Please help me understand flash sync speed.

Thanks all! You've been super helpful. I won't bother posting a picture as I think I have enough info to trouble shoot. Fyi- this isn't an actual "issue" as I've never needed to shoot above 1/160 just trying to understand a little better.

(Please excuse my inner mathematician he really likes to compute log base 2 of quotients)
1/160th is only 0.64 EV brighter than 1/250th. So approximately +2/3 EV that's not a huge advantage

I do think I have rear curtain sync on now that you mention it! I was testing it out with the boys on their dirt bikes trying to pan and then flash. Perhaps this is my issue!

This is still a hardware compatibility issue, but presumably not a big deal when you actually want rear curtain sync right? You'd be dragging the shutter anyway ;)
Shutter speed makes no difference when using studio flash the flash acts as your shutter speed, shutter speed only comes into play when there is ambient and flash
 
It helps to know the mechanics of how the camera is doing stuff, to understand what’s going on. Jazzie, if I’m covering stuff you already know, just skip over it.

The camera shutter in most DSLR’s is a two-curtain setup, with the two curtains rolled up at the top. To open the shutter, the first curtain descends, creating the opening through which the light from the lens can reach the sensor. Then second curtain descends, closing the opening.

Up to the sync speed, the opening is completely open (ie, the first curtain is down, and the second curtain is still up). Above the sync speed, the second curtain starts travelling down before the first curtain is all the way at the bottom. In effect, the two curtains form a travelling slit. The “faster” the shutter speed, the narrower is the slit.

When you’re using flash, the light pulse is usually very short (on the order of about 1/5,000 of a second or even faster, depending on whether you have a full-power or partial power pulse), and the camera and the flash need to synchronize so that the flash fires when the shutter is completely open (ie, the first curtain is completely down, and the second curtain is still up). If the flash fires with the travelling slit in motion, you’ll get black bands, which is the shadow of the curtain(s).

The two methods of synchronization are first curtain and second curtain. The first curtain flash means that the camera instructs the flash to fire as soon as the first curtain is down. The second curtain flash synchronization makes the camera fire just before the second curtain starts to travel down. The difference between the two synchronizations shows up when you are using a slow shutter speed, and you have enough ambient light to register.

A typical situation may be to have enough ambient light to give you an exposure at (say) 1/20 sec. If you move the camera during this time, you’ll get blur. If something moves during this time, you’ll also get blur. If you fire a flash, you’ll get a “frozen” subject (since the flash pulse duration is so short), but the ambient would produce blur. You can use this feature to do creative combinations, such as using a slow shutter with flash to record a moving object as a blur, but then “freeze” it with a flash. If you do first curtain sync, then the flash will happen at the start of the exposure, and the object will blur (due to ambient) after the flash has fired. With second-curtain sync, the blur happens first, and the “freeze” with the flash happens at the end. We tend to see things with the blur behind the moving object, so a second-curtain sync will give us the kind of image we expect to see.

As mentioned by other people, there is a time lag between when the camera initiates the signal for the flash to fire, and when the flash actually fires. If the flash is sitting in the camera’s hot shoe, the lag is essentially non-existent. When the signal is sent through a set of devices (such as triggers), the lag may become more significant, and the flash may fire later than the camera “expects”. This means that the camera will already instruct the second curtain to start coming down, before the flash is able to create the light pulse. This will show up as a black band on the lower part of the image (the image is inverted on the sensor). As mentioned already, when there is this time lag, you have to compensate by leaving the shutter open longer, so that your shutter speed is now slower than the sync speed.

Studio flashes generally have a much longer flash pulse than do speedlights, on the order of 1/160th of a second at full power, and they therefore sometimes need longer shutter speeds than the sync speed.

There is also another flash capability call "high-speed" sync, in which the flash fires many short pulses of light instead of a single pulse. This is used with shutter speeds higher than the sync speed, and is used primarily to provide fill with bright ambient.
 
It helps to know the mechanics of how the camera is doing stuff, to understand what’s going on. Jazzie, if I’m covering stuff you already know, just skip over it.

The camera shutter in most DSLR’s is a two-curtain setup, with the two curtains rolled up at the top. To open the shutter, the first curtain descends, creating the opening through which the light from the lens can reach the sensor. Then second curtain descends, closing the opening.

Up to the sync speed, the opening is completely open (ie, the first curtain is down, and the second curtain is still up). Above the sync speed, the second curtain starts travelling down before the first curtain is all the way at the bottom. In effect, the two curtains form a travelling slit. The “faster” the shutter speed, the narrower is the slit.

When you’re using flash, the light pulse is usually very short (on the order of about 1/5,000 of a second or even faster, depending on whether you have a full-power or partial power pulse), and the camera and the flash need to synchronize so that the flash fires when the shutter is completely open (ie, the first curtain is completely down, and the second curtain is still up). If the flash fires with the travelling slit in motion, you’ll get black bands, which is the shadow of the curtain(s).

The two methods of synchronization are first curtain and second curtain. The first curtain flash means that the camera instructs the flash to fire as soon as the first curtain is down. The second curtain flash synchronization makes the camera fire just before the second curtain starts to travel down. The difference between the two synchronizations shows up when you are using a slow shutter speed, and you have enough ambient light to register.

A typical situation may be to have enough ambient light to give you an exposure at (say) 1/20 sec. If you move the camera during this time, you’ll get blur. If something moves during this time, you’ll also get blur. If you fire a flash, you’ll get a “frozen” subject (since the flash pulse duration is so short), but the ambient would produce blur. You can use this feature to do creative combinations, such as using a slow shutter with flash to record a moving object as a blur, but then “freeze” it with a flash. If you do first curtain sync, then the flash will happen at the start of the exposure, and the object will blur (due to ambient) after the flash has fired. With second-curtain sync, the blur happens first, and the “freeze” with the flash happens at the end. We tend to see things with the blur behind the moving object, so a second-curtain sync will give us the kind of image we expect to see.

As mentioned by other people, there is a time lag between when the camera initiates the signal for the flash to fire, and when the flash actually fires. If the flash is sitting in the camera’s hot shoe, the lag is essentially non-existent. When the signal is sent through a set of devices (such as triggers), the lag may become more significant, and the flash may fire later than the camera “expects”. This means that the camera will already instruct the second curtain to start coming down, before the flash is able to create the light pulse. This will show up as a black band on the lower part of the image (the image is inverted on the sensor). As mentioned already, when there is this time lag, you have to compensate by leaving the shutter open longer, so that your shutter speed is now slower than the sync speed.

Studio flashes generally have a much longer flash pulse than do speedlights, on the order of 1/160th of a second at full power, and they therefore sometimes need longer shutter speeds than the sync speed.

There is also another flash capability call "high-speed" sync, in which the flash fires many short pulses of light instead of a single pulse. This is used with shutter speeds higher than the sync speed, and is used primarily to provide fill with bright ambient.
Thank you so much for taking the time to write such a detailed response. Some I already knew, but some was still new to me. it's very much appricated!
 
Thanks all! You've been super helpful. I won't bother posting a picture as I think I have enough info to trouble shoot. Fyi- this isn't an actual "issue" as I've never needed to shoot above 1/160 just trying to understand a little better.

(Please excuse my inner mathematician he really likes to compute log base 2 of quotients)
1/160th is only 0.64 EV brighter than 1/250th. So approximately +2/3 EV that's not a huge advantage

I do think I have rear curtain sync on now that you mention it! I was testing it out with the boys on their dirt bikes trying to pan and then flash. Perhaps this is my issue!

This is still a hardware compatibility issue, but presumably not a big deal when you actually want rear curtain sync right? You'd be dragging the shutter anyway ;)
Shutter speed makes no difference when using studio flash the flash acts as your shutter speed, shutter speed only comes into play when there is ambient and flash
And now I will have the knlowadge I need to use when the situation does arise. :)
 
...Shutter speed makes no difference when using studio flash the flash acts as your shutter speed, shutter speed only comes into play when there is ambient and flash
Unless your shutter speed exceeds the camera's sync speed. Then it can make a big difference!
 
or you shutter is so slow you get double exposures and still result with a blurry subject.
 

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