In HDR photography, the only varying factor controlling the exposure in your bracketed sequence should be the shutter speed. This means the camera controls the shutter speed to give us the median, darker, and brighter exposures. Because of the camera’s limitations regarding the highest possible shutter speed and the fact that shutter speed controls motion freeze, there are optimal shutter speed settings. The optimal shutter speed depends on a few factors, which we’ll discuss below.
Optimal Shutter Speed Settings
Issue #1: Shutter Speed Limitation
The first issue we need to consider regarding shutter speed involves a limitation caused by the camera itself. The fastest shutter speed that most DSLRs can shoot at is only 1/8000th of a second.
In a 3-frame/3-stop bracketed sequence, we cannot start our median exposure at 1/2000th of a second. Imagine our shutter speed is the only thing that is changing and our aperture and ISO are set to some arbitrary value. Going one stop darker would take us to a shutter speed of 1/4000th of a second. To go two stops darker, we’d hit 1/8000th of a second. In order to go three stops darker, we’d need to go to 1/16000th of a second. However, we’ve already hit the shutter speed limitation at 1/8000th of a second. This means that our camera cannot go any faster to further reduce the brightness of the image.
Optimal Exposure Value: Two Stops or Three Stops?
If we were to shoot a 3 frame-3 stop bracketed sequence, the camera would not be able to shoot the darkest image in the sequence if we started the median exposure at 1/2000th of a second since 1/8000th of a second is the darkest the camera can go and 3 stops darker needs to be at 1/16000th of a second. This is why the optimal exposure value difference should be at two stops instead of at three stops. At three stops, you start to run into this shutter speed limitation. In addition, you will also run into motion issues, which we will discuss below.
Where to Start Your Median Exposure for a 3-Frame/3-Stop Bracketed Sequence
If you want to shoot a 3-frame/3-stop bracketed sequence, the maximum shutter speed for the median exposure becomes 1/1000th of a second. At 1 stop darker, we will be at 1/2000th of a second and at 2 stops darker, we will hit 1/4000th of a second. Finally, at 3 stops darker, we will reach 1/8000th of a second, which is at the shutter speed limitation. Starting your median exposure at 1/1000th of a second will allow you to capture the darkest image in a 3 frame-3 stop bracketed sequence. At one stop brighter, we will be at 1/500th of a second. Two stops brighter will take use to 1/250th of a second, and three stops will land us at 1/125th of a second.
Where to Start Your Median Exposure for a 3-Frame/2-Stop Bracketed Sequence
If we’re shooting a 3-frame/2-stop bracketed sequence, the maximum shutter speed for the median exposure should be 1/2000th of a second because we will be at 1/8000th of a second at 2 stops darker. To go one stop brighter, we’d end up at 1/1000th of a second, and to two stops brighter would mean dialing in 1/500th of a second.
Issue #2: Motion Blur and Ghosting
The second issue that we run into with shutter speed is motion blur and ghosting, which are two different things.
Unintended Motion Blur
If we shoot a 3-frame/3-stop bracketed sequence, motion blur is very likely to occur. This is why it is difficult to shoot 3-frame/3-stop bracketed sequences in HDR photography. For example, if you start your median exposure at 1/500th of a second, we have to go to 1/250th of a second to go one stop brighter. We’d then drop to 1/125th of a second to go two stops brighter. Going three stops brighter would require a shutter speed of 1/60th of a second. However, at 1/60th of a second and 1/125th of a second, we’d likely see motion blur. With moving objects in our scene, motion occurs when the brighter shot is at a low shutter speed.
Using Motion for Effect
However, you can also use motion blur to your advantage by creating motion effects in your image. For example, by using a slow shutter speed, you can make the water in your scene apear smooth and silky in your image. It’s worth noting that we cannot go beyond 30 seconds, unless we are using a shutter release, when going brighter in our bracketed sequences. For example, a median exposure set at 7.5 seconds with a 3-frame/2-stop exposure sequence will take us to 15 seconds at one stop brighter. At two stops brighter, we hit 30 seconds, which is the camera limitation.
Regardless of the median exposure, a 3-frame/3-stop bracketed sequence is not the best choice when shooting HDR photography.
The optimal shutter speed will really depend on the overall scene that you are shooting. The shutter speed will also depend on whether or not there are moving objects in the scene and whether or not we want to create some sort of motion effect in the scene by dragging out the shutter speed. We also need to consider hitting the 1/8000th of a second limitation or the 30-second limitation. These are the shutter speed considerations that we need to think about whenever we approach a scene.
For the reasons mentioned above, we usually stick with a 3-frame/2-stop bracketed sequence. This sequence is generally the safest and will yield the best results. This bracketed sequence also gives us the widest variety in choosing shutter speeds. As such, the optimal shutter speed for 3-frame/2-stop bracketed HDR exposures falls between 1/500th and 1/2000th of a second. For 3-frame/3-stop bracketed HDR exposures, your optimal shutter speed will be 1/1000th of a second. Again, this allows the top end to peak at 1/8000th of a second, which is most DSLRs’ shutter speed limit. At the same time, the bottom end is at 1/125th of a second, allowing you to freeze as much motion as possible.
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