I was getting some ugly shadow interference with standard omni lights (shadows were on off and on shadow map)
when i changed the shadow map parameters from 512 to anything over 1024 they become less or completely gone, but the particles became very grainy,
i “fixed” this now by using photometric lights,
any information about this phenomena , or what lights are best used for certain effects and how they have an influence on the shadow and lighting behaviour of the particles?
The effect you are seeing can be best visualized by rendering your particles with output resolution of 640x480 vs. 1280x960 vs. 2560x1920 etc. The particles are drawn as 2x2 (or 3x3) pixel splats onto your view, and hence look smaller and further apart as the resolution increases. The same effect is occurring as you increase the resolution of your shadow map.
Krakatoa skill mostly involves knowing how to manage the resolution of your shadow maps, and the density settings to get the lighting you require.
When you say interference, are you referring to a moire pattern in the shadows on your particles?
Here is a discussion of how density and particle distribution can affect moire patterns:
software.primefocusworld.com/sof … ensity.php
i was just reading it and wanted to post it here 
software.primefocusworld.com/sof … ensity.php
thank you, yes it was the moire effect ,
funny i was playing with density too and realized they were gone with 0 or 0.005 density but that meant i had flat shading 
the more i read about this software the more i like it 