By expending the rendering equation, you will have a nice symmetric equation describing the light transport, please refer to the pbrt book at the page of 760. (Sorry about the large size of this equation, I've no idea how to scale it....)
What I'm wondering is that where is the cosine factor at the camera side of this equation? I never recall any renderer taking it into consideration, at least for real time rendering engine.
Take a real world example here, you are watching a movie, which is displaying a uniform white image, let's say radiance of each ray is exactly 1, obviously the ones that hit the center of the screen will reflect more lights to the viewer, while the ones hit around the edge of the screen will be a little bit darker depending on the FOV of the projector, no matter how small it is, it should be there.
So what is the real world solution for this issue? scale the radiance by a factor of 1/cos(theta)? Or totally ignore it since theta should be very small.