By using over and underexposed frames as sources for burning in highlights and dodging shadows major gains can be made in tonality over using adjustment layers to lighten or darken areas.
This is because the shadows in the overexposed frame have a good range of digital densities because they are strongly “exposed to the right”. Highlights in the underexposed frame have full detail and density because they are in the top of the tonal range.
The contrast enhancement is due to the distribution of gray tones in a digital original. In 12 bit format where the peak highlight is 4096 the top 3 zones, V-VIII, account for the majority of the tones in an image. i.e. 2048 plus 1024 plus 512 = 3584 or 87% of the tonal range of the original.
By getting certain tone ranges into this digital sweet spot and then using them in your completed image they make detail happen where it would not with a regional adjustment layer or luminance mask type of burn. This can actually add detail to highlights and shadows where there is none in the normally exposed frame.
To set up for this you want to have a 3 frame HDR stack which is shot from a tripod with either 1 or 2 stops difference between the frames. One frame should be a normally exposed version of the scene. By normally exposed I mean as light as possible without the highlights blowing out.
Load the HDR stack and drag the layers all into a single file. You can make individual contrast adjustments in the RAW converter. If you hold Shift while dropping the layer on the master file it will be close to in registration.
There should be no background layer in the master file. If there is make it into a regular layer by double clicking on the background layer.
You can also do this by loading a file with two different settings in the raw converter. This is not as good as having the actual over and underexposed frames but it will help in a pinch.
Select all of the layers and do an Edit->Auto Align Layers to make the layers be as lined up as possible. There are often subtle variations that come from the tripod.
The over and underexposed frames are placed above the normally exposed frame and revealed by brushing white onto the black transparency masks.
Name the underexposed frame’s layer “Burn” and the overexposed frame’s layer “Dodge” this is because we want to add/darken highlights with the underexposed frame and lighten shadows with the overexposed frame.
Create a transparency mask filled with black on the frames that are over and under exposed.
- Select the layer then press the Add Vector/Transparency Mask button at the bottom of the layers palette. A transparency mask is created for the layer.
- Set the foreground and background colors to B&W on the Tools palette.
- Hit Select->All then Delete and the mask should turn black. If not hit X to swap the fore and back colors and hit Delete again.
Select the Brush tool and pick a round brush with soft edges to use as a dodging tool.
Select the Burn layer and paint on the mask to burn in the highlights or select the Dodge layer and pain on its mask to lightened shadows. You need to avoid the areas that have out of range values in the over and underexposed frames.
This is a great way to condition the detail and tonal rendition of an image prior to luminance masking.
In the first example an over-exposed frame is combined with a normally exposed frame and the shadows are dodged out. This creates an interesting light in the dark areas because rather then having the shadow values become posturized and crunchy from brightening. In this case the tones are very supple due to their hard right exposure and the fact that brightening is not necessary.
The second example blends an underexposed frame over a normally exposed frame to add detail and tonality to the highlights.
Here are the HDR Burn and Dodge Examples...