Pre-Processing / Shooting
This is an outline of the technique for bracketing exposures for use in the creation of a high dynamic range photo. The information below shows the range of exposures necessary to get a good extended dynamic range image regardless of the software used to produce the HDR image or blended digital negative.
The bracketed exposures, taken from a Canon 5D, in this example were used to produce the developed image below. Getting a good series of bracketed exposures starts with proper shooting technique (for example using a tripod, mirror lockup, and a cable release) and using the lowest possible ISO on the camera. The key to a noise free image is in knowing exactly what range of exposures to take to insure a high quality dynamic range blend.
Each exposure from the typical digital camera contains about 12 bits of data which is good enough for a little over 5 stops of dynamic range without too much noise. Using the FFDD Batch script (in Photoshop) to blend exposure 3 through 6 of this series below will yield a resulting image with 15 bits of data which is enough for over 8 stops of dynamic range.
The series of exposures used to produce the above image is shown below. Each exposure is shown with no brightness and contrast adjustments. To the right of the exposure are two histograms. The top (white) histogram represents what the camera records (linear). The bottom (gray) histogram represents the histogram shown after opening up the file in Photoshop in the ProPhoto RGB colorspace (gamma 1.8).
Exposure 1 : Way Too Dark!
This exposure is way too dark. Only about 7% of the range of the camera's sensor is being used. Adding an exposure like this to a dynamic range blend will only serve to add noise.
Exposure 2 : Safe Darkest Exposure - 1/30 sec at f/16
This exposure is using about 50% of the range of the camera's sensor. This is a safe point to insure no highlights are clipped or distorted. Every camera has a point in the sensor where highlights start showing some color distortion. This can be in the upper 5% to 25% of the camera's sensor range, depending on the camera. It is a good idea to take this exposure just in case the next exposure is too bright. The perfect darkest exposure would use all of the camera's sensor range that is free of any highlight color distortion. In this case, using a Canon 5D, this the lower 85% to 90% of the sensor's range.
Exposure 3 : Correct Exposure - 1/15 sec at f/16
This is the traditional correct exposure using the camera's full range without clipping in the highlights. For bracketing this might be a tiny bit too bright to use as the darkest exposure because a few of the highlights are in the Canon 5D's upper sensor range which shows color distortion. However, if no color distortion is found when inspecting the exposure, this should be used as the darkest exposure to start blending from.
Exposure 4 : Expose to the Right - 1/8 sec at f/16
This is a little bit brighter than the correct exposure for the expose to the right method. If developing this as a single exposure, the color distortion and clipping caused by overexposing might be able to be fixed in post processing if not all the color channels are clipped. However for HDR blending this exposure is too dark to stop bracketing at because the shadows are still at the bottom of the sensor's range.
Exposure 5 : Almost There - 1/4 sec at f/16
Notice how the midtones of the scene are starting to spread out across the entire range of the sensor. This exposure is adding a lot in terms of dynamic range to the mid tones, but the shadows are still in the lower half of the sensors range.
Exposure 6 : Brightest Exposure - 1/2 sec at f/16
This is a good example of the brightest exposure that should be included. Notice how the shadows of the scene are spread across a majority of the range of the sensor. A majority of the dynamic range of the final blended output will be from this exposure, so it is critical to include an exposure that is this much overexposed for the shadows.
Shooting into the Sun Exception
One exception to the have no highlight clipping in the darkest exposure rule is when shooting into the sun. There is just too much dynamic range between the sun and the rest of the scene. In this case allow the sun to clip, but do not allow any other highlights to get in the sensor's highlight distortion range in the darkest exposure. The image below shows the darkest exposure used to blend on the left, and the resulting developed image on the right. Development will naturally compress the highlights so that any color distortion or clipping around the sun will not be noticed.
Example of Highlight Color Distortion
The sensor's limit before highlight color distortion is different for each camera. Early Canon digital cameras like the 10D tend to show color distortion as a hue shift, while early Nikon digital cameras tend to produce a graying effect. With later model cameras this tends not to be a problem.
Below is a crop of two exposures from a Canon 10D. Both images were processed with Auto Color Balance and all other settings set to zero, except Saturation which was set to 100 in order to amplify the color distortion. Levels was applied to the right image to match the exposure of the left image. The histograms were copied from Adobe Camera Raw before the Levels adjustment. The image on the left has a histogram that is not yet clipping in Adobe Camera Raw but has color distortion in the lightest parts of the sky (a green tint, see the gray arrow). The image on the right does not have this distortion.
