Greta Jimenez
Light pollution is a common issue in astrophotography, resulting in an ugly red-brown colour cast in images. This is caused by the foreground sky, which is created by light pollution in urban and suburban areas. To capture clearer images of the night sky, photographers can employ various techniques to reduce or eliminate light pollution. This can be done by adjusting camera settings, using filters, or utilising image editing software. While light pollution can be minimised, it is challenging to completely avoid it, even in the darkest locations on Earth.
Characteristics and Values of Removing Light Pollution in Photos
| Characteristics | Values |
|---|---|
| Image-editing programs | Adobe Photoshop, GIMP, Sequator, etc. |
| Camera settings | Custom white balance, RAW, JPEG, etc. |
| Filters | Light pollution filter, UHC filter, Astronomik CLS filter, etc. |
| Techniques | Blur, Gaussian Blur, Subtract blend, adjust light curves, etc. |
| Other | Use a dark location, crop the image, adjust black point, etc. |
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What You'll Learn
Utilise image-editing software
Image-editing software offers a range of tools to reduce or eliminate light pollution in photographs. Adobe Photoshop, GIMP, Paint Shop Pro 6, and Adobe Lightroom are some of the commonly used software for this purpose. Here are some techniques to utilise these tools effectively:
White Balance Settings:
Adjusting the white balance can help compensate for the colour cast caused by light pollution. Set the white balance to ''daylight' or ''custom' to achieve more accurate colours in your images. This can be done in-camera or during post-processing if shooting in RAW format.
Hue/Saturation Adjustment:
Once you've opened your image in the editing software, access the Hue/Saturation tool (usually under Image > Adjustments > Hue/Saturation). Tweak the sliders for the colour causing the cast to reduce its impact.
Layer Duplication and Blending:
Duplicate the background layer of your image and blur the new layer completely using a Gaussian Blur filter. This creates a smooth version of your scene with a similar colour to the light pollution. Then, blend the layers using the 'Subtract' blend mode to remove the light pollution.
Clone Tool:
Utilise the clone tool to edit out bright stars or nebulae by sampling nearby regions with similar brightness levels. This helps to create a more uniform image and reduce the impact of light pollution.
Gradient Tool:
Use the gradient tool to selectively adjust the exposure, brightness, or darkness of specific portions of the image affected by light pollution. Radial gradients can help blend these adjustments seamlessly into the rest of the image.
Dust and Scratches Filter:
Applying the 'Dust and Scratches' filter can help remove stars and other bright objects from the image, making it easier to isolate and edit the light-polluted areas.
Black Point Adjustment (Adobe Photoshop):
In Adobe Photoshop, adjust the black point by selecting the eyedropper tool, changing the sample size, and accessing the levels dialog. Double-clicking on the black-point eyedropper and setting the RGB values can help remove the red-brown colour cast caused by light pollution.
Remember that while image-editing software provides powerful tools, they are not magic wands. Some detail lost to light pollution may not be recoverable, so it's always best to start with images taken in relatively dark locations.
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Adjust white balance
White balance is the adjustment of a digital photograph to make its colours appear more realistic. It is a way to set a photograph to neutral, to make white objects appear white. For example, if you take a photo of a white object in certain lighting conditions, it can appear bluer than it actually is. To fix this, you can adjust the white balance to get rid of the blue cast.
In photography, white balance (also known as colour balance) refers to adjusting the colours in your photos to reflect how the object appears in real life. These adjustments can be made using preset settings on your camera or they can be done manually based on your location and lighting. If you are shooting with natural light from an overcast sky, your photos will have a cold colour temperature, more of a blue tone. Shooting in unnatural incandescent light with tungsten light bulbs creates a warm colour cast, which shows up as yellow or orange. Similarly, shooting under certain types of fluorescent lights can give your photos a green tint.
To achieve good in-camera white balance when shooting, shoot in the RAW file format. This file format preserves all the image data that is recorded by your camera's sensor, allowing you to adjust the white balance in post-production effectively. Aim for consistent lighting. Shooting photos with mismatched sources of light will make it more difficult to edit the white balance in post-production. You can use a grey card to be precise. This tool helps you find a perfect white balance for scenarios where being exact is the ultimate goal, like product photography or food photography.
To adjust the white balance of your photos in post-production, you can use Adobe Photoshop, Adobe Lightroom, or Adobe Camera Raw. In Adobe Photoshop, you can use the Temperature Slider in the Develop panel, or use the set white point" eyedropper in the Levels dialog box. In Adobe Lightroom, you can choose from various options in the White Balance (WB) drop-down menu in the Develop panel. In Adobe Camera Raw, you can manually adjust the white balance of a RAW image.
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Use a light pollution filter
Light pollution filters are a great way to reduce the impact of artificial light sources, such as street lamps, on your photographs. They work by blocking specific wavelengths of light in the visible spectrum, particularly the yellow and orange parts of the spectrum between 575nm and 600nm, which are the common colours of sodium vapour lights used in most big cities.
There are a variety of light pollution filters available, including broadband, dual-narrowband, and multi-bandpass filters. The UHC filter, for example, blocks a lot of light in the green, yellow, and orange parts of the spectrum, while allowing the main nebulae wavelengths through, which are blue and red light. The L-Pro filter is another option that is designed to remove airglow and mercury/sodium vapour lamps while retaining the normal colours. The Optolong L-eXtreme filter is also a popular choice for those shooting in heavy light pollution areas, as it features narrow bandpasses that isolate the H-alpha (Ha).
When choosing a light pollution filter, it is important to consider the specific wavelengths you want to capture, as well as the lighting conditions you will be shooting in. While light pollution filters can be effective in both dark skies and heavily light-polluted areas, they may not be sufficient to capture objects like the Milky Way Galaxy or the Orion Nebula from the centre of a big city.
Additionally, it is worth noting that light pollution filters may require longer exposure times, which can result in a slight loss in the number of stars captured. However, with the right filter and camera settings, you can capture NASA-quality photos even in light-polluted areas.
Overall, light pollution filters are a valuable tool for astro and nightscape photographers looking to improve their nighttime photos by reducing the unwanted effects of light pollution.
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Blur the image and subtract from the original
To remove light pollution from astrophotography images, one method is to blur the image and subtract that from the original. This method works well for light pollution since the focus is usually set to infinity in the original image, and when blurred, it will be set to just a few centimetres, diffusing every star but the brightest into the background.
Using image-editing software, duplicate the background layer and make sure the new layer is in front of the original. Completely blur this new layer until no stars are visible. In the floating palette that appears, set the radius to about 50 pixels to ensure that the star detail is completely gone. Click OK, and you will be presented with a smooth, averaged version of your scene, approximately the same colour as the light pollution you are trying to remove.
Now, blend the layers using the Subtract blend mode, which can be selected from the drop-down menu on the Layers palette. Subtracting the background will give a black background with the original stars. The background becomes black while the stars retain their colour, and become even easier to see on the new dark background. If you find the black too stark, experiment with the opacity of the top layer using the slider on the Layers palette to bring back some colour.
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Crop the image to only show the sky
Cropping an image to only show the sky is one of the ways to reduce light pollution in photos. This method can be used to remove light pollution from astro-photography images.
To do this, first, crop the image to show only the sky and no foreground images. Also, crop out clouds if there are any. This is a temporary step to help you get relevant information from the histogram. Next, set the contrast to 100% and push the exposure until the lower corner of the histogram is touching the right. This step is not related to cropping the image, but it is the next step in the process of removing light pollution.
The next step is what actually removes the light pollution. Select the coloured point curves (red, green, and blue) and drag the black point to where it touches the left of the histogram. Do this for each of the red, green, and blue colours. This is because space is dark, and there are parts of the sky that should have no light at all. The darkest pixels (the left side of the histogram) are lit, purely by light pollution. This lets you know how much to remove. Light pollution will add light to all pixels, so by dragging the black point to the left, you are subtracting that light from the image.
In addition to cropping the image, you can also use image-editing programs to remove light pollution. For example, you can adjust the Hue/Saturation sliders for the colour causing the cast in your image. Alternatively, you can use Adobe Photoshop to remove light pollution by adjusting the black point. First, double-click on the eyedropper tool and set it to 35,35,35, then click it in a blank area of the sky background. Select the eyedropper tool from the tool palette and change the sample size to 31 by 31 average. Call up the levels dialog from the pull-down menu (Image > Adjustments > Levels). Double-click on the black-point eyedropper on the left. In the Color Picker (Target Shadow Colour) window, set the R,G,B values to 35,35,35 and click OK. Now, simply click on an area of blank sky background with no stars or nebulosity. This will remove the ugly red-brown colour of the foreground sky by subtracting it from the image, resulting in a nice dark grey sky background.
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Frequently asked questions
There are several ways to remove light pollution from photos. You can use image-editing software, adjust the white balance on your camera, or use a light pollution filter.
You can use image-editing programs such as Adobe Photoshop or GIMP to remove light pollution. One method involves duplicating the background layer, blurring the new layer, and then using the Subtract blend mode to remove the light pollution.
You can set your camera to a custom white balance by pointing it at the sky. This will help reduce the colour cast caused by light pollution.
A light pollution filter, such as the Astronomik CLS filter, blocks certain wavelengths of light associated with light pollution, such as the light emitted by sodium and mercury streetlights.
Yes, it is recommended to shoot in RAW format so that you can adjust the white balance in post-processing. Additionally, try to find a dark location with minimal light pollution to capture your photos.
