Image Denoise via Stacking

Tools for Overcoming Photographic Limitations

Seattle Skyline Aerial, ISO 1600

City lights of Seattle skyline and space needle

Photographic gear are simply tools. They exist in order for us to fulfill our creative vision. But like any tool, there are limitations. Your choice of focal length, shutter speed, aperture, and ISO will shape your image, but even with the best settings and technique, certain images are impossible to create. Despite the improvement in modern sensors’ dynamic range, exposure bracketing in the field and exposure blending during post processing still is a useful technique. Larger sensors such as full frame and medium format have less apparent depth of field at a given focal length and aperture than smaller sensors. Even when stopping down to f16-f22, some images show a noticeable depth drop off on the near or far end of the frame. Not to mention those small apertures smudge fine details through diffraction. So, what do some photographers do? Focus stack several images at a sharper aperture and blend together.

One of the most popular modern drones at present is the DJI Mavic 2 Pro, of which I own and use frequently. It features a 1″ imaging sensor which is 4-5 times larger than most of the competition. The improved image quality is noticeable, but still lags behind APS-C and larger sensors, especially at higher ISOs (limitation #1). And when you are up in the sky, unique challenges to low light photography start to stack up (pun intended). Despite being relatively stable, getting sharp images at shutter speeds slower than 1/4 sec can be challenging (limitation #2). Recently, just before sunrise in Seattle, I recalled a technique I had heard about where multiple images at higher ISO values could be merged together to eliminate noise. I decided to give it a go. Some tips for drone pilots:

  1. With the DJI drones, set your flight mode to “tripod” or “T” to keep your composition steady
  2. Set your drive mode to burst and set it to 5 frames
  3. Keep your exposure settings constant
  4. Capture many frames…how many? Let’s find out!

I’ll use the image at the top of this post as my example. It was shot at ISO 1600, f3.5, 1/8 sec. I captured 30 frames total in this setting as I wasn’t sure how many I needed. Back home in Lightroom, make sure all your settings are zeroed out, especially sharpness, noise reduction, and lens profiles. Select your images you want to stack, right click and choose “Open as Layers in Photoshop”.

Once the images have been loaded and layered in Photoshop, select them all by going to Select>All Layers or use keyboard shortcut Alt+Ctrl+A or Option+Crtl+A. Go to Edit>Auto-Align Layers and be sure it is set to Auto and both Lens Corrections options are unchecked.

After the layers have been aligned, the next steps involve converting the stack to a Smart Object by going to Layer>Smart Objects>Convert to Smart Object. Once that is finished, you will then go to Layer>Smart Objects>Stack Mode>Median. Now, watch the noise disappear!

Comparison Between Stacks

5, 10, 20, 30 image stacks reviewed

Like I mentioned previously, I wasn’t sure how many images to capture in the field for best results. I only recalled reading how more images would yield better results. Watch the video below for a comparison of a single file vs stacks of 5, 10, 20, and 30 images. I’ve included crops of each image as well. I did notice a slight reduction in fine detail with the 20 and 30 image stacks, more so on the 30 image stack. This could be a result of the auto-align just not being able to create a sharp file or it could be my drone had a few slightly blurred frames that were exaggerated with so many stacked together. I personally see a noise improvement with each larger stack, but for the sake of time and storage, I’m pretty happy with 10 frames stacked. Granted this is at ISO 1600. I would imagine stacked images would be beneficial if using ISO 3200 or 6400 and 5-10 might be useful for ISO 800 and lower.

So what do you think? Is this a technique you think you might use? Let me know in the comments!

And I’m pretty certain this technique can open the door to sharp long exposure aerial photos as well, so I am excited to try this on moving water along the coast or at waterfalls. If I gather some examples, I’ll certainly share in another post!

Best,
Jim Patterson

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