how to change vtkcamera from perspective to parallel gradually

Hi, I can make the vtkcamera perspective or orthographic by using SetParallelProjection(False/True). But my requirement is to change the view from parallel to perspective or from perspective to parallel GRADUALLY

Here is my requirements
-The system will display my actor on the screen using perspective camera (done)

-Then user will press a “towards orthographic camera” button (done)

-Then system will decrease the “field of view” or something else by 1 on each click so that camera will display slightly more orthographic view of that actor in each click
When the variable become 0 then the actor will be displayed fully orthograhic

I am missing something but what, no idea
I really appreciated for your help, Thanks in advance

Try setting the focal distance with SetDistance(). As the distance approaches infinity, the projection will approach parallel.