Image Processing Tutorial- Page 2

All Images
Copyright Steve Cannistra

 

Step 3- Correction of Vignetting (next page):
Almost every image has a degree of vignetting that must be removed at some point during post-processing.  There are many ways to do this, each with its pros and cons.  I have tried almost all of them, and have concluded that the best and easiest is the "Background Compensation" feature present in ImagesPlus.  I am not affiliated with ImagesPlus in any way, and suggest that you check Mike Unsold's website for further details.

(Please click on image for a higher resolution view)
Correction of Vignetting

Steps (assumes some familiarity with ImagesPlus):
1.  Open the desired image that has been stretched in step 2 (the fact that it has been stretched will make it easier to detect vignetting).

2.  Open the Line Profile box; Open the Background Compensation box.

3.  Select the "Background Maximum" point.

4.  Draw a line that passes through the region of uneven luminosity (a diagonal was used in this example).

5.  Examine the Line Profile curve- you can see that it gradually rises towards the center and then falls, reflecting the distribution of uneven luminosity.  This is what requires correction, or "flattening".

6.  Using the Guassian option, adjust the Amplitude and Scale parameters to achieve the desired result, shown here:

(Please click on image for a higher resolution view)
Correction of Vignetting

7.  Save this corrected file in 16 bit TIFF format for use in subsequent steps.



Step 4- Levels Adjustment (next page):




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