You've probably seen some pretty comical behind-the-scenes images of the kinds of positions photographers put themselves in just to get a shot. They climb trees, hang off cliffs, stand in the middle of rivers, lay down in the dirt, all just to frame up that perfect shot. Well there's almost always a reason behind the madness and sometimes those reasons end up having a much bigger impact than most people might expect. Sometimes it's about getting a really intriguing angle on a particular subject, but I find myself laying in the dirt quite a lot just so I can create a composition that carries more depth. Let's compare a couple different shots that can help make some sense of this.
In this first set of example shots there really is only one single difference between the two images. I placed my subject in the exact same spot within the frame and I processed each image exactly the same way. The only difference is that I was standing up for the first image and in the second image I was crouching down with the camera not more than a foot off the ground. I used the Zeiss Apo Sonnar T* 2/135 ZE lens for each of these shots and was roughly 40 feet away from my subject.
For this first shot, you can see that the foreground doesn't really help us out much. The wide aperture blurs what's right in front of us just a little bit, but most of the foreground leading up to the subject is still mostly in focus. The subject doesn't stand out from the near surroundings as well.
Now, in this second shot, by crouching down and composing the shot a lot closer to the ground I was able to compress the foreground to the point where it is thrown solidly out of focus. By having the compressed, out of focus foreground, I was able to frame the shot so that my subject stands out from her surroundings that much more. This in turn allows the eye to focus more strongly on her since we have eliminated other distracting elements that were in the scene.
For this second set of shots I switched lenses to the Canon EF 70-200mm f/2.8L IS II USM lens which allowed me to move even further away from my subject and that in turn allows me to use the distance to create even more depth. Again, with these two shots, the only difference is the fact that I am standing upright for the first image and then I am laying down with the camera low to the ground for the second image. For both images I am roughly 50–60 feet away from my subject and I am zoomed in all the way to 200mm.
There are several things in this shot that aren't great simply because of how high I am above my subject. Standing up meant that the camera is pointing down at her and really doesn't allow for any foreground use at all. On top of that, it doesn't sell the story as well as it could. Her eye line, from our point of view in this shot, makes it look like she's just bored and staring at weeds.
Contrast that with this second image, where I am laying down prone with my camera not even a foot off the ground. By bringing the camera down to her level, even lower than her eye line, we change the composition to bring more depth into the shot and create more of a story instead of taking a glorified snapshot. With a lower viewpoint we bring more foreground into the shot and our aperture puts it nicely out of focus which makes our subject stand out all the more. The lower composition also gives us a completely different look for the direction that our subject is looking. Instead of it looking like she's merely looking at the plants next to her, it creates a more thoughtful pose, which gives the shot more of a story.
Your Choice of Lens Does Make a Difference
When it comes to making your foreground work for you, your lens choice really does play an important factor. The last thing that you want in a good portrait is to be shooting up your subject's nose, so the focal distance of your lens will be a determining factor when you're framing the shot. For a wider focal length such as a 35mm or a 50mm lens, if you get too close to your subject but keep the camera close to the ground, then you will likely end up pointing up at your subject to a degree. That might help you tie in your foreground, but it won't be as flattering for your subject. Simply put, the longer your focal length, the closer you can go to the ground. Since photography is largely personal preference, I don't have an exact formula for you to play with, but this basic diagram might help. Someone out there probably has a decent mathematical formula for this, but basically we're talking about angles and the distance between the top of the frame to the bottom of the frame.
With this basic chart you can see that the farther away you get from your subject, the less dramatic of an angle there is in relation from top of frame to bottom of frame. This means that you can get closer to the ground to maximize foreground without having to worry about shooting up the subject's nose. The shorter the focal distance, the closer you need to be to your subject, and the higher you have to raise the camera to avoid looking up someone's nose.
It's All Still Subjective
At this point, this really all boils down to personal preference and shooting styles. If you haven't tried shooting lower to the ground, give it a shot, see how you like it. Try playing with your distance between your subject and the distance from the camera to the ground. I think you'll find that simple things, like having your subject stand versus having them sit, will end up playing a decent part in your composition simply because of what the space in the frame allows you to do with your foreground and spacial alignment. The space between you and your subject can really be put to some fun and creative uses. Sometimes all it takes is just moving around to see what ends up working best.