Image Stacking vs. Long Exposure

The last time I was at Brushy Creek park, I wanted to do some long exposures but I didn’t have my ND filter with me. So, I just didn’t get the picture and thought that was that. A few weeks ago, I was watching a YouTube vlog by a photographer I like named Evan Ranft and when he wanted a long exposure of water, he set up his camera to take several shots and then stacked them in Photoshop to get the long exposure effect. My first thought was “why don’t you just use an ND filter?”. But, then I thought that maybe he didn’t have one, like I didn’t have one when I wanted a long exposure and that this was another way of getting this shot.

So, off to Brushy Creek park I went again to do this experiment and see if I can get a long exposure without an ND filter. It was not a day with beautiful clouds or anything and these are not breath-taking landscape photos, but I was there for this technical experiment. I mounted the camera on my tripod, framed the scene, and set the camera to take 32 pictures in succession at 5 second intervals. And then I waited about a minute and forty seconds for the camera to do this. Each individual image looked about like the image below; as you can see there are a lot of ripples in the water that a long exposure would smooth out.

Phote in sequence for stacking
Nikon D750 + 16-35mm,f/4; 16mm, f/8, 1/200s, ISO100

To set the camera to take a series of pictures like this, you go into the Photo Shooting Menu –> Interval Timer Shooting. You select how many photos you want and what the interval is and then you select “start”. There is an option for exposure smoothing if the light will be changing much during time that the camera is shooting and when this is enabled, the camera will try to even the exposures. This might be useful if you are stitching images together to make a time-lapse movie.

The post processing work is quite a bit more time consuming. When I got back to my computer, I had to

  • copy the pictures (RAW) from my memory card to my computer
  • import the pictures into Lightroom
  • find the series of 32 pictures, select them all, and then open them as layers in Photoshop (it took several minutes for Photoshop to do this)
  • once in Photoshop, select all of the layers and auto-align them (this takes several minutes)
  • convert all of the layers to a smart object (takes a few seconds)
  • stack the images: Smart Objects –> Stack Mode –> Mean (this takes several minutes)
  • save the resulting image and return to Lightroom for adjustments (quick)
  • export a JPG (see below)
Photoshop mean stack of 32 exposures of Brushy Creek
Mean stack of 32 exposures

For comparison, I also put on my 10-stop ND filter and took the same photo as a long exposure. You can compare the long exposure image below with the 32 exposure stacked image above. I think that the water came out a bit smoother with the long exposure.

Long exposure of Brushy Creek
20 second exposure with 10-stop ND filter, 16mm, f/16, 20s, ISO100

To take the long exposure, I had to start by setting the camera to a good exposure in manual mode without the filter, then install the filter, and then adjust the shutter speed to 1000 times the speed without the filter. I then triggered the shutter using a remote, and waited 20 seconds for the shot and another 20s for the noise reduction frame. When I got back to my computer, my workflow was:

  • copy the picture (RAW) from my memory card to my computer
  • import the picture into Lightroom
  • make my adjustments
  • export a JPG

The advantages of the ND filter, long exposure method was that it was a lot less work in Lightroom and Photoshop. In fact, it didn’t require that you have access to Photoshop or a program that can do this stacking. Also, I did get smoother water with the long exposure, though I might have just taken more photos to stack and watched the little spinny thing in Photoshop for even longer.

The advantages of the photo stacking method would be not needing an ND filter, which costs a lot of money and it is something else to lug around. Also ND filters can affect color, but the better ones minimize this problem. And perhaps a less noisy picture as noise tends to creep into the photo the longer the exposure, see the 4 and a half minute exposure below. This is not really a problem at 20s and ISO100, but if you look closely at the 270 second exposure below, you can see a few noisy pixels.

Long Exposure of Austin Texas after sunset
Nikon D750 + 16-35mm, f/4; 26mm, f/4, 270s, ISO400, 10-stop ND

I think that I would probably accept either photo but I prefer the filter method as it is a lot less work in post-processing and while I enjoy the photo taking experience, the post-processing experience is more of a task I have to do when I get home.

Please, tell me what you think in the comments below.

3 thoughts on “Image Stacking vs. Long Exposure

  1. Interesting experiment. From what a colleague told me a multi exposure is more useful to remove people from a photo. A person in a long exposure can leave a slight ghost trail but a multi exposure can remove the person completely if there’s a separate photo with a clear view of the background behind that person ( if you see what I mean! ) I haven’t tried either so I can’t comment further.

    Are there any other options for the “stack mode”?

    Liked by 1 person

    1. Yeah, there is a bird in there I thought about trying clean up.

      I will have to look in the menu to see what other stack modes are there. I have done the focus stacking before. That didn’t take as long as it was just a few photos.

      Like

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