From Wikipedia: “High-dynamic-range
imaging is a set of methods used in photography to capture a greater dynamic
range between the lightest and darkest areas of an image…”
Cameras take pictures at one exposure level with a limited contrast range. If you are metering for the sky, the foreground will be dark and if you are meeting for the fields of grass then the sly will be blown out of highlights that you won’t even capture the clouds.
At the age of film, this loss of detail in bright or dark areas of a picture, depending on whether the camera was metering, was neutralized by use of Neutral density Filters where half of the filter is dark enough to reduce exposure by 1, 2, or full 10 stops. See related blog posts:
Should you not have a ND Filter and you want to overcome this limitation, you basically need to take three or more pictures at different exposures and merge them into a final photograph. This increases the dynamic range and makes it possible to show detail in the highlights and shadows at the same time.
Will ND Filters be obsolete then in the age of HDR? Maybe and maybe not. I can't locate one in Beirut but I sure will be buying one from abroad. It is on my ever-evolving list of gear to buy... I guess it is much easier to capture the dynamic range in the camera than processing 3 to 5 pictures in software. That being said, sometimes you want to travel light during your photography expeditions, this is why you need to consider other post-processing options.
Auto exposure bracketing (AEB)
AEB is a function in your camera that automates taking three (or five; depending on your camera) shots in quick succession, each with a different amount of compensation. All DSLRs and even point-and-shoot camera nowadays have this option. It is helpful if you do not like the correct exposure but are unsure of how to change it. However, it does have another application; HDR.
Now the fun part… Lock your camera on a sturdy tripod, use timer to release the shutter or use a cable release, and start with the default setting of the camera then add plus and minus compensation at selectable levels (-1, 0, +1, or -2, 0, +2, for example).
The shutter speed and / or aperture will change with the compensation depending on the shooting mode selected. With Shutter Priority (Tv), the aperture will change; with Aperture Priority (Av), the shutter speed will change. Either or both values will change in Program AE (P) mode.
Blending it all together
After transferring all images to your computer and in order to blend all images together, you can use the utility that came with your DSLR. For Canon EOS cameras, they ship with a compositing too called Digital Photo Professional (DPP). I prefer however to do it in Adobe Camera RAW and Adobe Photoshop. Here’s how:
First in Adobe Camera RAW or Lightroom you should select all images that you are blending and make exact adjustments on all. So if you are doing any RAW editing you usually do, such as black and white conversion, white balance adjustments, and so on, click the “Select All” icon to have your adjustments apply to all your images.
With all images still selected, open then in into Adobe Photoshop. They will open in three different windows. The objective now is to combine them all into the same file using Layers.
Select your middle exposure file (0), Select All (CTRL + A) and Copy the image. Now select the brightest photo (longest exposure or +2) and create a new layer and Paste the middle exposure on top. If you executed your bracketing properly and used a tripod, you should have no alignment problems. Repeat the steps above for the underexposed image (-2) so that you have all three photos in one file and on their own separate layers. Your top layer should be your darkest image and the bottom layer should be the brightest.
Combining Exposures Using Layer Masks (it is all about revealing and hiding layers)
Assume now we have a shot of a landscape where the foreground is a field of grass positioned on the lower two-thirds and the sky sits on the upper third of the photo. First, we want to add a layer mask to blend in the sky. Click the Add Layer Mask icon located at the bottom of your layers palette.
Now select your brush tool (shortcut: B), and ensure that the foreground color is set to black, and background set to white (if not press X to toggle). With your new layer mask selected, adjust your brush to a suitable size ( “[“ and “]” shortcuts) and make sure your brush is set to 100% opacity, and Hardness is at 0%. What we will be doing now is basically erasing the some of the top layer so that we can see the middle layer’s sky. Instead of applying it directly to the layer, we are using a mask so that we can adjust or delete this later on.
Since I only want the sky to show through, I just brush over that particular area (i.e. I don’t brush over the field of grass).
If you made a mistake while brushing, you can always switch your brush color back to white (press X) to reverse the brush work you previously did and bring back the layer you are masking.
Now we need to improve our foreground because it is underexposed. Select the base exposure layer and add another layer mask and brush the foreground grass with a 100% opacity brush set at 0% hardness.
In the end, you will have three exposures in one well-balanced scene – an underexposed sky, a base exposure for the field of grass, and overexposure for the rocks.
After some additional detailed brushwork along the sky and the field of grass, your blending should be complete. This recovery of detail will also be reflected in the histogram (a nice arc distribution of tones).
Photos below were developed using the same Layers' technique in Photoshop. Notice the details across all contrasting areas