What is High Dynamic Range?
If you have ever tried to photograph a scene, that has a high-contrast between the lightest, and darkest areas, then you may also have been frustrated by what you see when you check the captured image. When you find that the image you captured doesn’t match what you are seeing before you. Even when you use what should be the perfect exposure there will always be scenes that will have overexposed (too light) highlights, underexposed (too dark) shadow areas, or both. The way to solve this problem is to use High Dynamic Range (HDR ) photography.
What does this mean?
The reason for this is that despite advances in digital cameras, and their image sensors there is a limit to the range of tones (dark to light) that can be captured on any one image. In an image, the dynamic range is a measure of the difference in exposure needed to capture the darkest of dark tones and the lightest of light tones.
An increase of one f-stop (opening lens aperture), doubles the amount of light hitting the image sensor. Whilst a decrease of one f-stop halves the amount of light. To bring out the detail in the darkest areas requires more light to hit the sensor. To prevent losing detail in the bright areas (blown out highlights), you need less light to the image sensor.
Obviously, the problem is that by opening up the aperture to expose the darker areas correctly will overexpose the lighter portions of the image. The converse is true if you expose for the lighter areas, as this will leave the darker areas lacking in detail.
As already mentioned, HDR stands for High Dynamic Range. HDR is a particular method of photography that produces an image containing a higher dynamic range than you could otherwise normally capture in a single photograph.
At it’s most basic, a HDR photo is comprised of several photos, which have been taken at different exposures and then combined together. This process can be achieved in several ways: Either by using in camera processing, if your camera has a HDR function (check your camera’s manual for this). Or by the use of software to create the High Dynamic Range image.
The three images above show both the captured image, and the histogram for the image. The centre image is the “correct” exposure. The images were then combined using Photomatix Pro HDR software to produce the final image below.
As you can see in the final HDR image, The sky is more dramatic, but foreground detail is exposed correctly to preserve detail as well.
That’s the basics of it. All you need to do, is take a range of bracketed photos, by that, I mean a series of photos of the same subject, from the same position, but taken with different shutter speed/aperture combinations.
When taking a photograph normally, if using manual mode, you would usually select the aperture to give the depth of field you want, and adjust the shutter speed to get the exposure correct for a given ISO.
If you use the histogram (to show the luminosity. (brightness)) on the live view screen. You can see the range of tones being captured in the image. To capture the full tonal range from dark to light of the scene, adjust the shutter speed to move the histogram. By using the histogram, you can ensure that the images captured contain the full range of tones within the scene being shot.
3 quick methods of taking bracketed shots for HDR images:
- Use HDR function on your camera / smartphone if it has it.
- Use auto exposure bracketing function of your camera. Simply set the range of exposures needed to capture the full tonal range of the scene, usually this will give 3 images correct exposure and + and – however many stops you dial in. Then either manually take 3 images, or use continuous shooting mode until all images captured.
- Use shutter priority mode. Set aperture for DOF, focus, check histogram to assess the bracketing needed to capture full tonal range of the scene. Then starting from either the darkest image (faster shutter speed) take photograph, adjust shutter speed, repeat until all images are captured.
Then, simply by using the HDR software , you are able to blend the photos together, and create a single image containing the full range of tones that were in the scene. If you haven’t taken a bracketed series of images, and one of images is too dark, or too light, if the images are in RAW format you can rescue the image
Simply adjust the exposure using the slider when processing your image. This can allow you to artificially create a HDR series which will hopefully rescue the blown highlights, or too dark shadowed area. Then simple use these images to create anHDR version of the image. Some HDR software will do this automatically from a single image.