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How to simulate long exposure using stacked image processing

The message How to simulate long exposure using Stacked Image Averaging first appeared at Digital Photography School. It is written by Rick Ohnsman.

Silky water effects, striped clouds, movement smoothed with an ethereal appearance; Long exposure photography seems to be in vogue when photographers discover the appearance that can be created. There are several ways to achieve this. The most basic way is to buy a standard neutral density photographic filter that cuts the light so that you can use slow shutter speeds without overexposing your shot. You can achieve exposures for minutes, especially when using 10-stop ND filters such as the Lee Big Stopper or even the 15-stop Super Stopper.

I recently did an article about an alternative way to take long-exposure photos: "Try this do-it-yourself filter with neutral density for long-exposure photos." I encourage you to read the piece and learn how a piece of welding glass can be a budget substitute for more expensive photographic ND filters.

simulate long-exposure stacked image averaging

This is the same location that I used for some of the other shots in this article, but taken when the river was much higher and faster. The biggest difference is that I use do-it-yourself welding glass ND filter to achieve this shot. See my other article for this technique.

This article teaches you a third method for creating images with a long exposure time without filter. In contrast to the trick of the welding glass, where your final image almost always has to be black and white in order not to have to fight against the heavy color cast, this works well in color, without filter and without color cast. It is a great method to simulate long exposure.

The technique uses a stack of multiple images from the same scene and then processed with a Photoshop process called Image Averaging. It is really quite simple and has some advantages over traditional methods with ND filters.

Advantages compared to the traditional ND filter method

When taking traditional photography with a long exposure time with an ND filter that you are about to take long exposures. (Duh!) There are a few challenges with this:

  • If you bump into the camera during the long exposure or move things in the shot that you don't want to blur, you have to take the shot again.
  • Long exposures can often take a few minutes. Double the time if you also enable in-camera noise reduction. If it takes 2 minutes to illuminate and another 2 minutes before the noise reduction works, take a picture only every 4 minutes. This can really slow down your work, and if the light changes during that time, you can miss it.
  • With very dark ND filters, you can no longer see through the lens once the filter is in place. You must compile your recording, focus in advance, then link the ND filter and create the image.
  • Determination of exposure will require some calculation. You check the exposure without the filter and then use a calculation tool to determine the new shutter speed that the ND filter requires. Often this needs to be adjusted after you've seen your photo and … yes, another attempt is needed.
  • When you're back in editing, you'll see the shots and you wish you'd used longer or faster shutter speeds to change the look, too bad. You have to go back and shoot again – if that's possible.
Image: in fairly bright sunlight, even with the ISO at 50 and the aperture at f / 22, 1/5 of a second w ...

In fairly bright sunlight, even with the ISO at 50 and aperture at f / 22, 1 / 5th of a second such a slow shutter speed was achievable while maintaining the correct exposure. This was without filter.

The benefits of the Image Averaging method

The benefits of using the image stacking method are essentially the opposite of the above mentioned things:

  • You are going to make multiple images instead of one long one. If one of the images in the group has a problem, you may be able to remove it and use the rest to still make the effect successful.
  • You can see what you are doing! If you are not shooting with a dark filter, you can still see, compose, use auto focus, auto exposure, and even image stabilization when shooting by hand.
  • No calculation! Without the addition of a dark filter, you eliminate this step.
  • Adjust the length of your "simulated slow shutter" later in post production. Do you want more or less blur? You can change your mind later.
  • Are the circumstances too clear for a standard photo with a long exposure time? You may only have a 6-stop ND filter and the daytime conditions are too bright to give you the length of exposure you want. You can combine both methods to simulate a longer exposure than possible with just the ND filter.
  • Are there people in the recording that you want to delete? Because they probably move during the multiple shots, they will disappear when the averaging process takes place!
simulate long-exposure stacked image averaging

Let people disappear! Note the bet on the people walking in the river, but on the completed recording, 15 images, each simulated 1/5 second = 4 seconds. They are gone.

Make shots

Setting up and recording the images you need for your average image creation is almost the same as any photography. Here are the factors and steps to keep in mind:

Composition still counts!

Because you introduce a blurry effect with long exposure, this does not mean that you have taken a good photo automatically. Still consider how to carefully compile your image. Keep in mind that moving objects fade into the recording and look easier with fewer details. Good images with long exposure often emphasize the contrast between static, non-moving objects (buildings, rocks, trees, etc.) and moving objects such as clouds and water. Include both in your photo.

Shoot a tripod

I said you could do this handheld and, well … maybe you could. Even with this technique, however, you still want to shoot with the slowest shutter speed. That way you don't have to take too many photos to combine. Once you become much slower than 1/30 of a second (and faster than that if you've just had coffee), holding your camera is likely to ruin your photos.

simulate long-exposure stacked image averaging

All images ISO 50, f / 22. Top left – No filter – 20 images every 1/5 second = simulated total = 4 seconds. Top right – No filter – 35 images every 1/5 second = simulated total = 7 seconds. Below – 6-stop ND filter – 15 images every 20 seconds = simulated total = 5 minutes.

How many shots?

This technique simulates long exposure by combining multiple shots. The simple formula is:

(# Shooting) x (shutter speed of each shot) = total simulated shutter speed effect (in seconds)

Let's connect some numbers to that and view the result. Set your camera to the lowest possible ISO. I can lower my Canon 6D to ISO 50. Some cameras have ISO 100 as the lowest. Use what you can. Set your aperture to the smallest possible aperture. Meter with those settings and see how long you can take each individual photo and illuminate it correctly. Suppose we could do this in the shadow: 1/4 second, f / 22, ISO 50. To get a simulated shutter speed of one minute (60 seconds), we have to take 240 shots.

240 shots x 1/4 second (.25) = 60 seconds

That is a bit of a log and stacking 240 images in Photoshop can cause your computer to choke. So what to do? You may not have an ND filter in your bag, but you do have a circular polarizer. It will help reduce the light. You mount it and now notice that you have lost 2 stops. So your exposure can be 1 second, f / 22, ISO 50. Put it in the formula and you get:

60 shots x 1 second = 60 seconds

When shooting in low light, you may be able to start with a slower shutter speed. This means that you can take fewer photos.

Always try to get the slowest shutter speed for your shots to make your task easier (and also the computers). That means you can take the simulated long exposure with fewer shots.

Suppose you had a 6-stop ND filter in your set. You mount that, and now your settings are 16 seconds, f / 22, ISO 50. Now, to get that simulated 1-minute exposure, you just needed about four shots. Why not make 10 while you are busy and you can simulate an exposure of 2.6 minutes (160 seconds)?

If you had done this traditionally and had a 10-stop ND filter, you could lower the unfiltered exposure from 1/4 second, f / 22, ISO 50 to 256 seconds (4.2 minutes), f / 22, ISO 50 So, to get the same effect with a 6-stop ND filter as you could with a 10-stop through image average, take 16 shots.

16 shots x 16 seconds each = 256 seconds (4.2 minutes)

simulate long-exposure stacked image averaging

35 images per 1/6 second are combined to simulate an exposure of 6 seconds. If you are shooting in the sun, it is probably impossible to make a 6-second exposure without a filter.

Forget the math, make the shots!

If all that math hurt your head (it hurt mine), here's the easy way to get what you need so Photoshop can do its magic:

  • Use a tripod. You don't want to do all this and take shaky photos. That will waste all your work.
  • Do what it takes to shoot with the slowest shutter speed you can get with the equipment you have. In the camera, that usually means setting the lowest ISO and the smallest aperture.
  • If you have a … polarizer or ND filter, Use this to make the shutter speed even slower if you can.
  • Take a lot of photos ' s for each stacked image that you create. Depending on how slow you could get your shutter speed, a few dozen are not too many. You don't have to use them all while you are editing, but if you have more, you can get a longer simulated effect.

Put everything together

This recipe assumes that you use Adobe Lightroom and Photoshop in combination. You do not need to use Lightroom. You can stack your individual images in Photoshop in a different way (although the use of LR is much easier). However, the use of Photoshop is mandatory. To use the Smart Objects feature described, you also need a version of Photoshop that is version 14.2 or higher. Older versions of Photoshop do not have this.

There are ways to do this with older versions in a more manual process. If you have an older version, you need to do some online research to learn that technique. I used the latest version of Photoshop during this writing (Photoshop CC 20.0.4).

Let's visually examine this step-by-step process …

simulate long-exposure stacked image averaging

1. In Lightroom, select the series of images that you want to use. Edit the first in the series as desired. Then select them all and use the Sync function so that they all have the same settings as the first.

How to simulate long exposure using stacked image processing

2. With everything selected, send the images from Lightroom to Photoshop by going to Photo-> Edit In-> Open as Layers in Photoshop. (Photoshop opens and the images appear as layers in a stack). If you have to open and stack many images, this may take a while. Let it work.

How to simulate long exposure using stacked image processing

3. Select with all selected layers in the menu Layer-> Smart object-> Convert to smart object. This can take some time to do its job. Be patient.

How to simulate long exposure using stacked image processing

4. Select the Smart Object layer and in the menu select Layer-> Smart Objects-> Stack Mode-> Average. This can also take a little time to work.

How to simulate long exposure using stacked image processing

Wait … wait … and …

simulate long-exposure stacked image averaging

Presto! You will have created a simulated image with long exposure from your stack of shorter exposures. 20 images each in 3.2 seconds, f / 22, ISO 50. No filter used. Simulated long exposure time of 64 seconds.

simulate long-exposure stacked image averaging

The water in this part of the river was pretty calm anyway, but look at the before and after areas indicated by the arrow where the original shots were 3.2 seconds versus the combined 20 shots x 3.2 seconds = a simulated 64 seconds.

How to simulate long exposure using stacked image processing

5. To finish, go to Layer-> Flatten Image. Then File-> Save As and save the finished image wherever you want. If you want to give the finished image some extra adjustments, you can do that with Photoshop or Lightroom as you would with any other image.

To remember…

That's the magic! Here are a few things to remember for the best results:

  • Think of your composition. Look for a scene with a combination of static objects that do not move during the sequence and objects that do move. An image with both will be more attractive.
  • Use a tripod. You can do this handheld if you have to, but know that every camera movement is translated as a haze in the end result.
  • Do what you can to get the slowest possible shutter speed with every frame in the series. Set your ISO to the lowest setting, use a small aperture and use polarizing filters or whatever ND filters you have. Longer exposures for each shot means that fewer images are needed to create a simulated long exposure.
  • Overshoot. You do not have to use all the images in a series if you decide that you do not want that much blur. However, if you don't shoot enough, you wish you had them later.
  • While going through the steps, some things can take a long time. Be patient and let your computer work. If the process crashes, you may not have enough computer resources and have to settle for a smaller stack.
simulate long-exposure stacked image averaging

5 images, every 6 seconds = a simulated exposure of 30 seconds. No filter used.

simulate long-exposure stacked image averaging

10 images, taken together every 1/4 of a second, give a simulated exposure of 2.5 seconds. This can be a great technique to get silky water effects if you don't have an ND filter and only need a longer exposure time of a few seconds.

Final thoughts

Is this a better method than using a real ND filter? Like so many photographic things, the answer is probably … it depends. Maybe you don't have a filter or have one with you. Maybe you don't need anything really long exposure, but only one few longer than you can get with a combination of low ISO / small aperture, such as when looking for blur on a waterfall. You may have to make people disappear and you do not want to make a single recording of several minutes for various reasons. Alternatively, you may have an ND filter but need an even longer exposure than it can give you.

There are many reasons to add this How to simulate long exposure using stacked image average technology to your bag with tricks. Give it a try and I am sure you will have fun. Share your images with us in the comments!

simulate long-exposure stacked image averaging

The message How to simulate long exposure using Stacked Image Averaging first appeared at Digital Photography School. It is written by Rick Ohnsman.

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