The following section describes the advantages of shooting
digital camera images in RAW format and gives a step-by-step example of using
RAW settings to get the most out of an astronomical image.
Why Shoot in RAW?
Digital SLR cameras can typically record images in two
formats: JPEG and RAW. JPEG is a compressed image format. This
gives each image a much smaller file size than a RAW file, but it compromises
the quality of the image. The difference in quality is subtle as long as
the highest-quality JPEG setting is used. So why bother shooting in RAW
format? The main advantage of RAW is that the camera does not permanently
apply any settings to the image. When a camera shoots a JPEG image, it
sets the white balance, exposure, brightness and contrast, sharpness, etc.
After the picture has been taken, none of these settings can be changed.
Using RAW format, these settings can be changed after the
fact, allowing the most detail to be extracted from the image. By opening
a file in the Camera RAW plug-in for Photoshop, settings can be adjusted before
the image is opened in Photoshop. This places the best possible
unprocessed image into Photoshop for further enhancement.
Camera RAW Settings
When an image is opened in Camera RAW, the image is displayed
within a separate window. Specifications of the image appear below the
image, while image settings such as white balance and contrast appear to the
right of the image. The most important tool in the Camera RAW window is
the histogram. The histogram
consists of three curves (red, green, and blue) in the upper right corner of the
Camera RAW window. The histogram tells the values of the pixels in the
image in each color channel. Almost all of the adjustments that will be
made in Camera RAW will be done using the histogram as a guide.
Above: The Camera RAW window in Photoshop.
Note the histogram in the upper right corner; this is the most important thing
to watch when adjusting RAW files. Horsehead Nebula image by James McGaha.
In the above example, note the default settings of some of
the image parameters. Temperature = 3800, Tint = -50, Shadows = 5.
These are the three settings that are most often adjusted in Camera RAW, but of
course the others can be used if necessary.
Setting the White Balance
Setting the white balance of the image is critical and should
always be done first. This will give a good base image to work from for
It is easy to adjust the white balance to the correct
settings by examining the histogram. Note in the above image, the default
settings create a histogram with three separate peaks. This results in a
pink sky background in the image. To create a neutral sky background, the
three peaks of the histogram must coincide. Begin by adjusting the
Temperature slider. This will usually bring two of the histogram peaks
together. In this example, dropping the Temperature to 2800 moves the red
and green peaks together.
Above: Adjusting Temperature in Camera RAW.
Value was changed from default of 3300 to 2800.
Next, adjust the Tint slider. This will bring the third
peak into registration. In this example, changing the Tint value to -38
brings all three histogram peaks together. This results in a neutral sky
Above: Adjusting Tint in Camera RAW.
Value was changed from default of -50 to -38.
Expanding the Histogram
Notice in this image that the histogram is shifted to the
right. In other words, there is a lot of empty space in the histogram
window to the left of the main histogram curve. This indicates that most
of the pixels in the image and relatively bright. In an astronomical
image, most of the pixels are the sky background. In this image, the
exposure was deep enough to register the background sky glow, even from a dark
location. This is ideal as it indicates that all possible information was
It is possible to simply adjust the Levels in Photoshop to
darken the background. However, it is better to make as much adjustment as
possible in Camera RAW, leaving as little as possible for Photoshop.
Recall that an image opened in Photoshop contains 256 levels of brightness.
The histogram values range from 0-255. If the histogram is left at the
Camera RAW default and the image opened in Photoshop, the left toe of the
histogram (the darkest part of the image) has a value of 107 in this example.
This means the black Levels slider will be brought up to 107 to darken the
background. The result is that the remaining values in the image (108-255)
are expanded into the 256 possible levels. 43% of the histogram (0-107) is
However, if the histogram is expanded in Camera RAW, less
Levels adjustment needs to be made in Photoshop. The best way to
accomplish this is by increasing the Shadows setting in Camera RAW. This
has the same basic effect as moving the black slider in Levels. The
histogram curve will expand as the Shadows value is increased. The
important thing is to avoid clipping the histogram. Ignore the appearance
of the image for now; just watch the histogram. Increase the Shadows value
but do not cut off the left side of the histogram. If you do, information
will be lost. In this example, the Shadows value was increased to 76.
Above: Adjusting Shadows in Camera RAW.
Value was changed from default of 5 to 76.
Most cameras apply a default sharpening to an image.
This one major drawback to using JPEG images. For regular photography, a
slight sharpening of the image usually looks great. However, for
astrophotography, in-camera sharpening results in blocky star images. It
is best to eliminate the in-camera sharpening and apply any sharpening in
Photoshop after other image adjustments have been made.
Select the Detail tab in Camera RAW. The top slider is
Sharpness. The default for the camera used in this example is 25.
Adjust the slider to 0 to eliminate any sharpening.
Above: Adjusting Sharpness in Camera RAW.
Value was changed from default of 25 to 0.
The Final Image
After making the Camera RAW adjustments, click OK and the
image will open in Photoshop. The standard aesthetic
image adjustments can now be made. Below is the final image from the
RAW file above. The image is a single 15-minute exposure using a Canon EOS
20Da and Takahashi Epsilon 180.
Above: Final processed image of the Horsehead
Nebula. Image by James McGaha.
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