# Thread: what's the theory behind high-sync flash in 420Ex can not freeze action?

1. ## Re: what's the theory behind high-sync flash in 420Ex can not freeze action?

Ugh. 19 replies and not a single satisfactory answer. And some outright wrong ones. The reason behind the phenomenon is in how FP flash works. I'm lazy to explain it fully, but in FP mode, the flash fires multiple times to cover the entire frame when the shutter is only exposing part of the frame. What that means is that in situations where the flash is the only, main or significant source of light, you'll get disjointed trails of the action which show the areas where the flash lit up portions of the frame during the course of the 1/2000s (or whatever shutter speed during the FP sync).

2. ## Re: what's the theory behind high-sync flash in 420Ex can not freeze action?

With the way FP flash works (with the multiple flashes to cover the entire frame as the open shutter moves across the frame), if your subject is moving fast enough, you'll start to see "jagged" edges on the edges of the subject being lit up by the flash.

3. ## Re: what's the theory behind high-sync flash in 420Ex can not freeze action?

Originally Posted by ST1100
The theory behind freezing action (flash or no flash) has to do with how much the subject changes relatively to the camera in the time the shutter is open, and whether captured subject movement falls within the circle of confusion (COC) tolerance to be considered as 'still'.

Take for example, a car moving across the frame at 100km/h. This translates to 27.8 m/s. If i open the shutter for 1/5000s, the car would have moved (27.8/5000=) 5.5mm in the time the shutter was open. Is this 5.5mm considered 'frozen'? We use the magnification factor to calculate how long this 5.5mm movement is on the film/sensor. If it is within the COC, the picture is considered 'frozen'.

In non-high speed sync, when the picture is *dominantly flash-lit*, the duration of the flash determines the exposure time. Eg, if the shutter time is 1/200s and the flash duration is 1/5000s, the 1/5000s will determine whether the action is frozen, not the 1/200s. The theory is based on the reasoning above, except that the flash duration determines the exposure time, not the shutter speed.

In high speed sync, the shutter speed determines the exposure time, and thus determines if the scene is frozen.

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