Preheat oven to 400° F. Drain the berries and reserve the syrup from one can.
In a saucepan stir the Boysenberry syrup into the combined mixture of the cornstarch and half of the sugar.
Cook over medium heat, stirring constantly, until thickened.
Remove from heat and gently stir in the remaining sugar, boysenberries and lemon juice. Let stand while preparing pastry.
Line a 9-inch pie pan with pastry and fill with fruit filling. Dot with butter or margarine. Adjust top crust on pie, cutting slits for steam to escape, and seal.
Bake for 30 minutes or until filling is bubbly and crust is golden brown.