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My lighting question

That's a chart. I understand charts if I know what they represent. This is something I can go with. I already know about the diagrams shown on a lot of product websites. I know, generally, what they represent, but not the importance.
A question about HID's and HP and those. Why is Old Head able to grow his plants past his lights? Like, straight up and leaning real close to the reflector? It seems like there isn't much of a need for distance using one of the HID types. I see the same thing in other grows. Plants getting superclose to lights, Two questions I guess. If the tutorial I linked uses a 250 watt in a 2x4x5, why wouldn't it work in a 3x3x6? I understand the recommended 50w/sf. So why are there good reseults when putting that 250 in 8sf? The 8' might not happen as I don't seem to be able to find one with the 3x3 footprint. Three questions. Last one, I swear.
I'm going to need a minute to go back and forth between your chart and the other diagrams for a better idea. I might print it out. As computer oriented I've been in my life, my brain is low tech. I understand printed media better than what I glean on a computer screen.

Thanks ZZ
 
Ginger, I can sense the trouble you're having trying to wrap your head around how much light is enough.
Plants don't give AF about watts, they only care about how MUCH light they get. It's measured in micro-moles of photons.
The wattage metric you keep hearing(50w/sq ft-30w/sq ft) is an easier way to estimate how many umoles of light the fixture can generate.
Current commercially available diodes generate around 2.8umole/watt while older blurple diodes are closer to 1.5umole/watt. That 1.5umole/watt is also the efficacy of most HPS lamps.
It's pretty simple to estimate how much light you'll need to achieve the light intensity you want.
Find the area of your grow space in meters(2) and multiply that value by the desired PPFD(700) level you want to achieve to get the amount of light(PPF).
Ex.: 1 sq meter of canopy x 700umole/m2=700 umolesPPF
If you use a HPS rig, you'll need: 700umoles PPF/1.5 umoles/watt= 467 watts hps
Using current tech diodes- 700 umoles/2.8 umole/watt= 250 watts led
 
Ginger_ I guess the pdf did get posted_ it eluded me the first time I looked at the post _the one with the two charts. The charts were so you could see what the best PPFD for different stages of growth are_ so you give your plants just the right amount of light at the right stage of growth. The pdf is a guide to choosing grow lights and explains how PPF and PPFD work together. I hope it helps. Just in case you didn't figure this out_ I just did!_ go back to my post with the charts and just above the "recommended PPFD" chart there is a small line in blue text_"how to compare grow lights pdf"_ click on that and a window will open that asks you if you want to open it_ click it and the pdf will download. Then you can even print it out! lots of good information. Cheers!
 
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