Okay Mr. SS, I went to the referenced site and lea... (
Ok Doc, thanks for going to the post, lots to see there.
I'll just repeat the basics so we're both on the same page...
The ambient light is controlled by the Shutter Time.
The exposure is controlled by the f-stop.
The ambient light is dark, so there is nothing to record. If it's dark, the shutter can be open for 3 seconds or 3 minutes, the result is still a black exposure.
So the shutter time is only controlling HOW MANY blast you will record in each image.
The f-stop will control how much light imprints on the sensor.
As you discovered, if there are a lot of blast, more light is created, so more light is recorded. If you don't control the light coming in, and if it gets to light, it will over expose and become completely white.
We must remember that anytime a very bright light source(usually the sun) is photographed we are dealing with an HDR scene and it's beyond the scope of the Range of your sensor. So we must stop down for that at the risk of underexposing the dark area of the scene.
You should only need an ND filter if you are stopped all the way down and you're already at ISO 100 and it's still overexposed.
So the 3 seconds or 6 seconds or whatever just controls how many blast will be in the image at one time. My experience is that if you try to get to many, they become somewhat jumbled up and not distiguiable individually and no longer is appealing. Worse, what might have been a great image is ruined by one of those that blast that zig-zags around!!
If you up the ISO, you'll need to stop down more
Hope all of this made some sense.
If you shot portraits using strobes, the sync speed is the duration and the blast is the strobe! The f-stop and ss do the same thing.
Hope this helped!
Again Doc, welcome!