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:

  1. Come back when the light was more balanced.
  2. Use a graduated filter to try to balance the light in the sky and the foreground.
  3. Use fill flash to try to bring out shadow detail.
  4. Bring small subjects into a studio where the light can be controlled.
  5. 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.

The dynamic range of this scene fit within 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.

The dynamic range of this scene is too great to contain in a single exposure.

How do I know how much I need to bracket for any particular scene? My procedure in the field goes something like this:

  1. With camera on a tripod, frame the composition and lock it down.
  2. With my camera metering set to “Evaluative” take the first exposure.
  3. 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.
  4. Begin taking successive underexposed images in one stop increments, viewing the histogram after each exposure.
  5. 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.
  6. 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.
  7. 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.

In the first exposure I noticed that both the shadows and highlights extended beyond the ends of the histogram.

By underexposing a stop I was able to contain most of the highlights.

Underexposing two stops enabled me to retain detail in even the brightest highlights.

Finally I overexposed by two stops to get an exposure in which no shadow detail was clipped. I did take an exposure one stop over exposed but didn't end up using it in the final image.

After careful blending using layers masks I did some additional processing for color and contrast to arrive at the final 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.

For information on image processing workshops or private instruction you can visit my website or contact me directly.

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.

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~ by photocascadia on September 8, 2010.

21 Responses to “Bracketing Exposures For Exposure Blending”

  1. Priceless info, thank you Sean!

  2. Great article.

  3. I love your work. Where is your seminar listing???

  4. Excellent article, thanks for the info.

    It will be interesting to try this out in the blinding sun of the Arizona desert.

    Have Fun,
    Jeff

    • Jeff – The bracketing is the easy part. The blending is where it gets tricky. I’m still not completely satisfied with the results I get from HDR software so I mostly do blending using layer masks. Let me know how it goes for you.

  5. Sean, good article, well written, clear and concise. Do you ever use the automatic bracketing?

    • Graham, I do use the auto bracketing fairly often, especially if something in the scene is changing rapidly, such as clouds, or if I am attempting to hand hold multiple exposures.

  6. Technically this technique to HDR makes perfect sense. The goal is to eliminate all clipping. The only problem I see with it is potential camera shake as you are adjusting the EV multiple times and may cause the images to shift. But I guess a good HDR program can merge the photos and align them properly. It seems to me you’d have to be very good (which I am not) in Photoshop to use layer masks to reveal the best parts of each photo. I’d rather rely on an HDR program (Photomatix, Nik, or Photoshop CS5) to create the final image.

    • TJ, Thanks for the comment. If my tripod is steady and locked solid I am very successful at bracketing exposures without any noticeable camera shift. You can also use the auto bracketing function, but my camera only auto brackets three exposures at a time. You are also correct that most current HDR software is able to align images if there is slight movement. When I am manually blending images with layer masks and need to align images I also use the Auto-Align function in Photoshop. Using the auto bracketing function on my camera I am actually able to handhold three exposures and successfully blend them thanks to auto aligning. It takes practice, but 99% of the time I can get more realistic and better quality results manually blending using layer masks then with HDR software.

  7. A very useful article. May be in the future a few tricks about the next step: manual blending?
    Thank you!
    Antonius

    • Antonius, I will most likely do some rudimentary posts about manual blending techniques in the future. However, the techniques are fairly involved and would be difficult to cover adequately in a blog format. I teach two separate 8 hour classes just to cover the various techniques in any sort of detail.

  8. Great tips! Doesn’t bracketing using the f-stop influence DOF within the range of exposures? Are there instances when you prefer to bracket with multiple shutter speeds instead?

    • Hi Ryan – When I refer to bracketing in one stop increments I’m referring to stops of light and not to the f/stop setting itself, otherwise known as aperture. One can vary the exposure by one stop of light by either adjusting the aperture or the shutter speed. You are correct that changing the aperture would change the depth of field and make blending unsuccessful. As you suggest, I always bracket my exposure by adjusting the shutter speed while leaving the aperture at a constant setting. To increase the exposure by one stop I double the shutter time and to decrease the exposure by one stop I cut the shutter time in half.

  9. The blending technique you use is very good….but there’s something you need to balance properly to get the result ever more realistic in the final result.(that is if realism is the goal when blending exposures)
    The brightest part in this scene you could see with your eyes at this location is the sky….you can easely verify that by looking at the most underexposed image. There is nothing thats brighter than the sky.
    In you final result the sky is clearly at least a stop or even two darker than the watersurface of the lake. A watersurface can never be brighter than the lightsource it is reflecting. A watersurface has no light amplifying power….at best it can only reflect the light as a mirror. The green trees and bushshes at the left of the frame are clearly a bit bright as well. It is very tempting to bring out powerfull fresh colors and all detail in the dark parts of a scene …but even with your own eyes and brain there’s no way of getting everything this bright colored and detailed. IMHO lower the exposure a bit on both greenery and watersurface and this photo will be a lot closer to the real scene.

    • Peter, thanks for your very astute comments. Everything you say about relative sky, foreground and reflection brightness is correct. Your are also correct in noting that absolute realism is not the primary goal of my photography. My goal is to express my creative visions and impressions through my images. I approach much of my photography less like documentation and more like an expressionistic painting. The technique of rendering the sky and other areas of the image darker in order to make other parts of the image appear more bright is called countershading. It is a way of intentionally enhancing drama, creating depth and leading the eye. Many of the renaissance painters, most notably Rembrandt, used this technique. It was also one of the main techniques taught by the Hudson River School which sparked a genre all its own. Countershading was also widely used by the classic black and white photography masters such as Ansel Adams, Elliot Porter and Edward Weston. They used the zone system and dodging and burning techniques to achieve the effect.

      • Sean, This is a terrific response to Peter’s comment. You are obviously well educated and very talented, diplomatic, too!. It’s truly wonderful that you have such control and output your images exactly as you see them and for all of us to enjoy. I guess what we have to remember is that this is art and art is subject to opinion. And in the end he who is selling holds a positive opinion from the masses and gets to keep on making his art. There will always be purists especially from the old film crowd, but as I recall, we used to retouch and hand color plenty of those images then, didn’t we? Hmmm!
        Keep on going!

  10. Thanks for another awesome tutorial Sean. I really enjoy reading this blog and all the useful information you guys kindly share.

    Cheers

    Neal

  11. Thank you Sean for a concise explanation of your exposure bracketing technique. If there is one thing I keep learning over and over as an amateur photographer, it is that the histogram is my friend, always 🙂
    Have you considered offering your seminars and workshops on DVD? I would certainly be very interested in purchasing a tutorial in advanced layer masking in Photoshop (or other processing techniques you use for your images).

    Thanks!
    Diana

  12. Thank you Sean for a concise explanation of your exposure bracketing technique. If there is one thing I have learned as an amateur photographer, it is that the histogram is my friend, always 🙂
    Have you considered offering your workshops on DVD? I would certainly be very interested in purchasing an advanced layer masking tutorial (or any other processing techniques you use).

    Thanks!
    Diana

  13. Very interesting, I have just written some tips on using Photoshop with Exposure Bracketing, all by the Brighton Photographer, I thought you would find them useful.

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