Slow synchro flash vs long exposure

[thetoxicmud]

Can someone explain me please key differences?

I saw videos explaining how slow synchro works, but people used it even during the day. One video shows that using 2nd curtain gives you nice trails of movement behind the subject. But so does the long exposure without the flash - traffic trails lights being the most common example. So why do I want to use slow synchro flash especially if there is sufficient light instead of just long exposure and tripod ?

[p4pictures]

Slow synchro flash and long exposure are similar.

Slow synchro flash combines a pop of flash at either the start of the exposure or at the end and a slow shutter speed. This creates a sharp image of the subject at either the start or end of the exposure and allows blurs to happen in between. Depending on the camera settings and exposure mode, you can have shutter speeds up to 30 seconds and select either 1st curtain (flash fires at the start) or 2nd curtain (flash fires at the end) sync for the flash. If you photograph a car, in low light and use first curtain the resulting image looks like the car is going backwards.

Slow synchro with 2nd curtain sync flashSlow synchro with 1st curtain sync flash

Long exposure - just records the blurs for the moving subject over time. This is the kind of picture where you might want to be on a bridge over a busy highway and show the long streaks of light as cars pass through the scene.

[shadowsports]

Brian's explanation is wonderful as usual.  😀

My add: Slow synchro flash can be used both day or night.  A tripod is recommended.  Night time is most popular since it allows you to use lower ISO resulting in less noise.  

During the day its effective for close subjects or action shots. It can help you equalize the exposure if you are shooting a subject with shadows against a brightly lit background. Its also used to fill and remove shadows when the sun is at a high angle.

When shooting at night it allows you to illuminate the main subject in your foreground or create light trails behind.  This is determined by the curtain sync you choose.  The slower shutter speed allows the camera to capture more ambient light in the background your flash would not typically be able to illuminate.  

When you shoot first curtain, the flash fires at the beginning of the exposure putting the light trails in front of a subject.  When second curtain is used the exposure occurs first, then the flash fires which puts light trails behind your subject.

[thetoxicmud]

Thank you both for explaining. Just one more small question, I will do some tests myself at some point to compare both long exposure and slow synchro, but can someone give one example of a situation , assuming we have day and subject is close, where you would prefer to use one technique over another? Or are they similar enough that it wouldn’t matter?

[p4pictures]

Taking the car pictures as an example the long exposure no flash result would not show the blue car, only the light trails. So then it’s a case of what you want to show.