In this article...
We've all seen those beautiful long exposure photos. Crashing waves reduced to a fog on a rocky beach. Waterfalls that look more like mist than water. Clouds flowing carefree across the sky.
How do you take a photo like that?
Well, the technique is known as long exposure. Simply put, it means keeping your camera's shutter open for a much longer time than usual so that the scene becomes blurry wherever there's movement. Essentially, the movement is averaged out.
In low light (e.g. at night), this technique works beautifully. But if you kept your shutter open in daylight for 5 seconds, 10 seconds or even longer, there would be far too much light and your image would be totally washed out - likely almost completely white. There's simply too much light even at low ISO with a narrow aperture.
That's where solid ND, or Neutral Density, filters come in. They simply block out some of the light - without changing the color. They're measured in stops, and the higher the number of stops, the more light they block out. Each stop doubles the shutter time needed to achieve the same exposure.
Let's say that a scene is well exposed at 1/125th second shutter speed. With a 3-stop ND filter the shutter speed would slow to 1/15th second, and with a 10-stop ND filter it would slow to 8 seconds - all at the same ISO and aperture!
It's a tried and tested approach, and people have produced some amazing photographs using this technique, and I don't mean to discredit the approach by any means. But there are some drawbacks:
- ND filters aren't cheap, and are either designed for a specific lens diameter or you'll need adapters to fit each lens you want to use;
- ND filters are one extra thing to have to carry with you when hiking;
- Slow shutter speeds can increase the noise in an image as it adds up over time;
- If something moves in front of your frame (e.g. an animal, person, etc) during the exposure then it may cause an artifact on your finished shot;
- You have to expose (and oftentimes, focus) the shot without the ND filter in place, then calculate what the shutter speed should be to take the shot;
- Any movement of the camera while the shutter is open will ruin the shot.
Is there another way?
Yes, we can fake it!
I don't mean fake it by adding blur effects in Adobe Photoshop, I mean using our camera to take the photo a different way.
In traditional long exposure photography, the ND filter reduces the amount of light that reaches the sensor, so that it adds up more slowly to create the final image. What if instead we could collect more light in total and just average it all out? So rather than capturing all the light in one exposure, we capture it in multiple.
That's exactly how this technique works.
We'll take lots of individual photos, each one with the same settings and correctly exposed. Then we'll use software to average out the color and luminosity of each pixel - it's mathematically almost the same as long exposure, but with no need for an ND filter.
Plus, an ND filter reduces light going into the lens (the signal), but noise from the camera sensor & electronics still accumulates. This means the signal-to-noise ratio is higher on longer exposures. With this "fake" technique our shutter speed is still fast, so the signal-to-noise ratio remains low.
There are times when this technique doesn't work well, for instance:
- In dark conditions when you need a slow shutter speed to capture enough light;
- When you want uninterrupted trails of light - they'll appear as dashes with this technique due to the brief time when the shutter closes.
But generally speaking, this is a very versatile technique!
I was inspired by this great blog post by Pat David to try this technique, and have learned to really enjoy it!
To keep things simple, I've broken the process down into four steps:
We'll start at the beginning and work our way through, using an example throughout.
It all starts here. No amount of processing later will compensate for poor capturing.
Once you've found the scene you want to photograph, take your time to set the camera up and compose the shot like you normally would. The camera needs to be on a very stable tripod or base where it's not going to move whatsoever for the duration of the shoot - e.g. no bouncy footbridges (he says, from experience!). There are methods to align the images later, but it's much easier to stabilize things now if you can.
You're going to take lots of photos in rapid succession, so make sure you're on manual mode with manual focus - you don't want the camera settings to change between shots. You want the shot to be well exposed - just pretend you're taking a normal photo.
Once everything is ready, take a test photo to make sure it all looks good. Again, all the normal things - good sharp focus, no highlights blown out, shadows lost, etc.
When it's all set up the way you want, it's time to take some shots. You'll need to use an intervalometer for this. Some cameras (like our Sony a7R III have one built-in), or for others you may need an external intervalometer like this one - make sure it's compatible with your camera. You can also use a remote release and just put the camera into burst mode, then hold down the button on the remote release.
The number of shots you take depends on the shutter speed of the individual frame and the amount of total exposure time you want.
In this example, I had exposed the shot so that the shutter speed was 1/13th second. 100 exposures would give me just under 8 seconds (1/13 multiplied by 100), and somewhere in the range of 5-15 seconds generally works well for moving water.
I usually set the intervalometer for a slight delay before it starts shooting, just so any vibrations from pressing the shutter release don't impact the image. If you're using an intervalometer with remote release, you likely won't have this issue.
One of the downsides of this approach is the amount of data it produces! Our Sony a7R III creates 83MB RAW images, so 100 shots is over 8GB of photos! Make sure you use a large SD card like the 128GB UHS-II card we use.
Before wrapping up, click through and make sure the photos all look good.
I use Adobe Lightroom to edit my photos, but use whichever software you're most comfortable with.
I start by checking through every frame, and making sure they all look good. If any have obvious artifacts or problems (e.g. a stray flashlight flashing across the frame), then simply delete that photo. Minor issues will disappear during the compositing phase anyway.
Pick a photo to edit (I just use the first one for convenience), and process it however you'd like. Don't forget to include things like lens correction, but don't add sharpening or noise reduction.
Ensure that the histogram is where you want it - typically no shadows lost or highlights blown out.
I'd recommend against any spot edits (e.g. clone or heal) at this stage, but you can crop the image if desired.
When you're happy, synchronize your edits to all images in the set and export them. I export full-resolution 16-bit TIFF files with the Color Space set to ProPhoto RGB.
This is where the magic happens!
We're going to use free software called ImageMagick. It's available to download from their site for Windows, MacOS and Linux. Follow the instructions on the Download page to install it on your machine. When you install it, make sure to check the box to install legacy components such as convert.
While this is a powerful library with lots of features, there's just one that we need - its command for averaging a set of images.
To run the command, open a terminal. On MacOS, use Cmd + Space to start Spotlight, then type in terminal to open a terminal window. Browse to wherever you exported the images.
Assuming you exported the images as TIFF files with the .tif extension, then run the following to composite the images.
convert *.tif -evaluate-sequence mean -alpha off composite.tif
Be warned that this command will use a lot of RAM. Compositing the 100 images I took used around 40GB of memory, so if your computer doesn't have enough, it will take a long time to complete as it relies on swap files. Be patient and it'll get there eventually. This will create a file called composite.tif that you can then re-import into Lightroom.
Onto the final straight: editing the finished photo.
Here is where you can let your artistic talents loose - editing the photo as little or as much as you want.
In the case of this image, I made some further changes to the overall image, and then dropped into Adobe Photoshop to remove some distractions.
If you zoom in on the final image, it should be very sharp in areas of no movement, and assuming you shot at low ISO, almost no noise whatsoever.
This technique doesn't need to be a replacement for long exposure photography with ND filters. But I believe that the more tools and techniques you have as a photographer, the better.
As well as compositing individual photos, you can also composite frames from a video. Although you are limited by the resolution of the video, with higher and higher resolutions becoming standard, that's less of an issue.
I love using this technique because it means one less piece of equipment that we need to buy, store in our RV, and carry around with us on hikes. As long as I have my camera and a stable platform or tripod, I can get the shot.