You can enhance the dramatic mood in portraits using a combination of blurring, blend modes and layer masks in a few easy steps.
I'm using Photoshop CS6 in this tutorial, but you can do the same thing with Elements or Pixelmator. Let's start with our portrait out of camera. I shot this image a few years ago at Rollins College.
Using Photoshop to Enhance Dramatic Mood in Portraits
Here's one of the problems I'm trying to correct in this image. Ideally, I would like to separate my subject from the background in-camera. That requires using a large aperture to keep her in focus while letting the background fall into a blur. I also would have liked to under-expose the background, to provide a bit more contrast between my subject and the background.
This was exposure Hell for me at the time. I had too much ambient light to open the aperture up enough for a shallow depth of field and and not enough flash power to under expose the scene while leaving her appropriately lit. It's slightly under-exposed, but not as much as I would have liked.
This simple technique lets me kill two birds with one stone in post processing.
1: Duplicate the Layer
The first thing you want to do is create a duplicate of the image. Just hit Command or Control-J to duplicate the layer above your original. We're going to tweak that one a bit and then change its Blend Mode. It also lets you compare your before and after progress along the way.
2: Blur the Snot Out of Layer 1
Select the duplicate layer. We're going to blur this thing into oblivion, so you won't even be able to identify the model standing in front of you.
From the Filter menu, choose Blur -> Gausian Blur. When the dialog box appears, change the Radius to 50 pixels, as shown below.
3: Change the Blend Mode
We're going to do a few things in this section. The most important part is to change the Blend Mode of the burry layer. Which Blend Mode do you choose? Most of the time, I really like using Soft Light. Some other folks also have a preference for Overlay. In this case, I want to darken the background, so I chose to use Multiply.
We aren't going to want this blurry effect over the entire image, so let's also add a Layer Mask, as shown here.
Once you have those steps in place, you'll see a result like the one below.
4: Reduce the Layer Opacity
OK, that's a bit too much drama. While not so blurry that you can't recognize the subject, we've still lost detail due to the darkness and blur filter. We need to back that off a bit. There's no special number to choose. This is a matter of taste.
My preference was to lower the layer opacity to about 75% for this image. That value could be very different for another image. Just move the slider and see what you like. All you need to do is pay attention to how it affects the background, since we're going to use the Layer Mask to let our subject appear from the layer below.
Here's how the image looks with the layer opacity reduced to 75%.
5: Selective Elimination of the Effect
Now you take your brush and paint with Black to bring your subject out from the layer below. Before you get started painting, think about how much of that image you want to appear. If you brush in 100% from the original image, it may not blend naturally with the darker and softer version of the layer we created above.
Fortunately, you can also change the Opacity of your brush. I decided to use 50% for this image, with a Flow of 80%. The Opacity determines how much of the image below to reveal with each stroke. I opted for 50%, but that's cumulative with additional strokes. If I want 100% of the layer below to show up, I need two strokes. That gives me a little time to evaluate as I brush, rather than just have everything pop through at once.
The Flow value determines how fast you brush through the layer. I'm old and my reflexes are slow, so I slow down the Flow to 80%. It's a personal preference, but I like to let the masking build up rather than appear all at once.
The thumbnail image to the side is a bit small, but you may be able to see the outline of my brush strokes to see what I chose to reveal. I didn't reveal every part of her through the image. Some areas looked better in a bit of shadow. I also didn't reveal all of her completely. Some areas certainly need more definition, particularly the face and hair. Other parts of the image are left well enough in the effect we created.
6: Crop it Down
This image has a bit more headroom and space than it needs, so I decided to crop it just a bit closer. If this Crop view looks a bit unfamiliar to you, it's from Apple Aperture. I like Aperture's Crop tool better than Adobe's tools in Photoshop or Lightroom. Also, it's non-destructive. I can always go back later to change or remove the Crop is I need.
If you want to add a bit more drama to your images, make 'em dark and obliterate the background details. Your viewer's mind knows she's in a garden area without needing to see all of the detail in the flowers. They know there's a building behind her with Spanish Mission architecture. You don't have to club them over the head with detail, particularly when those things aren't the subject. They only exist to provide a sense of place. Your mind will fill in what it needs, so the details of the background are unimportant.