How do I change the field of view of camera while keeping amount of shadow ?

Please excuse my poor English.

Please look at these pictures.
These have different field of view.


The former(A) is an angle of 90 degrees.
The latter(B) is an angle of 26 degrees.

Why are there different shadow?

I want to adjust amount of shadow A as B.
And keep field of view of 26 degrees.

Is there any good method?