Creating An HDR Image

By Wayne Bennett
July 5, 2010

HDR! You can hardly pick up a photo magazine today without some mention of HDR! What is this HDR and why should I be concerned about it?

HDR stands for High Dynamic Range. Unlike the human eye, which can see in the range of 10-15 stops of light, depending on the person to whom you speak, the digital sensor can record in the range of 5-6 stops of light, very similar to the range of slide film. The human eye, because of its range of light recognition, can see details in both shadows and highlights in the same scene, the digital sensor cannot, if the range is more than 5 stops. Before HDR you had to choose…do I expose for the highlights or do I expose for the shadows? You couldn’t do both when the range of light exceeded the range of light, which could be recorded by the digital sensor. If you have a clear cut division, like a horizon line of mountains or beach, you could use a graduated neutral density filter to even the playing field, but you do not always have such a clear cut choice. Creating an HDR image gives you the opportunity to capture the complete range of light you see by merging multiple images.

There are a few programs available as plug-in or stand alone applications for HDR. Photoshop has its own version, which is much improved in Version CS-5, although I must admit I have not tried it yet. The other two popular programs are FDR Tools and Photomatix. Both have their strong and weak points, however I know more photographers who use Photomatix as a routine, as do I. I feel very comfortable with Photomatix and use it routinely.

In this article I do not have the time or space to go into specific details regarding the myriad of settings you can use to create an HDR image, but I will go over the basic technique I use and will illustrate the results. I can highly recommend the book “Practical HDR” by David Nightingale. He goes into detail on the three aforementioned programs and discusses in detail all the sliders and buttons with which you will be confronted and how they are inter-related.

When selecting a scene to photograph as an HDR image there are a few basic things you have to consider. The first consideration is the range of light. Take a reading of the brightest area of your scene as well as a reading of the darkest area. If the range is 5 stops or more, then you have a viable candidate. That being said, even if the range is less, you can still do an HDR. It’s your choice (as you do more HDR images, you can “eyeball” the range of light and be pretty close). Is there any movement, i.e., water, people, clouds, which will affect the image? Too much movement is basically a no-no, but minor movement will work, since the programs will make corrections for minor movements.

When shooting HDR, the cardinal rule is…always have your camera firmly mounted on a tripod. You cannot adequately align images if they do not line up with one another. So, the tripod is a must!

Depending on the perceived range of light I will shoot anywhere from 3 to 9 images, however, unless I see a dramatic range, I usually shoot 5 images at 1 stop intervals. If I shoot only 3 images, I will shoot at 2 stop intervals. The sequence I use is; one

      Image #1 Metered Image                           Image #2 One stop Underexposed

metered exposure, one underexposed by 2 stops and one overexposed by 2 stops. With 5, 7 or 9 images, I will shoot at 1 stop intervals. I set my bracketing menu to shoot 1 exposure at the metered value, then 1 stop under, 2 stops under, 1 stop over and 2 stops over (for 5 images). Below are 5 images shot in this manner. They are a sunrise shot at Waterton Lakes National Park I took when touring with my good friend David Lilly, a wonderful photographer from Calgary. The images were shot using a Nikon D700 with a 24-70mm lens and a Singh-Ray polarizer attached. My camera was firmly mounted on my Gitzo tripod. Images 1-5 illustrate the sequence of exposed images I used to generate this particular HDR image.

Image #3 2 Stops Underexposed                   Image #4 1 Stop Overexposed

Image #5 2 Stops Overexposed

As you can see, the orientation of each of the 5 images is identical. As I stated, my camera was firmly mounted on a tripod and I used a cable release to expose the images, so as to minimize vibration. For the metered image, I was in manual mode at f22 to get the greatest depth of field and metered for the brightest highlight in the scene, then set my shutter speed accordingly. I then bracketed the five images in the following order….M, -1, -2, +1 and +2 to get my 5 shot sequence.

Once you download your images to your computer, open your HDR program. With Photomatix, the procedure is as such…

1. Open Photomatix, then click Generate HDR Image
   
   

2. In the next box, click Browse, then select the images you want to use to create your HDR image, click select, then click OK
   
   

3. HDR Options window opens; check the options you want, then click Generate HDR. An HDR image will be generated as a 32 bit image and will appear on your computer screen. It will look awful, but that’s OK. Your computer screen cannot display a 32 bit image.
   
   

4. Click the Tone Mapping button and a new image will appear. It will be a better image as it is a 16 bit preview of your final image. Note: at this point there will be a large palate of sliders and buttons, which will be used to set the values of the tone-mapped image you generate. The selections you make will determine the image you will see. It takes time to fine tune your image, but after trial and error you will decide on a starting point for every image, then the choice of making changes from subtle to radical are yours. All of the parameters you choose must be made before you actually generate your HDR image. Next, click Process.
   
   

5. Save and rename the processed image to a folder on your computer. See image #6.
   
   

Image #6 Tone-mapped image                          Image #7 Final image

As you can see the tone-mapped image does not look too bad and I could probably use it as it stands, however I like to make a few adjustments to make the image “pop”. The differences between Images #6 and #7 are minor, but there is a distinct difference and Image #7 has more appeal to my eye.

In this case I made a few adjustments in Photoshop CS4 using the following tools in the order mentioned…Levels, Brightness and Contrast, Vibrance, Shadows and Highlights, then finally Unsharp Mask (but only a small amount of sharpening).

Every image is different, but it will be an exciting adventure in bringing your images to a new and different level.

LInks to Resrouces:

• Photomatrix
• Puffin Pad - Window mounts - Wayne is one of the creators of this device
• Sing-Ray polarizers
• Waterton Lakes National Park, Alberta