What Is It?

In photography, your medium (film or digital) has a limited range of brightness levels that it can record. As parts of the image get too bright for the medium, the medium records absolute white and all subtleties in the bright area are lost. Similarly, as parts of the image get too dark for the medium, the medium records absolute black and all subtleties in the shadows are lost.

A solution to the limited range of the medium is to take more than one photo of the same scene, and for each photo adjust the exposure so that one is underexposed, one is a normal exposure, and one is overexposed. The under and over exposed images will capture the details in the highlights and shadows, respectively. The next step is to combine the images into a single image that captures the full range of values of all three exposures. This is accomplished using software, such as: Photoshop CS2 or Photmatix.

Now you have a High Dynamic Range photo. The problem with this photo is that its dynamic range exceeds that of your display devices (monitor, printer, etc.). The dynamic range needs to be converted into a lower dynamic range to be useful. This is accomplished using software. You can either scale the range of values down to a limited set of values, or you can “tonemap” the image. Scaling results in a dull, flat looking image. Tonemapping can increase the saturation and contrast in the image such that local subtleties in the image are preserved or accentuated.

HDR Pros and Cons

Pros

The ability to capture a scene that has portions that are too bright or too dark for a single shot.
Can improve the contrast and color saturation.

Cons

Extra steps in taking the photo.
Extra steps to process the photo.
Consumes lots of hard disk space.
Requires purchase of additional software.
Can exaggerate image noise.

Pro / Con

Does funny things if parts of the scene move between shots.

Helps

Extra RAM and CPU horsepower speed up processing and helps avoid “out-of-memory” errors.
RAW image file format helps reduce noise, but JPG’s can be used also.

My Workflow

Using a Canon 5D camera and Photoshop CS2, Bridge, and Photomatix software.

Taking the Photo

Setup the tripod.
Mount the camera to the tripod.
Connect the cable release.
Power up the camera
Set the Auto Exposure Bracket (AEB) to -2 to +2 EV.
Set the camera mode to aperture preferred.
Select desired aperture.
Set the ISO as low as I can.
Make sure I’m set to generate RAW output.
Put lens in manual focus mode.
Turn lens image stabilization off.
Focus.
Snap three shots.

Processing the Photo

Copy the RAW image files to the hard disk.
Rename the files.
Using Adobe Bridge, tag the images with the keyword ‘hdrpiece’. (This facilitates image searching at some later date.)
Using Bridge, add other keywords and descriptions as desired.
Using Bridge, select all RAW images and batch convert them to JPG. (This is not necessary for HDR but I like to keep a non-proprietary format copy of all of my RAW images.)
Open Photomatix stand-alone program.
Select: HDR > Generate, and select a set of three related RAW images.
Check “Align source images”. (May not want to do this if part of your scene moved during the taking of the different exposures.)
Select OK.
Select: HDR > Tone Mapping.
Select Output depth of 16-bit.
Adjust various sliders to get the desired results.
Save the settings to file with the same name as the original photo file. (This helps if you want to go back and reprocess that image again at a later date.)
Click Apply.
Review results. Undo and repeat steps 10-14 again until you get the desired results.
Select: File > Save As
Select: Save as type to “.tif <TIFF> 16-bit mode”
Save image. (Note: Photomatix has the ability to do contrast adjustments and sharpening, but I prefer to do that in Photoshop.)
Open up the TIFF image in Photoshop.
Adjust contrast (typically using “curves”), clone out any dust, and sharpen (unsharp).
Save adjusted TIFF image.
Select: Image > Mode > 8-bits/Channel
Save As a JPG image with quality 12 (maximum)
Using Bridge, set the keyword ‘hdrblend’ on the new JPG image. (Again, this for searching at a later date.)
Using Bridge, set other keywords and descriptions on the new JPG image. (Photomatix looses these when it processes the original images.)

Camera Selection

I use a Canon 5D so I don't have any specific recommendations for a mid-priced camera, but if you're going to do a fair amount of HDR photography, here's a few things to consider.

LOW NOISE! The HDR software tends to exaggerate noise artifacts.
Auto Exposure Bracketing (AEB). This allows you to take multiple exposures without having to manually tweak the shutter speed. (The 5D has AEB but it resets itself to off each time you power down the camera.)
AEB range of -2 to +2 EV. Some only do -1 to +1 EV.
Aperture preferred mode. Not essential, but helps. You only want to vary the shutter speed, not the aperture, for each exposure.
Cable release. You don't want to put your hands on the camera between exposures. (Most cable releases are electronic these days.)
Mirror lockup. Only on DSLR's and not a critical feature.
Five-exposure AEB. Not essential. Most (e.g. 5D) take three exposures, but it something I wish the 5D had.
RAW capture mode. Less noise than taking JPG images.
Sturdy tripod.
Rapid-fire AEB, that is, taking three exposures with one click of the shutter. Not essential. (On the 5D I have to press the shutter release three times.)

Another recommendation is to go with either Canon or Nikon. Some of the HDR software will accept "certain" manufacturers' RAW file formats. If you go with either of these you won't get caught short.

Hard Disk Consumption

Here is a typical set of files involved in creating a single HDR image:

(Note: the 5D camera is a 12.7 mega-pixel camera, so files tend to be somewhat larger than other photographers might experience.)

13,488,516 070804115704_.cr2 (RAW image)

5,896,951 070804115704_.jpg (backup image)

4,067 070804115704_.xmp (RAW settings)

11,630,683 070804115705_.cr2 (RAW image)

4,847,530 070804115705_.jpg (backup image)

4,068 070804115705_.xmp (RAW settings)

15,257,072 070804115706_.cr2 (RAW image)

5,246,723 070804115706_.jpg (backup image)

4,067 070804115706_.xmp (RAW settings)

8,638,027 070804115704__5__6__tonemapped.jpg (final 8 bit img)

76,850,616 070804115704__5__6__tonemapped.tif (16 bit result)

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141,868,320 – 142 Mega-bytes

Dealing with Image Noise

When converting the HDR using tonemapping you will occasionally get black splotches in your results. There are several things you can do to try and eliminate this problem:

Shoot with a lower ISO setting.

Use RAW instead of JPG for your input images.

Undo, and try tonemapping with different settings.
Preprocess the input images and add just a pinch of Gausian blur.
Proprocess the input images and adjust the curves slightly to eliminate absolute black pixels.