well man thats simple if your light are lets say 1000 watt's then u have to leave them a fair distance from your plants or they are going to burn, but if your lights are weaker then you will see there are always going to be closer to the plant when u see hydo set ups on tv the neally always use very strong light thats can be keep on the roof cos there are so many of those powerful lights thats they dont need to be moved