diff --git a/doomsday/client/data/cphelp.txt b/doomsday/client/data/cphelp.txt
index 3a1b3f0d3e..dadeb848bc 100644
--- a/doomsday/client/data/cphelp.txt
+++ b/doomsday/client/data/cphelp.txt
@@ -274,7 +274,7 @@ desc = Play a sound effect.
 
 [postfx]
 desc = Set or clear the frame post-processing shader.
-inf = USAGE:\npostfx (console) (shader) [(time)]\nEvery player has their own frame post-processing effects. The first argument specifies which player will be affected.\nThe frame post-processing shader is changed to "fx.post. (shader) ". If (time) is specified, and there is no shader currently in use, the new shader is faded in in (time) seconds. Otherwise the new shader is taken immediately into use.\nAs a special case, if (shader) is "none", the post-processing shader is faded out and removed.\nEXAMPLES:\nFade in the "fx.post.monochrome" shader for player 0 in 2 seconds: 'postfx 0 monochrome 2'
+inf = USAGE:\npostfx (console) (shader) [(time)]\nEvery player has their own frame post-processing effects. The first argument specifies which player will be affected.\nThe frame post-processing shader is changed to "fx.post. (shader) ". If (time) is specified, and there is no shader currently in use, the new shader is faded in in (time) seconds. Otherwise the new shader is taken immediately into use.\nAs a special case, if (shader) is "none", the post-processing shader is faded out and removed.\nAnother special case is when (shader) is "opacity". This will set the opacity of the effect to the value of (time). However, hote that the shader does not necessarily implement opacity as simple alpha blending.\nEXAMPLES:\nFade in the "fx.post.monochrome" shader for player 0 in 2 seconds: 'postfx 0 monochrome 2'
 
 [quit!]
 desc = Exit immediately and return to the OS.
diff --git a/doomsday/doc/engine/command/postfx.ame b/doomsday/doc/engine/command/postfx.ame
index 780db1efd6..a2508ab781 100644
--- a/doomsday/doc/engine/command/postfx.ame
+++ b/doomsday/doc/engine/command/postfx.ame
@@ -17,6 +17,11 @@
     As a special case, if @help_arg{shader} is "none", the post-processing
     shader is faded out and removed.
     
+    Another special case is when @help_arg{shader} is "opacity". This will 
+    set the opacity of the effect to the value of @help_arg{time}. However, 
+    hote that the shader does not necessarily implement opacity as simple 
+    alpha blending.
+    
     @examples
     Fade in the "fx.post.monochrome" shader for player 0 in 2 seconds:
     @code{postfx 0 monochrome 2}