Adjusting Levels

Levels control the brightness of different parts of the picture.  If you want to make the background sky darker, or if you want to make faint detail more apparent, modifying Levels is the place to start.  Levels also provides a very effective method for removing sky glow due to light pollution.

We'll start the example with an RGB image of the Whirlpool Galaxy, combined in MaxIm DL but otherwise unprocessed.

Above:  Unprocessed RGB image of M51.

In this image, note that the background is not completely black and that the galaxy itself is pretty dim in the picture.  Select Image > Adjust > Levels from the main menu (or press Ctrl-L).  This will open the Levels window.

The Levels window displays a histogram.  A histogram is a plot of the number of pixels versus pixel value (or brightness).  In other words, the curve shown in the Levels window runs from dark to light (left to right) and the height of the curve indicates the number of pixels with a particular value.  Usually the curve in astronomical images will be similar to that shown above, with few or no pixels at either extreme end of the scale, and most of the pixels crowded around a value closer to the low end (left side).

Levels Adjustments

Adjustments are made by dragging the triangular sliders under the Levels curve.

Left Slider (shadows):  This controls the background level of the image, or the black point.  To darken the sky background, move the left slider toward the right.  Usually bringing the slider to the very left end of the Levels curve will give a sufficiently dark background without sacrificing detail.

Right Slider (highlights):  This controls the brightness of the highlights in the image, or white point.  To brighten the overall image, move the right slider toward the left.  This can cause the brightest regions of a picture to appear overexposed, and bringing the slider too far left can significantly increase noise, but some adjustment is almost always necessary with faint celestial objects.

Middle Slider (midtones):  This should be used after making adjustments to the left and right sliders.  The middle slider will move when the right slider is adjusted but its relative position stays the same (and the value remains at 1.00).  The middle slider controls the gamma of the image.  Adjusting the middle slider toward the left will brighten the image, moving the middle slider right will darken the picture.  It affects the midtones only, so the black point and white point retain their settings from the left and right sliders.

Tip:  It depends on the particular image, of course, but moving the left slider to the right end of the Levels curve and moving the right slider toward the left until the middle slider reaches the right end of the curve often results in a good picture.

An Example

Below is the Levels window of the Whirlpool Galaxy image above with the left slider moved to darken the background:

The resulting picture:

Now, moving the right slider toward the left until the middle slider reaches the right end of the curve brightens the overall image.

Above:  Final Levels adjustment

Above:  Image after final Levels adjustment

Histogram Clipping

This is one of the most common mistakes made by beginning imagers.  Histogram clipping happens when the screen stretch is adjusted so that part of the lower end of the histogram is cut off.  This results in a loss of faint detail in an image as well as a less aesthetically pleasing result.  The lost data can now not be recovered.

Above:  An image of NGC253 with a proper screen stretch setting.

The image above has the proper histogram settings.  The histogram is not clipped and the very left end of the histogram curve is visible.

Above:  Correct histogram appearance.  The left end of the histogram is not cut off.

When Levels are adjusted with the black point slider to the right of the left end of the histogram, the faintest detail in the image is lost.

Above:  Moving the black point slider in Levels past the end of the histogram darkens the background but cuts off data.

Above:  NGC253 image with clipped histogram.  Note the lack of faint details compared to the first image.

When Levels are set in this manner, all the data below the black point slider will be lost.  For astronomical images, it is critical that the faint detail not be lost.  The resulting histogram after adjusting Levels this way will look like the image below.  This is a clipped histogram.

Above:  Clipped histogram.

A clipped histogram is most likely to occur when trying to darken the background of an image.  It is imperative to not over-darken the background, causing the loss of detail.  If bringing the black point slider up to the very end of the histogram curve does not remove all the skyglow from an image (which can happen in an image with severe vignetting), it is preferable to use a tool such as Curves which will darken the image without sacrificing the black point setting.  This will retain the faint detail you've worked so hard to capture in the first place.

Removing Skyglow with Levels

Below is an image of M27 that suffers from some serious light pollution problems.  (It was taken from the Starizona parking lot, 20 feet from a 6-lane road.)  With Photoshop we can easily salvage what appears to be a lousy image and show that modern technology lets us successfully capture deep-sky images from just about anywhere.  Technology is so cool!

Open the Levels window (Ctrl-L) and notice the unusual appearance of the histogram, as shown below.

Above:  Notice the 3 peaks in the histogram

In an image with severe light pollution problems (or other severe color balance issues), there will be multiple peaks in the histogram.  This is because the brightness of the background is different in each color.  Here the background is a putrid orange color, so we should expect the background to be brightest in red and darkest in blue.

Using the Channel pulldown menu at the top of the Levels window, you can select each channel (red, green, or blue) individually.  Below is the Red channel.  It is the rightmost peak in the histogram, indicating that it is, indeed, the brightest channel.

To remove the background color cast, simply bring the black slider up to the left end of the histogram curve in each channel.

Above:  Levels adjustment in the Red channel

Repeat this adjustment, bringing the black slider up to the left toe of the histogram curve in all three channels, red, green, and blue.  Return to the RGB channel and the Levels histogram should appear as below.

Above:  Corrected Levels

Above:  M27 with sky glow removed

The final step is then to adjust the white slider in the Levels window to brighten the image, as shown below.

Below is the final image.

Above:  Final adjusted image of M27.

Next, Color Balance...

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