Case Study: Editing a Milky Way Image in Lightroom CC--Updated
January 25, 2018
Iwrote the first version of this blog post two years ago. Since then, my thinking on the best way to edit Milky Way images has changed dramatically. In my old approach, I shifted the color of the sky blue by setting Temp in Lightroom's Develop module, Basic panel, to 3200 K. While this certainly produced a pretty blue sky, it also turned the stars and the Milky Way blue. In fact, practically every color in the entire image became some shade of blue. Although we are essentially color-blind at night, some stars are bright enough to excite our cones, the cells in our eyes that enable us to see color. Antares, Aldebaran, Arcturus, and Betelguese, for example, exhibit hints of orange. Rigel and Sirius are blue-white. In my new approach, I shift the color of the sky toward blue while preserving the colors of the stars and Milky Way. Here's how I re-edited this image using Lightroom CC.
First, the tech specs: I shot this image with a Canon EOS 5D Mark III and Canon EF 24mm f/1.4L II USM lens, exposing for 20 seconds at f/2.0, ISO 3200, using a daylight white balance. The light on the foreground rocks came from two F&V HDV-Z96 flat-panel LED video lights, each with an 85B warming filter. The image above shows the original raw file, straight out of the camera.
The first step was to open the Lens Corrections panel, Profile tab, and check the boxes labeled Enable Profile Corrections and Remove Chromatic Aberration. Chromatic aberration is an optical flaw that can cause red or green fringes to appear around dark objects set against very bright backgrounds. By enabling profile corrections, you eliminate falloff, the tendency of lenses to produce an image that's brighter in the center than in the corners. You also correct geometric distortion, a flaw that causes parallel lines to bow outward (barrel distortion) or inward (pincushion distortion).
Be sure to check that Lightroom has identified your lens correctly. If Lightroom doesn't offer an exact match, try to find a lens that is close in terms of focal length or zoom range, or choose the Manual tab and correct the falloff manually by adjusting the Vignetting slider. Notice how enabling profile corrections has brightened the corners of the image.
I also reduced noise by setting the Luminance slider in the Details panel to 20. The setting you need will vary depending on your camera and the ISO you used. Use only enough noise reduction to control the noise without smoothing out all the fine detail and making the subject look like injection-molded plastic.
The next step is to use the Tone Curve panel in Lightroom to shift the sky color toward blue by manipulating the individual red, green, and blue channels. Note that the Tone Curve panel has two modes. If you see the word Region underneath the graph, with four sliders below that, click the small icon in the bottom-right corner of the dialog box to switch to the Edit Point Curve mode.
Set the Point Curve preset to Linear. Next, click the disclosure triangle next to Channel: RGB and choose the red channel. Click to place a point on the curve about two-thirds of the way up, but don’t move the point in any direction. This point serves to anchor the top part of the red curve, preserving the color of the highlights (the stars and Milky Way) as captured in the original file. Next, click to place a point about one-quarter of the way up the curve and drag downward a bit. Switch to the green channel and place the same two points. Again, drag the lower point down a bit. Finally, switch to the blue channel, place the same two points, but this time drag the lower point upward a bit. The figure below shows screenshots of the three channels.
If you find that it’s hard to make small, precise adjustments to the curve, hold down the Alt key while you drag the point. The point will now move a smaller distance for a given mouse movement, giving you better control. The goal is to shift the color of the sky to a pleasing blue while preserving the original colors of the stars and the Milky Way. It can take a lot of trial-and-error to get this right. Once you have a group of settings you like, click the disclosure triangle next to Point Curve: Custom and click Save to create a preset. You’ll probably find you need different settings for different images, so you may want to create a series of presets you can click through to see which one gets you closest. You can then perfect your settings if necessary. On nights when the airglow is particularly strong, such as this one, you may find it impossible to completely eliminate the green cast without shifting some portions of the sky purple. Your best compromise may be to shift the upper regions of the sky blue while allowing the sky near the horizon to retain its greenish hue.
The next step was to make the Milky Way stand out more by adding contrast and Clarity to the sky with the Adjustment brush (farthest right icon, just above the Basic panel). I set Contrast to +60 and Clarity to +20 and brushed over the whole sky with a broad, soft-edged brush (Feather, Flow, and Density all set to 100; Auto Mask unchecked).
The final step was to select the Adjustment Brush again, make sure all adjustments were zeroed, then set Temp to 10 and Tint to 5. When changing color with the Adjustment Brush, you can't paint on a specific color temperature or tint; instead you paint on a correction to the current color. In this case, I determined by trial-and-error that setting Temp to 10 and Tint to 5 would add the right amount of additional warmth to the foreground rocks. To check my work, I pressed the O key to see a translucent red overlay over the regions I had painted. As a final step, I enlarged the image to 100 percent, pressed the Home key to go to the top left corner of the image, then pressed the Page Down key repeatedly to examine every square inch of the image. I used the Spot Removal tool to remove a few bright pixels and several small jet streaks, and the image was complete.
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