After much digging I did discover that I was wrong, somewhat anyway. Yes there are 2 curtains, one opening, the other closing. My deepest apologies, but yet, my sincere thanks as well. It seems that I learned something again tonight. Now I can go rest. Now, back to the original question; what was it again?

"So it seems that higher shutter speed is a mechanism for controlling the amount of light that gets onto each photosite of the sensor to control overall exposure settings, but it won't work for freezing motion."

No we did not move into discussing HSS just to argue. The above part of the OP postulates that a fast Shutter won't "work for freezing motion" and then asks if he is right or wrong. That was answered in about a second (he was wrong).

Then we went off to try and explain that his understanding of 1st and 2nd curtain and how "mechanically" it worked trying to pass along some information that was absolutely pertinent to the topic if you want to begin a serious discussion about how the Shutter freezes motion. HSS came into it because some reply's thought that a strobe was the thing that froze the motion. The difference between the Eye and the Shutter came in because some didn't understand how the Flash worked with HSS (because it worked for their eye to see fast things). The OP was basically correct about the Shutter controlling the amount of light that reaches the Sensor but the topic (some of us felt) could stand a little expanding. My original reply answered the question that was asked and then tried to quantify my reply, the resulting frenzy occurred when certain other people disagreed with some of my point and we discussed them, BIG DEAL.

Like I said in my earlier reply after some people started to basically argue with what is generally an accepted fact (that a high speed Shutter does stop motion), if we cannot expand on a topic and hear from those that might be more informed why have a forum? Is this site like a court where certain "facts" are not allowed because they were not part of this exact topic? Even though they might just take the topic in a direction where people can pickup some good information? Then there are those that feel it necessary to post a reply about people that posted a reply so they can accomplish what? And within their "sensible" reply they draw the same conclusion we did but provide nothing additional. Seems to me those are the people that just want to bicker.

I actually thought that by exploring the topic of freezing motion in its entirety we might have some really cool input from people but that is far from what I got. So I apologize for trying to go too deep and trust me it won't happen again. I have to prepare for a big shoot now and It just might involve a fast Shutter so wish me luck.

KG, I think you have been given some bad information. There are not two physical curtains in your shutter. 1st curtain refers to the opening of the shutter, 2nd curtain refers to the closing of the shutter. So, when you are setting your flash to 2nd curtain, you are actually forcing the flash to activate just before the shutter closes. This is very effective when using slower shutter speeds to blur the action but freeze one subject in the image. Try taking a shot of someone walking towards you in the evening with a 1 second shutter speed using 2nd curtain flash. You will see the them nice and clear with an action blur behind them. Or try first curtain and you will see the blur in front of the clear image. This is what I've perceived from the turorials that I've read.

M

I took this at 1/3200 shutter speed.
It froze the action of the bird.

Have you ever seen a strobe tachometer? While in the Navy on the aircraft carrier USS Kitty Hawk (CV63) years ago I have seen spinning propeller blades at full throttle look like they were standing still to the naked eye, it is quite old technology dating back to WWII and comes in quite handy when you need to see how something is working at high RPM’s.

If a flash is powerful enough and, more importantly "fast enough" to illuminate and then de-illuminate the spinning object in a square wave pattern (meaning instant on and then instant off) it can be made to look motionless without any distortion. Now, during that split second of time during illumination if you can expose the "entire" surface of a sensor equally to the light of the subject then the picture will show the subject frozen in time without any distortion. When using a flood light you simply need a shutter speed as fast or faster for an amount of time that the blade spin will be a sufficiently short distance at it extremities that the resultant blur will be imperceptible to the naked eye, something that even a “slit” of exposure will do if it is fast enough.

I know what was said but the "fact" that was quoted about how flash and Shutter speed are linked was incorrect. So is their understanding of front and rear Curtains I might add. So what I was trying to do was interject the correct mechanical explanation of how high speed Shutters effect motion stopping abilities and how to achieve it.

The idea of a forum is for people to share their experience and knowledge with each other to gain a broader knowledge base. When I see someone build a scenario based on incorrect data and then reach a conclusion I simply try and provide a more technically accurate version. There are people on here that are very new to this so their "filters" are more open to clogging and I like to help clear the air if I can. Just my 2 Cents worth.

I love how we all perceive what others are saying. The use of a strobe to give the illusion "to your eye" is one thing. Your eye and your camera are not the same thing. Like I have said, and others have confirmed, to stop motion you need a fast shutter. To use a fast shutter requires proper light. To achieve proper exposure requires either very bright lights or a strobe. To use a strobe requires that you sync that strobe with the motion of the Shutter so that it is at its zenith (brightest point whatever that "output" is to achieve your proper exposure) during each "slit". I have a feeling that some people here have never actually done a HSS shoot on a fast moving object and are shooting from the hip. Guys you can stop a Hummingbirds wings dead stop if you can get the Shutter speed up there. And to do that just might require a HSS strobe. :thumbup:

"First, take into account the fact that shutter speeds above camera's flash sync speed are achieved purely by moving the second curtain before the first one fully opens. So it seems like anything above the camera's flash sync speed (which is the highest speed at which the sensor gets completely uncovered) would not work for freezing motion. Different parts of the sensor would be exposed at different times."

The OP's explanation of how the Flash works in conjunction with the fast Shutter speed is not accurate skewing the rest of the discussion. The question is "can a fast shutter stop motion", the answer is YES. The SUPPORTING details are that in order to use the fast Shutter you must provide sufficient light. In most cases that requires HSS.

I'd like to see an example of that. I can show you hundreds of examples of that prop NOT freezing, but curving, even at 1/4000. That being said, in the real world, we don't want to freeze that prop at all because it looks terrible to see a prop frozen on a plane in flight. A nice blurred disc on the other hand is very nice.

However, if I'm flying and want to freeze the prop for a picture I usually pull the mixture and raise the nose to below 60mph...that usually does the trick ;-)