I was playing around with Glass BSDF shader to see what it look like and how it interacts with its environment. Bellow, you can see a test scene.

Previewtest render of a cube sphere and suzanne

Final enter image description here The preview render on the sphere looks fine but when a final render was done the refraction of the checker-board was pixelated and distorted. The final render was done with 400 samples. Is that too little or am I missing something. Is there a way to improve render quality of the refraction in the glass sphere?

File here :

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    $\begingroup$ Are you using flat shading on the sphere and suzanne? Can you upload the scene, e.g. on pasteall.org? $\endgroup$ – B.Y.O.B. Mar 18 '18 at 22:32
  • $\begingroup$ @B.Y.O.B. here is the url pasteall.org/blend/index.php?id=49181 Sorry, new to this and not sure what you mean by flat shading. $\endgroup$ – Dzyuv001 Mar 18 '18 at 22:46
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    $\begingroup$ @B.Y.O.B. in pasteall the links expire after a few months. For permanent links use blend-exchange.giantcowfilms.com $\endgroup$ – user1853 Mar 18 '18 at 23:17
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    $\begingroup$ Select the sphere, go into edit mode (Tab), press W, click "Shade Smooth" and render again. @cegaton Good point. edit: looking at the .blend, the subsurf modifier is set to 4 in viewport and 2 in render, that's why it looks ok in viewport (the high subdivision hides the flat shading) $\endgroup$ – B.Y.O.B. Mar 18 '18 at 23:23
  • $\begingroup$ @B.Y.O.B. thank you, it worked. I will edit the port with a permanent link. $\endgroup$ – Dzyuv001 Mar 18 '18 at 23:55

The problem is that you are using flat shading on the mesh, as opposed to smooth shading that interpolates normals: https://docs.blender.org/manual/en/dev/modeling/meshes/editing/normals.html?highlight=smooth%20shading#shading

The reason why it looks ok in the viewport is that your subdivision surface modifier is set to 4 iterations for the viewport and 2 iterations for the final render. So the high subdivision in the viewport hides the flat shading.

Solution: Set viewport subdivisions equal or lower than final subdivisions, and enable smooth shading on the mesh (see above link).

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