CalculateFrustumPlanes - and screenshot

Hello. I have a question about multiple cameras and CalculateFrustumPlanes features. I have a first camera (main) which is used for the first person character. And I have a second one in order to create some screenshots in game and each of them is displayed in the corner. Everything works as expected, but I am trying to take a picture with a reduced panorama. I don’t want a full screen picture - only the center (more or less 50% of the screen). I modified the screeview port rectangle of my second camera, but it does not work as expected. Is there a way to get a modified CalculateFrustumPlanes? I read something about the scissors rectangle, but it seems to me that Unity does not provide it. Any ideas? Thank you for your help ++