pretty, but the photo I was really seeing in my mind's eye would have required the third factor: getting closer. And that would have required a fourth factor: a photographer willing to stand up this his chest in snow to get the shot. That day, it wasn't me!