Why Does Dragging the Shutter Work?

It seems this discussion is getting too focused on the three inputs that control exposure and losing sight of what one is trying to accomplish when "dragging" the shutter.

The OP suggests that the desire is to underexpose the background 2 stops. So, if that means ISO 400, F5.6 at 1/200 then setting the flash power/distance to provide F5.6 will mean the subject is under exposed as well. So, the only thing that can be done to get the subject exposed properly i.e. two stops brighter, is to move the speedlite closer or increase its power.

Now, going to the image I posed, I wanted to get the surf sharp, and also to have the background reasonably well exposed. That meant finding an exposure that would reasonably expose the scene. 3.2 sec at F5.6 at ISO 1600. The next step was to find a power setting for the strobe that would properly expose the surf. That turned out to be 40Ws with an 8.5" reflector at 30 feet.

The key to "dragging" the shutter or any other approach to combining strobe and ambient light is to determine what is the primary exposure consideration. Then adjusting the other light source based on the exposure determined for the first light source and its relationship to proper exposure.

BrandonSi wrote in post #11742768
The impact of shutter speed on a flash exposure can have minimal impact from ~1/100 to 1/250, etc.. but it's still present.

No, it's not. A good electronic flash fires its light in less than 1/500s, and when the power levels are lower, in significantly less time than that, so if you use 1/100 or 1/200 is irrelevant.

Large studio strobes tend to have a longer duration, and then you may be correct. But they aren't that common for most amateurs.

Which in that picture is totally irrelevant. It would look the same with normal sync.

Anders, not if they were moving.
That's the only reason to set the camera to RCS. Just in case you get the subject moving.
Although with the shutter open for that long it probably is academic.

dmward wrote in post #11746126
Anders, not if they were moving.
That's the only reason to set the camera to RCS. Just in case you get the subject moving.
Although with the shutter open for that long it probably is academic.

Cracking shot btw. Did you gel your strobe to match ambient?

No,
The off camera strobe blew over and broke the hot foot connection. The bride was cold, so i just fired away with on camera 580EX. Camera was on a tripod. I told them to stand still, hold onto one another and smile.

Thanks everyone. I fell off this thread for a couple of days owing to the relentlessness of our winter weather lately, but appreciate all of the responses and insights.

This thread wandered pretty wildly while I was away, but a few people honed in on exactly my question and gave me my answer.

To those who weren't clear on my OP, I already have ample knowledge of the concept of dragging the shutter, what it can do for me, and how to control it. What I didn't understand is why everyone always spoke of raising ambient exposure almost exclusively in terms of dragging the shutter.

In my mind, adjusting any of the three settings should allow me to raise ambient. I know you'll want a given aperture for DOF control and also want the lowest ISO setting possible leaving SS as the most likey choice for making the adjustment, but I wanted to confirm what I thought; that adjusting any or all of these three settings ultimately have the same and equal effect with respect to raising ambient exposure.

Thanks.

Peacefield wrote in post #11762934
In my mind, adjusting any of the three settings should allow me to raise ambient. I know you'll want a given aperture for DOF control and also want the lowest ISO setting possible leaving SS as the most likey choice for making the adjustment, but I wanted to confirm what I thought; that adjusting any or all of these three settings ultimately have the same and equal effect with respect to raising ambient exposure.

Thanks.

You got it.