Bracketing Exposures For Exposure Blending
By Sean Bagshaw
As photographers we frequently struggle to overcome the limitations of our equipment in order to create the photographs we envision. One of the biggest limitations of traditional photography is the narrow dynamic range of light that can be contained in an image compared to what we see. To date, neither film nor digital sensors are able to come close to capturing the range of light we can see with our eyes. Often the eye sees a scene with good detail in both the shadows and the highlights, but when you take the photograph you find that the shadows have gone completely black and the sky or other highlights have gone completely white.
Such wide dynamic range light conditions happen in nature more often than not. In the past photographers had a few options. We could:
- Come back when the light was more balanced.
- Use a graduated filter to try to balance the light in the sky and the foreground.
- Use fill flash to try to bring out shadow detail.
- Bring small subjects into a studio where the light can be controlled.
- Resign to the fact that certain images in certain light just aren’t possible and go shoot something else.
These options are still available to us, but we now also have the option to bracket multiple exposures in camera to record all the light values in a scene. Digital processing techniques can later be used to blend the exposures together and yield a final image with a much greater dynamic range than could be achieved in a single exposure.
The techniques for blending exposures vary from simple to highly complex. They can be accomplished using skilled layer masking techniques in Photoshop. We also have the option to use one of a rapidly growing list of exposure blending programs commonly referred to as High Dynamic Range (HDR) software. I teach many of the techniques in group workshops and through private instruction.
As people become aware of the exposure blending options available to them they inevitably start experimenting with bracketing exposures for future processing. One of the most common questions I’m asked in classes and workshops on this topic is, “how do you know how many exposures you need to bracket?”
The technique I use is very simple and straight forward. Most digital cameras on the market today have just the feature you need; the histogram.
A histogram, in digital photography terms, is the graphic representation of all the tonal values in an image from shadows to highlights. The specific shape of the graph is determined by the tonal values in any particular scene. The shape of the histogram itself isn’t necessarily of interest to us in this case. What we are concerned with are the ends of the histogram. The very left side of the graph represents the darkest shadow tones in the image and the right side of the graph represents the brightest values in the image.
If all the tonal values reside within the confines of the graph you know that you can contain the dynamic range of the scene in a single exposure.
When you notice that the tonal values extend past and get clipped off at both the shadow and highlight ends of the graph at the same time you know that the dynamic range is too great for a single exposure and bracketing exposures will be required.
How do I know how much I need to bracket for any particular scene? My procedure in the field goes something like this:
- With camera on a tripod, frame the composition and lock it down.
- With my camera metering set to “Evaluative” take the first exposure.
- Without moving the camera at all, view the histogram on the back of the camera. If the values extend off both sides of the histogram commence bracketing.
- Begin taking successive underexposed images in one stop increments, viewing the histogram after each exposure.
- Continue underexposing by one stop as many times as it takes to no longer see any highlight clipping on the right side of the graph.
- Now return to your initial exposure and begin overexposing the scene in one stop increments. Continue until the histogram shows an exposure in which none of the shadow values are clipped on the left side of the graph.
- Depending on how much dynamic range exists you will have bracketed anywhere from two exposures to as many as nine or more. Three and five exposures are the most common. Depending on your blending technique you may use all of your bracketed exposures or only some of them, but you often don’t know which ones until later.
Below is a sequence of exposures I took in order to capture all the tonal values in the scene. The dynamic range was far too great to be captured in a single image even if I had used a three stop neutral density filter. I later blended these exposures together using layer masking techniques to create the final extended dynamic range image.
As you can see, the number of exposures you need to take is dictated by the range of light values in the scene. The bigger the difference between the darkest and brightest values the more you need to bracket. Your camera’s histogram is the perfect tool for determining when you have bracketed enough to contain tonal information in both the darkest shadows and brightest highlights. When you have obtained one exposure in which no highlights are clipped and one exposure in which no shadows are clipped along with incremental exposures in between you can be sure you have enough dynamic range data to later create a successful extended dynamic range image.
I’m sure there are other methods and variations for determining how many exposures are needed to contain an extended dynamic range. Share your technique or let me know if you have any questions.