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I am new to blender and have recently created my first model. I created the mesh and then textured it, added a world environment texture, and rendered it only to find a very dark harsh "shadow" around the rear of my model. However, the shadow only appears when I add a face to the back of the model to complete the whole mesh. I have included pictures so you can see what I am talking about.

Without the rear face
This is what the model looks like when the rear face of the cylinder is deleted

With the rear face
This is what the model looks like with the rear face included

In the first picture you can see the model's cylindrical body WITHOUT it's rear face resulting in no shadow. However, in the second picture, a face was added to the rear of the model resulting in a harsh shadow as shown.
I am using blender version 2.73

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    $\begingroup$ Let me guess, Subsurf modifier, and then a flat many-sided face on the back? That would be called an N-Gon, and is highly frowned upon in Blender as it brings a host of issues into the picture. Try adding an edge-loop near the N-Gon face on the back (Ctrl+R) $\endgroup$ – VRM Jan 18 '15 at 19:26
  • $\begingroup$ Yes that sounds about right although, the rear face is only made up of one side. However, your solution fixed it, thank you! :) $\endgroup$ – Andy Jan 18 '15 at 19:36
  • $\begingroup$ I'll add an answer then, glad it helped :) $\endgroup$ – VRM Jan 18 '15 at 19:59
  • $\begingroup$ BTW, I meant side as in edge ;-) $\endgroup$ – VRM Jan 18 '15 at 20:04
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It appears that you are using a Subsurf modifier, and a flat many-edged face on the back. That would be called an N-Gon, and is highly frowned upon in Blender as it brings a host of issues into the picture. Quads (faces with 4 edges) are much prefered, as it they work best for almost all of the relevant calculations done by Blender.

This particular issue can be resolved by adding an edge-loop near the face on the back by pressing Ctrl+R

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    $\begingroup$ It looks to me as though the blackened part is caused by the smooth shaded normals being interpolated across the sharp angle, not necessarily the n-gon. In which case the extra loop cut you suggested would fix it. If my theory is correct, adding an edge split modifier or enabling auto smooth should fix it as well. $\endgroup$ – PGmath Jan 19 '15 at 20:13

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