There is intensity of light, focus of light and containment of light you need to think about,
The light spread and depth.
Light is not one dimensional.
Light as it travels from the arc of the bulb gets weaker as it leaves the bulb, the manufactures of HPS and MH test the bulbs intensity when it is new one foot from the center of the tube,
The hoods immediately turn the light then aim it from the back side of the lamp down into the plants,
We all know light hits the plant concentrating it into energy through the FAN leaves.
The light leaves the hook in a arc or angle steadily decreasing in intensity until there is nothing left, if you take the light and turn it into the plant to before it loses intensity it becomes usable light that makes bud, the more you can contain light to the growing area the more bud you will produce.
The light before it's effective intensity falls off on a 1000 watt bulb is about three ft.
Because many growers are putting their plants up against a trellis and letting side Branch's grow up on the terminal stem are screwing up, if they are growing for production in a couple of ways, one they are putting the plant into stress causing the plant to use energy taking away from that energy to go to bud development.
Then they are blocking the use of the effective depth of the canopy by using the length and width of the production , completely ignoring the depth.
The side Branch's you are growing for example normally only form a small bud at the end of it, if there are buds further back on the branch they are normally garbage,
If that whole branch is removed and top pinched once a week for the first five weeks of bloom on most strains ,it will form long deep colas into the canopy, and because as the plant is developing you can contain the plants energy to the terminal colas, you do not want anymore colas then the light can penetrate effectively , but you want the maximum amount it can, .
A 1000 watt HPS will provide almost 10k lumens per sq ft, when NEW in a 4x4 area, you want to keep the light and plants within that area, contain the light to that area and intensify the light in the area,
It is very important your hood focus the light to that area,