I want to talk about isolation in your images. This post addresses single subject isolation for effect.
Any good photo has a subject. (I know, painfully obvious statement.) Sometimes the subject is rather obvious, while it may be less obvious and/or more abstract in others. One of our jobs as photographers is to tell the viewer where to look. Good composition does this.
Now, you've surely seen photographers who shoot everything wide open (widest possible aperture on a given lens) because of the bokeh. Yes, bokeh is nice but it can very easily become a crutch. That's because shooting wide open all the time doesn't require a photographer to carefully consider the background. It's thrown so far out of focus that your eye cannot help but go right to the subject, which is often the only in-focus object. Easy. And sometimes just plain lazy. Whether you shoot wide open, stopped down a couple stops or at "f/8 and be there," you should know WHY you're doing it.
1. Choose your backgrounds carefully. To isolate a subject, the simpler the better.
Here is a shot of my son while on a trip to Texas. His great grandmother had given him a new skateboard and this is all he did for the few days we were there. I stayed back to let him play and used a 135mm lens. I shot from a side angle because I liked the shape of the houses behind him and because the background was uncluttered, he stood out well.
I shot this is my son while in Texas. The sun was brilliant and warm. I was shooting close to wide open but I still carefully placed him clear of the sign posts and poles.
3. Get close.
Getting close to your subject can separate him/her from the background. Part of this lies in the fact that the closer you are, the faster depth of field drops off, even stopped down to f/4-f/5.6. One of the biggest benefits this has is proximity to the lens often catches the eye of the viewer more quickly. This shot below captures a quiet moment of a cousin with her puppy. She and the dog share a moment, completely separate of anyone playing around her. The image conveys that.
4. Choose your angle.
Below, you see a girl on a slide, alone in her thoughts. What you don't see is the house behind that slide, surrounding trees, other playground equipment or children playing. The low angle eliminated those distractions and set her apart from them all. The result is another quiet, contemplative image.
The shot below illustrates isolation at its rawest. Simplicity. White walls, a backwards glance, and nothing else. These moments are always both fortuitous and equally intentional. You can't expect them to fall into line regularly and you can't wait until the stars align. As a photographer, your job is to MAKE it happen.
Hope this is helpful.