Cycles shadows different in live render and final render I'm a beginner in blender and i can't figure out why the shadows in my scene are different in live render and final render. The shadows seem to be less pronounced in the final render. That makes it really time consuming to fine tune the lamp size and strength values. I'm using a shadow catcher and a background image with one lamp (sun) and a world background with the strength of 1.0.

Color management settings: Color Space &amp; Display Device: sRGB, View: default, Exposure: 0, Gamma: 1, Look: none, Use curves: off


Live render:





Final render:





Composition nodes:




EDIT:

When changing the world background strength to 0.0 the images become more similar but still not the same and i need the background for reflections.



EDIT:

I reproduced the problem in a new file. Download for blender 2.79.

The shadow in the preview is clearly darken than in the final render:
This probably not a bug, but differences in settings between viewport/preview and the final render. 

Checklist:


Check that all objects that appear in the preview also appear in the final render, especially lamps. See: https://blender.stackexchange.com/a/1114/245 

Check that the sampling settings are the same for preview and final render. See https://blender.stackexchange.com/a/111591/245 for reference.
For completeness but probably not relevant: Check the modifiers for any difference between preview and final render.


Given your description there seems to be a problem in the rendering section so you might also use this answer for tracking down your problem. 

I specifically think that the amount of bounces is lower in the preview then in the final render. The increased diffuse lighting makes the shadow lighter.

EDIT:

The problem seems to be in the node setup:

The problem is, that the shadow has a strange mix between alpha and color:

The diameter of the shadow changes with the threshhold on the alpha channel!



Maybe you could try to use a shadowcatcher material/setup as described here. 

Investigating further, I think that you set up everything the right way, but this might be indeed a shortcoming of blender as discussed in the developemnt group here.