main window
This image is taken by a usual camera with 
zooming mechanism. Estimated angle of  
vertical field of view  is 60.3 degree.
Choose a line in the image for reference, which is used to estimate the coefficient value. The criteria of choosing are: 
  1. the longer the better
  2. the farther from the center of image the better
Activate the "Stretcher Window", which is an auxiliary window where a thin rectangular portion of the original image is mapped  into a square by an affine transformation. The Strecher Window enables you to recognize the slightest distortion by exaggeration. 

In the image on the left, you can see a thin rectangle bounded by blue lines, and center line in red. By pick and drag adjust the end points of the red axis to those of the chosen line in the image. 
 

     
    Strecher Window
In the Strecher Window an edge of the building is displayed as a curve, while the red axis is drawn in straight line because the distortion coefficient is set zero by default.
     
    regulated axis 
Regulate the value of coefficient so that the red line is distorted to fit the chosen line in the image.  The red curve means that  if the distortion coefficient is as indicated by operation, straight line in the real world should be taken into the photo as the red curve. Thus we have estimated a value of the coefficient.
     
    corrected image 
By invoking the correction function, we get the corrected image in the Strecher Window.
     
    corrected full image
 And also in the main window. Thus we have got a steady start point to work.