OnRenderImage with 16 bit framebuffer

I am running on an android s3 device. I have a post effect that I am rendering from OnRenderImage. When I sample the main texture(framebuffer) in my shader, the samples always result in black. If I change the player settings to use a 32 bit framebuffer, then the shader works and the sampling of the main texture results in the expected colors.

How can I get the colors to be sampled properly when using a 16 bit framebuffer?

Update: I have found that I can get the effect to render properly when the effect is applied to a perspective camera. It's only when the effect is applied to an orthographic camera in a 16 bit framebuffer environment that I end up with a framebuffer texture that appears to be black. The investigation continues.