Multiple exposures can be used for several purposes. Well-known reasons for making multiple exposures include the creation of a panorama, focus stacking and creating high dynamic range (HDR) photos. But what not so many people know is that multiple exposures can also be stacked in Photoshop to reduce noise or to extend the effective exposure time. In this article I will explain the use of stacking multiple exposures to increase the effective exposure time.
The first step consists of taking multiple photos. For the example that I consider here, I used a 9-stops ND filter, such that the exposure time of a single photo was 10 s. I took many photos with the same camera settings and I stacked in the end 32 of them.
Once you have processed your photos in Camera RAW or Lightroom, the next step is to open the photos into Photoshop as layers. A way to do that is through Adobe Bridge: select the photos in Bridge and in the menu go to Tools – Photoshop – Load Files into Photoshop Layers.
In Photoshop, select all the layers and create a smart object.
Once you have combined all layers into a smart object, render a single image based on all the layers by selecting stack mode ‘mean’.
This stack mode creates a nice and smooth image with an effective exposure time of all photos combined.
In this case I used 32 images, each with an exposure time of 10 s, so the total effective exposure time was 320 s. 32 images corresponds to 5 stops (2^5 = 32). This means that effectively the stacked image has an exposure time as if a 14 stops filter would have been used: 9 stops from the actual ND filter that I used, plus 5 stops from stacking.
In summary: if you don’t have an ND filter with many stops or if you want to increase your effective exposure time even more, stacking multiple exposures is a great way to do so.
I further processed the stacked image in Photoshop by cropping it and adjusting contrasts and colors. The end result is shown below.