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


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dRebelXT

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May 14, 2005
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I was told that even if one can flash at 1/2000s in high speed sync, it's unable to free actions.
what's the theory behind that? :confused:
 

sk.images

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Dec 9, 2005
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#3
1/2000 can freeze action as long as the ambient light isn't that strong (relatively) that it lights the subject enough to register significantly. At night you should be fine, during the day you will get ghost images.

Obviously you need to through the shutter speed and the speed of the subject into the equation as well.
 

kietgnoel

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In normal flash, the shutter would be opened for a certain amount of time. During this period, a single burst of light fills the scene only for a fraction of that time. Assuming that the ambient light is insufficient to significantly register on the sensor, the subject would appear to be to be frozen by the flash.

In FP flash or high-speed sync flash, multiple bursts of light are emitted (high frequency) for the duration that the shutter is opened, instead of a single burst of light. Therefore, light from the flash fills the scene for the entire duration that the shutter is opened. So, if the shutter speed is low enough and the subject moves, you get subject movement blur instead of a "frozen" subject.

You can think of FP flash like a strobe, pulsing light many times per second e.g. 50 Hz. The downside to this is that you lose flash range i.e. the GN is lower. The higher the shutter speed, the lower the flash range.
 

Artosoft

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#6
This is the flash's duration data from Nikon flash SB-800:
Full M/1 = 1/1050s
M/2 = 1/1100s
M/4 = 1/2700s
...
M/16 = 1/10900s
...
M/64 = 1/32300s
M/128 = 1/41600s

I know, I know, the TS asking about Canon 420EX, and how can high speed sync at 1/2000s can't stop an action.

Like kietgnoel said about high-speed flash, yes, the flash will strobe, so the entire open window travel (from top to bottom, or from bottom to top) of shutter at 1/2000s is 'kena' by flash's illumination. Thus, we can say it is 1/2000s speed.

When we use flash with iTTL, eTTL, Auto or GN mode at sync speed or below, the flash duration will vary from full power to lowest possible power (in my example SB-800, from 1/1050s to 1/41600s). This flash's duration depends on Lens' aperture, Camera's ASA/ISO, Distance from Flash to subject, and flash's GN power (, and whether the subject absorb or reflect the flash's illumination). Same thing if we use flash in Manual mode, we vary the flash's power from M/1 to M/128, thus varying the flash's duration from 1/1050s to 1/41600s. We can say, if we use SB-800 at night (or dark environment) even if you set camera's shutter speed to 1/10s, your shutter speed is varying from 1/1050s to 1/41600s (depends on flash's duration).

Say when the flash is use only 1/16 of its power, we get 1/10900s, which is enough to stop an action (compare to 1/2000s).

Please do correct me (nicely) if I am wrong ;) .

Regards,
Arto.
 

imouyang

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#7
Arto, you are most probably right. And i dun understand y 1/2000 such a fast speed cant freeze motion due to ambient light and the flash is also emitted in a shorter time faster than 1/2000. The only thing that i would worry now is whether the high speed flash is strong enuff to illuminate my target a not, since hi-spd flash has reduced power..
 

dRebelXT

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Thank you all for the explanation. Assume a subject needs 1/4000s to perfectly freeze,
am I right to say,

1) At max sync speed (1/200s) of 420Ex under normal mode, the flash only emit high power light for maybe 1/5000s (determined to E-TTL). The subjects is frozen.
2) At 1/2000s under high speed sync, the the fpash emits low power light for 1/2000s, and subject is not well done.
 

Artosoft

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#9
imouyang said:
Arto, you are most probably right. And i dun understand y 1/2000 such a fast speed cant freeze motion due to ambient light and the flash is also emitted in a shorter time faster than 1/2000. The only thing that i would worry now is whether the high speed flash is strong enuff to illuminate my target a not, since hi-spd flash has reduced power..
Ok, maybe 1/2000s is fast enough to freeze motion of walking man, but not fast enough to freeze motion of running man or cheetah.

If you use high speed flash at 1/2000s, actually the flash will strobe (few flash's bursts instead of single flash's burst) for period of 1/2000s. Let say it will strobe 10 times for period of 1/2000s. Each burst is faster than (1/2000s / 10 =) 1/20000s, but the power of each burst (from the strobe) is also divide by 10 (compare to single burst), thus explain why the range of high-speed flash is also reduced. The duration captured by camera's sensor is all the bursts of strobe (1/2000s), and not the single burst of the strobe (1/20000s).

So, if you want to freeze a motion, use normal flash mode instead of high-speed flash mode.
High-speed flash mode is to fill limitation of low flash's sync speed of camera (so you can use flash for fill-in during strong ambient light when you need to use big aperture and faster shutter speed than camera flash's sync speed).

Regards,
Arto.
 

Artosoft

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Aug 31, 2005
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#10
dRebelXT said:
Thank you all for the explanation. Assume a subject needs 1/4000s to perfectly freeze,
am I right to say,

1) At max sync speed (1/200s) of 420Ex under normal mode, the flash only emit high power light for maybe 1/5000s (determined to E-TTL). The subjects is frozen.
2) At 1/2000s under high speed sync, the the fpash emits low power light for 1/2000s, and subject is not well done.
1) Yes.
2) Depending on Flash's GN power, Distance from flash to subject, Camera's ISO/ASA, Lens' aperture, the subject maybe not or maybe well illuminated.

Regards,
Arto.
 

user111

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Jul 27, 2004
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#11
dRebelXT said:
I was told that even if one can flash at 1/2000s in high speed sync, it's unable to free actions.
what's the theory behind that? :confused:
thats rubbish
 

user111

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Jul 27, 2004
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#12
what on earth is so fast that 1/2000 cannot freeze?
 

user111

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Jul 27, 2004
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#15
maybe u r right

lol :thumbsup:
 

Artosoft

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#16
user111 said:
what on earth is so fast that 1/2000 cannot freeze?
Bullet release from snipper shotgun... Special bullet that is more than speed of sound ;p .

Regards,
Arto.
 

mpenza

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Jan 18, 2002
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#18
if 1/2000s is not fast enough to freeze action, using highspeed sync flash also won't be able to freeze. but if 1/2000s is fast enough to freeze action, whether u use highspeed sync flash or no flash, you can freeze the action.
 

ST1100

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Jun 18, 2003
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#19
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.
 

user111

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#20
cant get more detailed than that :thumbsup:
 

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