This adapted from an article on HDR found in the "Garden Route Photo" web site, written by Hougaard Malan”
HDR is popular because a camera can’t always capture the dynamic range of light in one exposure. This results in areas of the image that have no detail due to shadows or highlights. Traditionally we would use graduated neutral density filters to balance the exposure, but they are rather expensive and limited by many situations found in landscapes everywhere, like uneven horizons, tall objects like trees, mountains etc. When digitally blending 2 or more images we are basically simulating an ND filter, but we have full control over how the graduation of the filter would have been.
HDR opens new doors in landscape photography. Moving objects within an image creates a lot of problems for HDR blending, because your images have to be identical.
The basic concept of HDR is to capture the total dynamic range of the scene ( when it is greater than the camera can capture in one exposure ) in multiple exposures and combine select parts of the various exposures using Photoshop.
The first thing we need is the multiple exposures of a scene. In most natural scenes, the sky is brighter than the land/foreground and if you expose for the sky, the foreground is too dark and if you expose for the foreground then the sky is too bright. So we take a separate exposure for each.
The easiest way is to use your camera’s AEB (auto exposure bracketing) function. It allows you to capture a sequence of 3 exposures, each a certain f-value apart. In most situations, you simply need one exposure for the sky and one for the foreground.
If this is the case, meter the sky and foreground separately, check the difference and set the bracketing so that the exposures are that f-value apart so that you get an ideal exposure for each. So if the sky is 2 stops brighter than the foreground, set the bracketing for 0;-2;+2 and expose it for either sky or FG and you will have the 2 necessary exposures in your sequence.
Sometimes however, a nasty highlight or shadow requires a 3rd exposure.
In a short sentence : You need multiple exposures of which the darkest image may have no highlights and the brightest image may have no shadows.
A final shooting tip, use a tripod and ensure you have a common apperture to ensure continuous depth of focus.
Blending a 2 exposure image in Photoshop
1. We start by just opening our 2 images that we are going to blend. RAW adjustments should already have been applied and make sure your layers window is open (F7)
2. Now, stack the exposures ( in this case 2 ) from darkest to brightest by using the move tool (v) and simply dragging the one image onto the other one. Hold shift in as you drop the image and it will align itself within the frame. You have now stacked the 2 layers and you will see the 2 thumbs in your layers window. Clicking on the eye to the left of the top thumb will hide the top layer and reveal the layer below.
3. You can now close the bright exposure as you have it in a layer on the darker one. Maximize the window simply for better viewing. You will be working on a layer mask so that any mistakes can be easily corrected. At the bottom of the layers window there’s a rectangle with a circle in, click on it to add a layer mask (make sure your top layer is selected ) to the top layer.
4. To simulate the graduated filter, we use a gradient on the layer mask. Press G to select your gradient tool. Check at the top of the window for the following (if everything is on default, it will be right )
a. The gradient type must be foreground to background ( top left )
b. Orientation of the gradient must linear ( left )
c. Mode – normal
d. Opacity – 100%
5. Press D to make sure your FG/BG color is on default.
You will now blend the two exposures. Where you click the gradient tool, the gradient will start, you then drag it to where you want the gradient to end. You can drag the line at any angle and the gradient will be in that direction. The gradient starts where you first clicked, transitions over the path you dragged and ends where you released the mouse. The black pixels will reveal what’s on the layer below them. The pure black will reveal everything with the gray pixels resulting in a smooth transition that fades the 2 exposures into each other.
6. You can now touch up the gradient by painting on the mask with a brush. Once you are happy with the blend, you should flatten the image and treat it as one exposure. Doing separate adjustments to the blended layers can sometimes make a nasty unwanted transition between the layers visible.
7.You can now do your usual processing steps to get to the final result
When starting out with this stuff, the most common errors people usually make is simply not having the correct layer or gradient tool or palette colors selected so always double check your tools when something isn’t working as it should!