For a long time I wanted to explore long-exposure photography. The price of some Lee filters, the conflicting opinions about expensive filter systems and the mixed reviews of cheaper alternatives such as Camdiox made me hesitate. Some filters were also difficult to use with my Samyang 14mm f/2.8, and the available solutions could be expensive and cumbersome.
Then I came across a technique that quickly became one of my favourites for photographing water: taking a sequence of exposures and combining them in post-processing, rather than using a neutral-density filter to make one continuous long exposure.
The technique explained
Imagine taking ten consecutive photographs, each exposed for 30 seconds at f/22 and ISO 100. If the camera remains perfectly still and the frames are captured without significant gaps, the sequence records activity over a total period of around five minutes.
When the images are combined using the Mean stack mode, the result averages the frames. This can reduce random noise and can produce a smoother appearance in moving elements such as water or clouds. It is not identical to a single five-minute exposure: gaps between frames, changes in light and the way moving elements are averaged can all affect the result. The noise improvement also does not mean that ISO 100 becomes ISO 10. Averaging multiple frames improves signal-to-noise ratio, but it does not change the camera’s actual ISO setting.
A tripod is essential. The photographs need to be aligned accurately, otherwise the final image may show unwanted ghosting or soft edges. Use a remote release or intervalometer if you have one, and keep the exposure and white balance consistent across the sequence.
Photoshop steps
- Open the photographs in Camera Raw and apply the same basic adjustments to each frame.
- Load the images into Photoshop as layers.
- Select the layers and convert them into a Smart Object.
- Choose Layer > Smart Objects > Stack Mode > Mean.
- Inspect the result at full size. If the camera moved between frames, align the layers before stacking.
Change the stack mode to Mean:
The technique is not a complete replacement for an ND filter, but it is an interesting alternative when you have time to make a sequence of exposures and the scene is suitable.
Historical note: The screenshots refer to an older version of Photoshop. Menu names may differ in current releases.
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