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Photography

How to Make a Long Exposure High Dynamic Range (HDR) Photo

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This post is part of a series called HDR Photography.
Creating an HDR Time Lapse with SNS-HDR Pro

Your eye has a greater dynamic range than any consumer camera. When you look out at almost any scene, you can make out large amounts of both highlight and shadow detail. Unfortunately, cameras can’t. If there is a lot of contrast in a scene, they’ll only be able to correctly expose either the shadows or the highlights. Enter high dynamic range (HDR) photography. High dynamic range photography is one of the most common forms of computational photography.

In HDR photography, multiple images—or plates—of a single scene are shot, each with a different exposure value. To change the exposure values, different shutter speeds are used for each frame. At least three shots are taken: one exposing for the shadows, one for the midtones and one for the highlights; it’s not uncommon for five or even seven shots to be used, each exposed for different elements in the scene.

Once all the necessary images are taken, they are combined using computer software into a single final image. While each frame has a limited dynamic range, by combining elements from each plate the dynamic range of the final image can be extended to retain detail in both the extreme shadows and highlights of the scene.

Long exposure HDR is the same as regular HDR photography, except that longer exposure times are used. Long exposure HDR creates a very particular look. Some subjects, such as water or clouds, benefit from a lengthened exposure time that blurs their movement, while others, like the night sky, require long shutter speeds to be correctly exposed.

In this tutorial I’ll look at how to make LE HDR images.

When to Shoot a Long Exposure HDR Photo

Not all situations are appropriate for a LE HDR shot. Unless there is a creative or technical reason to use a long shutter speed, shorter shutter speeds are generally better: there is less risk of subject motion blur, camera motion blur or long exposure noise being introduced.

LE HDR images are almost always landscapes, cityscapes or architecture shots. It’s when making these kinds of images that scenes with static subjects and a large dynamic range are most commonly encountered.

le hdr image
A LE HDR image I shot for this tutorial. The sky, sea and foreground are largely composed of different frames.

Even when there are moving subjects in these kinds of images—like a body of water, clouds or cars—introducing motion blur is a pleasing creative effect rather than jarring distraction. Seascapes look a lot better when shot at 60 seconds rather than 1/60th of a second. Instead of having harsh edges, the waves blend into a serene, soft sea.

Similarly, unless there are distinct shadow and highlight areas in the image, the additional dynamic range from the HDR process won’t be needed. One of the most useful times to use LE HDR is when you’re shooting at night. You can expose for both the moonlit foreground and the stars in the night sky.

With that said, I like to shoot bracketed exposures whenever I’m doing long exposure photography anyway. Even if I don’t use the extra frames, it’s a useful fall back to have.

What You Need to Shoot a Long Exposure HDR Photo

LE HDR has the same basic requirements as regular long exposure photography: at a minimum you need a camera that has bulb mode, a stable tripod and an intervalometer. If you can bring along a weight to anchor down your tripod, all the better.

If you’re shooting during the day, you might also need a neutral density filter to get long enough shutter speeds.

setup
My set up for shooting an LE HDR image.

Instead of an intervalometer, I prefer to use a Triggertrap mobile dongle which I’ve written about before. With it, I can control my camera from my iPhone or iPad. It even has a specific LE HDR mode that does all the camera control for you.

Shooting a Long Exposure HDR Photo

To shoot a LE HDR image, head out to a suitable location and start by securing your camera to the tripod. Put your camera in manual and set your ISO to your camera’s lowest native setting. For most DSLRs, it’s 100 ISO.

Next, frame your shot. Pay particular attention to objects that move in the scene. How will their movement look when it’s blurred out over a minute long exposure? If it won’t look good, like people walking in the distance, can it be easily removed in post? If it’s not going to work out, recompose your shot to eliminate the problem.

dark frame
One of the frames used in the HDR image above. It was shot at f/16, ISO 100 for 20 seconds. 

At this point you should also decide what ballpark shutter speed you’re looking for. Is 30 seconds enough to get the creative effect you’re looking for, or do you need exposures that are five or six minutes long? There’s no need to settle on an exact number yet, but having a rough idea is important for setting everything else up.

After deciding on your framing and ballpark shutter speed, you need to consider what depth of field and sharpness you require. For most LE HDRs, you’ll want everything from a few metres out to infinity to be sharp. While you could stop your lens down to its minimum aperture, which will give the greatest depth of field and slowest shutter speeds, it will not give the sharpest images. Every LE HDR image requires balancing these three factors.

The best solution is to select the sharpest possible aperture that meets your depth of field requirements—which I’ve covered how to do—and use a neutral density filter if your shutter speed is still too fast. In most situations, however, you’ll find that the optimum aperture is between f/11 and f/16 which will give reasonably slow shutter speeds. If, with the sharpest aperture for the scene and a neutral density filter, your shutter speed is still not slow enough you’ll need to sacrifice some sharpness and stop your lens down more.

base plate
One of the frames used in the HDR image above. It was shot at f/16, ISO 100 for 40 seconds. 

Set your lens to manual focus, then focus either a third of the way into the scene, or at the distance your depth of field calculations require.

With everything else decided, you can start to work out an exact shutter speed for the base plate. Every other shot will be calculated from this. The base plate should correctly expose the scene’s midtones; it’s okay to lose shadows or highlights as other plates will capture them.

The simplest way is to use your camera’s built-in meter to find a starting shutter speed. Take a test frame and inspect the histogram. If the midtones are correctly exposed, use it as your base plate; otherwise adjust the shutter speed to increase or decrease the exposure as required.

overexposed
One of the frames used in the HDR image above. It was shot at f/16, ISO 100 for 80 seconds.

If you’re shooting extremely long exposures (upwards of a few minutes), one way to speed up the test shots is to crank your ISO way up. Find a shutter speed that works at the high ISO and then do a bit of basic maths to work out what the equivalent shutter speed is for the lower ISO.

The last thing to do before shooting is calculate the shutter speeds for the other plates. The most common HDR settings are to take two additional frames, one a stop overexposed from the base plate and one a step underexposed from the base plate. To find the required shutter speeds, double your base shutter speed and divide it by two respectively.

If you’re using Triggertrap, there’s no need to do the additional calculations. You can just enter the base shutter speed and the steps you want between each exposure.

Finally, it’s time to shoot. Most cameras don’t allow you to select shutter speeds above 30 seconds so you’ll need to use an intervalometer. Put your camera in bulb mode and use the intervalometer to get each required shot.

For Triggertrap, connect it to your camera, enter the settings you want and push the red button.

While the camera is shooting, step away. Don’t go near it. The last thing you want to do is move the camera when it’s taking long exposure photos.

Processing the Long Exposure HDR Photo

Once you’ve shot everything you need, it’s time to combine all the plates into a single frame. There are countless applications, including both Adobe Photoshop and Adobe Photoshop Lightroom, that you can use to merge the plates. Andrew Childress has recently looked at how to do it in Lightroom. Use whatever you’re comfortable with.

While the automated processes in software like Photoshop will get you some of the way to a final image, it’s important to perform additional post-processing—or at least consider it. I like to use Photoshop’s automation as a base and then tweak what it’s done. From there, I’ll perform techniques like dodging and burning, and colour toning to get the final LE HDR image.

Final Thoughts

Shooting a LE HDR image combines the tools and techniques of both long exposure photography and HDR photography. The most important part of making an LE HDR image is choosing a suitable subject. Landscapes, seascapes and cityscapes are all perfect, while portraits don’t normally work.

If you’ve a great subject, the technical part is easy enough. Although there are some extra things you need to think about, the process of making an image is still much the same. Follow the steps I outlined above and everything will work out. Just make sure not to knock the tripod!

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