strange camera output effect when looking at emissive objects

I am working to get an image of an emissive plane(unlit) using a orthographic camera. But I found that left and right sides of output image will become darker due to some unknown effects. It looks like a dark corner effect(but up and bottom sides are normal). Then I tried to render a white emissive plane, I deleted all the light sources and got a picture below:
Although the material is unlit, it still seems to be affected by some light. I am so confused.

Thanks in advanced!

Just go to the post process settings and disable vignette effect, its the culprit causing darkening of your screen edges.

Thank you! By the way, the solution is not disable the vignette because the default value of camera is 0.4. We must enable this effect and set it to 0 in order to cancel this.