Earlier today, while I was going through the list of unconfirmed bugs, I 
came across what appears to be a longstanding bug in the arealight 
implementation.  Render the following scene in either v3.5 or v3.1 and note 
that the shadow on the right hand side of the sphere appears to be roughly 
cut off rather than smooth.

plane {y,0 pigment {rgb 0.5}}

sphere {y,1 pigment {rgb 0.75}}

camera {location <0,4,-8> look_at <0,0,0> rotate y*45}

light_source {y*10 rgb 1 area_light <5,0,5> <-5,0,-5> 16,16 adaptive 1 
jitter rotate z*45}

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It's not a bug. You have just misunderstood how area lights are defined.
  You define two axis vectors as the dimensions of the area light, not its
corner coordinates (as it seems to be the case in your example). What your
example does is (I think) a 1-dimensional area light instead of 2-dimensional.
This is causing the smoothing to be in one direction only.

  What you probably meant was this:

light_source {y*10 rgb 1 area_light <5,0,0> <0,0,5> 16,16 adaptive 1
jitter rotate z*45}

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

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