noise

Processing Star Point Images with the Help of Starry Landscape Stacker

We all love shooting under the stars, but oftentimes we are pushing our cameras and lenses to the extreme. In last month’s article we talked about “Shooting for the Sharpest Stars” and how that forces us into even higher ISOs and shorter exposures. Most cameras have difficulty at 6400-plus ISOs, and the shorter exposures make it tricky to get any detail in the foreground.

Enter the Mac-only software Starry Landscape Stacker (SLS), which blends multiple high ISO star point shots to reduce noise while keeping your stars sharp! (For PC users, your solution is Sequator, which operates and yields similar results to SLS. We will take a closer look at Sequator on our blog a little later this year.)

What is Starry Landscape Stacker?

SLS is a very intuitive piece of software that gives you smoother final images by recognizing and aligning the “tracked stars,” and then stacking the files while applying noise reduction to the other areas of the sky. This is probably the best piece of software to squeeze the most image quality out of multiple files instead of just working with one.

In this post, we’re going to look at the basics of how to use SLS. At the end, you can watch a video of me working through the details of processing an image this way.

Shooting Considerations for SLS

The key for both SLS and Sequator is that we shoot multiple images in the field—at least 10, but 20 is even better. The more information the program has, the better it will work.

So, once you settle on your star point composition, check focus and attain a good exposure, don’t just take one or two shots and move on. Instead, set your intervalometer to take 20 shots with a 1-second interval between. And you can certainly take more than 20 shots. If I have settled on a really nice composition, I might shoot it for an hour or so to get different alignments of the Milky Way as it moves across the sky.

We also have to consider the foreground. In the Figure 1 below, the exposure is good for the sky, but there’s hardly any information in the foreground.

Figure 1. Nikon D5 with an Irix 15mm f/2.4 lens. 25 seconds, f/2.5, ISO 6400.

How can we fix it? Three solutions:

1. Take a twilight shot.

If you arrive to your location prior to nightfall, take a few shots as the light varies throughout the twilights. You can use those later to blend into your star shot. Of course this approach works for only one setup per night (unless you have two or more cameras), but it can create a unique look. (In previous blog posts you can see examples of when Matt and Tim have done this.)

2. Light paint your foreground.

This works really well if we have something dominant in the foreground that is easily reachable with our light painting tools. However, if we have a big swathe of landscape, that will be difficult to paint.

3. Take a longer foreground-only exposure.

Lower your ISO, turn on your long-exposure noise reduction, and take a shot that is three to six stops higher than your sky shot. By using the a lower ISO we can get a cleaner foreground. But don’t expose so that it looks like a daytime foreground—my rule of thumb is to shoot a foreground exposure that gets the histogram off the left side and more in the middle (Figure 2).

Figure 2. Nikon D5 with an Irix 15mm f/2.4 lens. 13 minutes, f/2.5, ISO 1600.

My thought process on for this image: It was very dark, with no moon in the sky in Capitol Reef National Park, which is a Gold Tier Dark Sky Park. I figured an exposure of 4 minutes at ISO 6400 (three stops brighter than the sky shot) would start to reveal foreground. However, I also didn’t want the 6400 noise in the darker foreground, so I lowered my ISO two stops to 1600, which is incredibly clean on the Nikon D5. Adding five stops to my original image gave me an exposure of 13 minutes (should have been 16 minutes, but oh well), f/2.5, ISO 1600.

Don’t look at how bright the sky and the trailing stars are—the piece we want from this image is the foreground detail of the road cutting through the landscape.

Preparing Your Files for SLS

There are a few key things to do to your sky files in either Lightroom or Camera Raw to best prepare them for SLS. The idea here is to remove any chromatic aberrations and have a nice flat file that will help SLS align and combine the dimmer stars.

1. Lower the contrast of the image. I generally set my contrast to -100 and increase the exposure to +.30 (Figure 3). It won’t look good on the screen but that’s OK—just be careful you are not blowing out any stars.

Figure 3.

2. Turn off any sharpening and noise reduction. Bring the sliders all the way to zero (Figure 4). SLS will handle the noise reduction for now and we will do the sharpening after SLS stacks the image.

Figure 4.

3. Correct for lens aberrations. Under the Lens Correction section of Lightroom, check Remove Chromatic Aberration (Figure 5) and manually adjust any vignetting to even out the exposure of the image (Figure 6). I choose to manually adjust the vignetting instead of turning on “Enable Profile Correction” (EPC) because I’ve noticed weird artifacts/moire patterns in the stacked images when doing so early in this process. EPC adjusts for distortion and vignetting, so it’s best practice to apply the Lightroom adjustment (if you want) to the finished image after stacking in SLS. Again, our goal here is only to prepare a nice flat file that SLS can use to easily recognize the stars, sky and landscape for blending. Minimum pre-processing equals maximum results.

Figure 5.

Figure 6.

That’s it! You don’t have to remove airplane trails or satellites. SLS will help you remove them from or add them into the final image.

Once you have worked on the first image of your “batch” you can then quickly sync the adjustments to other files. Choose your best 10 to 20 consecutive images that have the best positioning of the Milky Way or stars, and export them as full-resolution 16-bit TIFF files that include all metadata (because SLS uses your exposure information when stacking). (Figure 7.)

Figure 7.

Processing in SLS

I’ll go into more in depth in the video below, but here are some overall processing tips:

1. Open the files in SLS. It will stack them together and there will be a bunch of red dots that identify the stars.

Figure 8.

2. Follow the “Workflow” instructions at the top left.

3. Adjust Dots in the Sky. Remove any red dots on the ground, along the horizon, in cloudy areas, or in any spots where there weren’t actually stars. You can even add dots into the sky if there are major areas in the sky that don’t have them.

4. Click Find Sky. That will create the blue mask of the sky; it doesn’t need to be perfect but should include all the areas where there are stars. If you have a horizon that has lots of trees cutting into the sky, choose “Mask with Islands of Sky” and that will let SLS know to create masks for the stars between the branches. You might need to clean this up, but SLS does an excellent job to get you started.

5. Align With. This will remove the blue mask. At the bottom of the workflow section it says Current Image. Click on Next and Previous to choose which Milky Way/star-point alignment you’d like SLS align to.

6. Align and Composite. This stacks all the images. You have to select a composition algorithm to choose:

  • Min Horizon Noise is the default and works in most cases. It brightens the Milky Way stars a bit more than the surrounding stars and minimizes the noise along the horizon.

  • If you do notice any star trailing or duplication along the edge of the mask, use the Min Horizon Star Dupe. There is also a “mean” version of the Horizon Noise and Star Dupe—this uses the older “median” algorithm that was seen in previous versions of SLS.

7. Click Save. SLS saves the stacked file to your folder of TIFFs. SLS gives you the option to save a copy of the mask; I generally don’t need it as the masked file works fine. You’ll need to import this new file into LR and then do any image massaging there.

Tip: You can save multiple algorithms of the stack. The Max algorithm reveals any airplanes, satellites or meteors. Save a clean Min Horizon Noise for minimal noise, but then save any meteors that came through and you can blend them together in Photoshop!

Walking Through an SSL Edit

That’s it! You now have the cleanest night skies out there! Check out the video below, where I go into detail and walk through the whole process. I also take the SLS night sky image and blend it with the foreground from the longer exposure in Photoshop.

Milky Way season is here and the temperatures are rising, bringing more noise into our shots. Use Starry Landscape Stacker to create even cleaner jaw dropping night images!

Gabriel Biderman is a partner and workshop leader with National Parks at Night. He is a Brooklyn-based fine art and travel photographer, and author of Night Photography: From Snapshots to Great Shots (Peachpit, 2014). During the daytime hours you'll often find Gabe at one of many photo events around the world working for B&H Photo’s road marketing team. See his portfolio and workshop lineup at www.ruinism.com.

Keep the Noise Down (Part I): How to Take an ISO Test with your Camera

One aspect of night photography that is particular to the camera you own is its tolerance for or its ability to repress, avoid or use high ISO noise to your advantage.

In this blog post, I teach you how to run your own High ISO Noise Test, resulting in a series of images that reveal when your camera has undesirable high ISO noise.

But first, let's identify High ISO Noise. (For those of you who already know, feel free to jump to "Preparing for the Test," below.)

What is High ISO Noise?

But what about quality?

Your digital camera provides the best-quality image at its native ISO, or the lowest number on your camera's ISO scale. On my current camera, the Nikon D750, this is ISO 100. On my previous camera, the Nikon D700, it was ISO 200.

But we can't always use the highest-quality ISO to capture star points and the Milky Way, because the amount of time required to get a proper exposure exceeds the 400 Rule.

How about saving time?

The Six-Stop Rule, or High ISO Test, also saves you valuable time in determining a correct exposure. More about that in our blog post "Save Time by Using High ISO Testing to Set Up Your Night Shots," by Lance Keimig.

Even more about High ISO Noise

In another of our previous blog posts, "Testing Your Camera’s Tolerance For Long-Exposure Noise," Tim Cooper points out, "Long-exposure noise is virtually impossible to fix via post-processing." That's why it's essential for you to test your camera's high ISO capabilities at many temperatures, and to know when and at what temperature it creates undesirable results.

But what about LENR?

LENR (long exposure noise reduction) is necessary only when heat buildup causes hot pixels—the red, green or blue pixels scattered throughout a long exposure brought on by heat building up on the imaging sensor. You can combine LENR with an educated high ISO practice, but its purpose is different and separate from high ISO noise reduction or avoidance.

Preparing for the test

You'll need the following gear:

  • your camera + lens + battery with enough power
  • tripod
  • intervalometer

You will need your intervalometer to minimize camera shake when activating the shutter, and for those longer exposures at the lower ISOs.

Where to perform your test

I suggest testing in a few scenarios. Choose a scene with some deep shadow areas, some midtones and some highlights to gauge when and where high ISO fails or excels. Possibilities include:

  • at home, in a dim to dark room
  • outdoors
  • in a rural or wilderness area
  • with your lens cap on

When to perform the test

I suggest testing your camera at many temperatures, including the following ranges:

  • 90 degrees and above
  • 80 to 89 degrees
  • 65 to 79 degrees
  • below 65 degrees

Camera settings

Here are some vital settings to help you establish a consistent high ISO test:

  • RAW capture (not JPG)
  • manual color balance (match the main light source in front of your camera)
  • manual mode (or B mode for certain Canon models)
  • one focal length (no changes if you use a zoom lens)
  • one aperture (no changes)
  • if DSLR (not mirrorless), cover the back window of your eyepiece to prevent light from bouncing in from the rear (some cameras have this capability via a small switch near eyepiece)
  • long-exposure noise reduction (LENR) disabled
  • high ISO noise reduction OFF (or if off is not available, set to Normal)

What to photograph

Ideally, I'd include an X-Rite ColorChecker Passport or ColorChecker in your test scene, but if you don't have one, no sweat.

In your scene, don't stress composing for aesthetics. Frame your image to include sky and foreground, highlight and shadow, and those transitions from light to dark. It's in those gradient areas in bright and dark areas, and in flat areas of same color and brightness, that you'll see high ISO noise appear first (and worst!).

Shooting the test

Shoot a test exposure at ISO 6400. Ideally you'll try for an aperture that yields you somewhere around 1 to 4 seconds at ISO 6400. Why? Well, when you get to the longer, quality exposures, each second at 6400 will become minutes, so it's just a matter of budgeting your time and how much performing an ISO test is worth to you versus shooting something non-technical.

I like testing all available ISOs on my camera, even if some of them are ridiculous. It's good to know by seeing what the results are.

On my D750, the ISO I tested are: 51,200, 25,600, 12,800, 6400, 3200, 1600, 800, 400, 200 and 100.

That's ten exposures. Each of them (in this order) is double the previous one's time in shutter speed.

My first test exposure in this scene tested at 6400 at 8 seconds. So I ramped up to 51,200 and started there, and then proceeded in order from shortest exposure to longest exposure.

Here is the table of exposures I made at 50 degrees F:

ISO 51,200 25,600 12,800 6,400 3,200 1,600 800 400 200 100
Aperture f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6 f/5.6
Time (seconds) 1 2 4 8 15 30 60 120 240 480

Here's the RAW (unprocessed) image sequence:

And after doing some minor post-processing, here is the resulting series of ten images:

So what do I do with this?

Great question! Examine the images and learn from what you observe. I'll go into deeper detail in the next post, including:

  • How to spot high ISO noise
  • When to avoid high ISO noise
  • When high ISO noise is OK

Homework

  1. Go shoot at least two high ISO sequences at temperatures that differ by at least 10 degrees F.
  2. Import into your editor of choice. (Power tip: I add all my high ISO tests into a collection in Lightroom called "High ISO Tests" so I can review them quickly.)
  3. Start looking for when noise changes to a level you find acceptable.

After doing your homework, read Part II of this article: Examining Your High ISO Photographs.

See more about Matt's photography, art, workshops and writing at MattHillArt.com. Follow Matt on Twitter Instagram Facebook.

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