This is a common question.
But not a very well answered one, at least from what I've seen so far.
The very foundation of HDR photography is exposure bracketing.
This is to capture the entire dynamic range of the scene, an attempt to overcome the deficiency of the image sensor in recording the full dynamic range.
But what's the optimal number of bracketed exposures for a given HDR image?
In this post, I’m going to show you a foolproof, data-driven method that allows you to capture the full dynamic range without any guess.
Why Bracket Exposures?
Let’s consider this typical scenario.
You’re shooting a beautiful landscape. You compose the frame and notice the light is coming right from the center.
If you click the shutter release now…
You’re going to get one of these two results:
(1) The foreground is perfectly exposed but the sky is too bright (image on right), or...
(2) The foreground is too dark (no appreciable details) but the sky is perfectly exposed.
What went wrong?
The dynamic range of the scene is just too much for the camera to handle...more specifically, the digital image sensor.
It’s a known issue with ANY camera...but folks in the lab are working on it.
Hopefully, it won’t be long before we have the technology to record the full dynamic range within a single frame.
But for now, we have to know how to bracket exposures properly.
When To and Not To Bracket Exposures?
I see many people bracketing exposures unnecessary all the time.
Landscape, cars or even picture of their kids playing in the backyard.
Yes, you can dump all the bracketed images into a software and let it do the hard work…
But you’re taking extra steps and spending more time in post-processing to achieve what could have been done with a single shot (and less time!).
So, how do you know when to and not to do exposure bracketing?
You use the histogram!
I won’t go into too much detail about histograms because I have already covered it thoroughly in this tutorial - Histogram: How To Visually Extract and Interpret Data.
But here are the essential steps:
- 1Take a shot and look at the histogram on the LCD screen.
- 2You want the graph to be within the frame, not touching the far left or the far right.
- 3If the graph is touching one end but with space on the other, adjust the exposure settings to shift the graph. The quickest way is to use exposure compensation.
- 4If this fails to keep the graph within the histogram, or the graph is already touching both ends, you should bracket exposures.
It may sound like a lot of trial and error...
But once you get used to it, it only takes seconds to decide if exposure bracketing is necessary.
How Many Bracketed Exposures?
Although most people seem to be shooting three bracketed exposures, I have seen eight or even 12 exposures before…
So, how many bracketed exposures do you actually need?
Remember at the start, I told you about a foolproof, data-driven method that helps you determine the optimal number of bracketed exposures for a given scene?
The data comes from the histogram!
How on earth does it work?
Exposure Bracketing With Histogram
Pretty simple actually…
It’s the same principle as deciding when to bracket exposures.
What you do is to make sure the entire graph is within the histograms.
Let me show you an example:
This is a histogram of a high dynamic range scene.
It's what I call the U-shape or "smiley face" histogram 🙂
We know that because both the highlights and shadows are clipped (touching both ends).
To start off, take three bracketed exposures.
You can use the auto exposure bracketing function in your camera or do manual bracketing.
I tend to keep by bracketing at +/-2EV if the contrast isn’t too extreme.
This is to make sure there isn’t too much gap between each bracketed images (some cameras can bracket at +/-3EV). Otherwise, it can be tricky in post-processing sometimes.
Once you have taken the images, check the histograms of all three images on the LCD screen.
Can you see both ends of the graph? (like the histograms above)
This means paying attention to the -2EV and +2EV images.
Visualize All Histograms As One
The key here is to visually merge the histograms of all images into one.
In the example above, here’s how you should picture the histogram in your mind:
In the example above, you can see the graph of all histograms combined is still within the "frame".
This means the full dynamic range is captured!
But when either or both ends of the histogram is still touching the far left or the right, you need to extend the range of exposure bracketing.
If you have previously shot three bracketed exposures at +/-2EV, you now need to shoot five bracketed exposures at +/-2EV.
This means you have to bracket exposures at a total of 8EV range, which captures the full dynamic range in probably 99% of the cases in my opinion.
Know How Your AEB Works
The auto exposure bracketing (AEB) of my Canon 5DM2 allows me to bracket a maximum of three images at a maximum of 2EV within a +/-4EV range.
Know how many images your AEB can shoot and the range will help you decide if you can count on it or need to do manual bracketing.
Here is a good resource that lists the maximum number of images you can shoot with AEB and its EV range for most cameras on the market today.
When AEB Is Not Enough
When your camera’s AEB hasn’t got enough EV range to capture the full dynamic range, manual bracketing is the next step forward.
“Manual” sounds daunting, I know…
But to be honest, it’s not as hard as you imagine!
All you need is to know how to dial up and down the shutter speed manually in 1-stop interval.
Stop of Light
“Stop” is how light is measured in photography (maybe in other fields too?).
In the context of shutter speed, an increase in 1-stop (less light) means doubling the shutter speed; reduce in 1-stop (more light) means halving the shutter speed.
Let’s say your image has a shutter speed of 1/250sec and it looks underexposed. You want to increase exposure by 1-stop to make it brighter.
What you do is halving the shutter speed to 1/125sec. If you were to increase it by 1-stop, it would be 1/500sec.
Bear in mind that 1-stop means the same in ISO but different in aperture. You can learn more about it here.
How To Bracket Exposures Manually
Now that you know the essence of manual bracketing, let’s go through the steps:
- 1Set your camera in aperture priority mode, take a shot.
- 2Review the histogram on the LCD screen, take note of the shutter speed, aperture and ISO.
- 3Set your camera to manual mode, dial in the shutter speed, aperture and ISO from above.
- 4(Optional) Take a shot to make sure the exposure is still correct.
- 5Dial the shutter speed up by 1-stop, take a shot and check the histogram to see if any highlights clipping.
- 6If so, dial the shutter speed up further by 1-stop, take a shot and check the histogram. This step may be repeated as many times as necessary.
- 7Now dial the shutter speed back to the initial setting (the ones you set in step 3).
- 8Dial the shutter speed down by 1-stop this time, take a shot and check the histogram. Basically, similar to step 5 and 6 but dialing the shutter speed down.
In the steps above, I dial the shutter speed up first. You can certainly dialing it down to start with if you like.
I also mentioned dialing it at 1-stop instead of 2-stop (I said I tend to do this earlier). The only reason for using 1-stop in manual bracketing is the ease in calculation!
Once you’re done, review all images and histograms.
You would have covered the full dynamic range!
Histograms Are Often Underrated
Histograms take the guesswork out of bracketing exposures.
It’s definitely the way you should be doing it, not just in HDR photography but in everything else you shoot.
The data helps you nail exposure consistently in every single shot!
If you planning to master the ins and outs of histograms, check out my ebook - Dissecting Histogram: The Unwritten Manual. You can find it from the top menu under Courses.
For more tutorials on HDR photography, don't forget to check out the HDR resource page!