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Manual Correction

Click on the Radial button to permit you to correct radial distortion manually.



Numeric fields for the three constants which control distortion correction appear as to a set of track bars which permit you to change the values positively or negatively around zero by a small amount.



Make a test image showing regular geometric structure and display it.

  

Use the grid check box button   to draw a grid over this image 

The calibration image may be slightly rotated relative to the sides of the screen picture. You can correct this prior to beginning the calibration process by clicking on  Custom  and then on Rotate: 

  

and the image will be rotated automatically by the programme so that the center area is as parallel as possible to the sides of the screen.

Start by changing the value of the b constant first by a small amount. If the lens has considerable distortion, you can see the effect of the change immediately, but if not click the image first to enlarge it with repeated left clicks or using the mouse wheel, and then click the Apply button to see the effect at full resolution. Redraw the grid as necessary. This mainly affects the middle areas of the image. The outer areas are affected primarily by the a constant, and the inner areas by the c constant.  Experiment with all three until you find a satisfactory combination. 

Then click on the Custom menu item and save the result to a file which will enable you to identify the correction. If your lens has a zoom function, start with the widest angle image (shortest focal length) first, since this will usually require the most correction. Longer focal lengths generally require little or no correction.  The d constant is shown for information purposes only. It is computed as 1 - ( a + b + c ) and need not be set by hand.  Very small corrections may be typed directly into the appropriate editing boxes, but usually the values produced by the track bars will be good enough.  



The b constant will usually be negative for most lenses (drag the slider to the left). The a constant will usually be positive. The c  constant is nearly always zero except for so-called "fish-eye" extreme wide angle lenses.

When you are satisfied with the result, click on Custom and 



save the result.