I'm thinking specifically with photos...there are obviously drastic differences with autos, so I'm not saying it's a 100% rule across the board, but MH for example creates much tighter nodes and can be positioned closer to a young plant, therefore it's used in veg. Once a plant gets flipped to 12/12, the stretch start and most growers switch to HPS...compared to CFL or other forms of MH, HPS creates more of a pre-flower stretch than other spectrum. All of this is repeated knowledge, I don't have enough grows to verify, but this is what I've discovered on the internet and in books.