I am going to show you a quick method for adding "fill flash" to your photos, using Photoshop. Fill flash is required when you are shooting a portrait with a bright background scene. If you accidently neglect to turn your flash on during the shot, you will find your background to be well exposed, but your subject may appear quite dark in the scene. All is not lost – here's how we can use Photoshop to correct it!
Step 1: Begin by opening a portrait photograph where your subject appears too dark in the scene.
Step 2: Duplicate the layer: Control-J (PC) or Command-J (Mac).
Step 3: Next, open your Channels Palette. In the Channels Palette you will want to select the channel with the most contrast (most darks vs lights). To view the channels, simply click on them in the channel palette, and you will be able to view them (they will appear in black and white). In portrait photography, the channel with the most contrast is typically the blue channel, as it is here in this example.
Step 4: Once you have found the channel with the most contrast, we are now going to duplicate this channel. You can easily do this by clicking on the channel, and dragging it down onto the "Create New Channel" icon at the bottom of the Channels Palette. What we are doing here is creating a channel mask, which will be used as a layer mask shortly.
Step 5: With this new channel active, open the Levels dialog box (Menu: Image> Adjustments> Levels …). What we want to do here is increase the contrast even further. You will have to experiment as it will be different for each image. Simply slide the light slider inwards, and do the same with the dark slider.
Notice the extreme boost in contrast in the image? Now click OK.
Step 6: Now we are going to add a little bit of Gaussian blur to soften the edges between the blacks and whites. Go to the menu: Filter> Blur> Gaussian Blur … and add a blur of approximately 0.5 to 1.5 pixels, depending on your image.
Step 7: In your Channels Palette, click on the icon of the RGB channel to make it active again, which should now display the original image.
Step 8: Switch back to your Layers Palette and ensure that your duplicated layer (top layer) is the one that is still active. Now go to the menu: Select> Load Selection …
When this dialog box appears, ensure that the Channel option is showing the channel mask you created earlier (in this case the "Blue copy", not to be confused with the actual blue channel).
Also, it is important to check the Invert option ON, because (since this will be used as a layer mask in our next step) we want to use the inverse (or negative) of the channel mask ON we created.
Step 9: Now you will see your image with the selection "marching ants" showing. To load this selection as a layer mask, all you have to do is click on the "Add layer mask" icon at the bottom of the layers palette, and the mask will automatically be created for you based on your selection. Here's what our layer mask looks like in the layers palette now:
Step 10: There's always a step where "the magic" happens in Photoshop, and this is finally that step!
In the layers palette, change the blending mode of this top layer to "Screen" and you will see the virtual flash light up your subject. How illuminating!
You may find with some images, that the effect is not quite right, so there are some refinements which are optional and are very easy to make.
If the image is still too dark:
Simply duplicate your top layer again: Control-J (PC) or Command-J (Mac). You can do this as many times as necessary until there is enough lighting.
If the image is too bright:
Reduce the opacity of your top layer until you have your desired result.
The "fill-flash" affected other parts of your image:
You have a layer mask already, so all you need to do in this case is refine it. Click on the layer mask in the Layer's Palette, and (using the paintbrush tool), paint with a BLACK brush where you want to hide the fill flash effect. You can also try changing the brush opacity to 50% along the top of your screen in the brush options.