Distant Lights (Studio) or Infinite Lights (Poser)
Many light presets for Poser and Studio use Infinite/Distant lights. As a result, some users fall in the habit of considering Distant/Infinite light as legitimate light fixtures for lighting.
Distant Lights (Studio) or Infinite Lights (Poser) should never be used with Reality and LuxRender
Let me repeat this: never.
Those lights are harsh, unrealistic and can create fireflies and high noise. One Infinite/Distant light is one too much. Two of those are a disaster and three are the render apocalypse.
Combine those lights with others and you have a real mess that will never clear to a satisfactory results.
Infinite/Distant lights are an invention of 3D biased renderers that don't have any connection with real life. Think about it. With the exception of the Sun, where can you find an Infinite or Distant light in nature?
The key to realism if to replicate lighting conditions that are based in real life, not to apply biased tricks to an unbiased renderer.
Use Mesh Lights, IBL, spots when appropriate, and you will get the best results in the shortest time. Use the Reality's Sun light, which appears as an Infinite light in Poser and a Distant light in Studio, but which is different in nature. The Sun works fine but remember that the sun, as it is in real life, is one of the worse lights for portraiture. Every photographer know this and will avoid the harsh light of the sun for portraits, except for very special cases.