What does flash sync means

[novetan5]

I suspect this may hv asked before. But a search on flash sync turns out many threads but surprisingly most of it not quite related.

Here's my question.

When is states flash sync at 1/250, what does it means? What will be the result if I use slower or faster speed than what's stated? And how abt EV adjustment on the flash. If we adjust lower or higher EV on the flash, does it means it will not sync at 1/250?

Tks.

 

[Leonard Migliore]

I suspect this may hv asked before. But a search on flash sync turns out many threads but surprisingly most of it not quite related.

Here's my question.

When is states flash sync at 1/250, what does it means? What will be the result if I use slower or faster speed than what's stated? And how abt EV adjustment on the flash. If we adjust lower or higher EV on the flash, does it means it will not sync at 1/250?

This is sort of complicated. You're almost certainly referring to a camera with a focal plane shutter, which is typically found on cameras with interchangeable lenses. A focal plane shutter is near the sensor, so you don't have to put a shutter on each lens.

The way the shutter works is that it has a pair of curtains that run across the sensor. This video shows what happens in great detail:

In your case, the curtain takes about 1/250 second to go across the sensor. If you have a shutter speed slower than 1/250, there's a delay before the second curtain starts moving. If your shutter speed is faster than 1/250, the second curtain starts before the first one is completely across the sensor.

Now lets imagine you have a flash. Electronic flashes are pretty fast, being on from about 1/1000 second to as short as 1/25,000 at low power. If you want the whole image to be exposed to the light from the flash, the whole sensor has to be exposed at some instant in time. That can't happen if the shutter speed is faster than 1/250 because then there won't be any point in time where the entire sensor is exposed.

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

If your shutter speed is slower than 1/250, then you can fire the flash any time between the first curtain finishing its path and the second curtain starting. Some cameras allow you to select between first curtain and second curtain triggering to get different motion effects.

Flash EV has no effect here; it just changes how much light the flash puts out.

 

[FrancoD]

It means that the fastest shutter speed you can shoot at to get the full image is 1/250th, however any speed below that is OK.

This is what you would get at 1/320 or thereabout :



from

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www.

bhphotovideo.com/explora/photography/tips-and-solutions/sync-speed-versus-flash-duration

 

[Chris R-UK]

The only thing that I can add to Leonard's excellent explanation is that some cameras/flashes can get round the problem by using something called High Speed Sync or HSS. In this mode the flash fires a series of short bursts to ensure that the whole of the image is illuminated, but it uses a lot of power.

 

[Knoxis]

Flash sync speed is merely the fastest shutter speed at which the shutter curtains will not obstruct any of the light coming from the flash. If you remove the lens from your camera, you might be able to see the shutter mechanism. The shutter mechanism works by having one curtain cover the sensor, then drop down, and another shutter curtain follows afterwards. During a slow shutter speed, like 1 second, the first curtain will drop, and the sensor will stay completely exposed for 1 second, and then the second curtain will drop. In essence, the flash sync speed is the fastest shutter speed at which the camera has the entire sensor exposed, with neither shutter curtain in the way — i.e. the first curtain has dropped but the second curtain hasn't. For most cameras this speed is 1/250. Anything faster than that, i.e. 1/500s, and the second curtain will start to drop before the first curtain even completes its journey. Therefore, at shutter speeds faster than your flash sync speed, you will only get a small slit uncovering the sensor a time, moving across the frame and exposing parts of the sensor, but by bit, vertically. This results in light cut off, as the light from the flash is unable to hit parts of the sensor due to the sensor not being completely uncovered, and thus result in black bars across the frame.

Increasing the flash power tends not to have any effect on the flash sync speed, as the flash will generally fire light at a very high speed (around 1/1000s or faster) and is faster than the flash sync speed, and therefore all the light will hit the sensor regardless of shutter speed (so long as it remains below or at the flash sync speed). Increasing the power of a flash can make the flash output a bit slower, but it tends to remain quite fast (at least 1/1000s) and so there isn't any problem.

 

[John Sheehy]

When is states flash sync at 1/250, what does it means?

It means that 1/250 is the fastest shutter speed (shortest exposure) at which the camera guarantees to have the entire sensor exposed at one time, and use a single pulse of flash to illuminate the entire frame. At higher shutter speeds, the entire sensor is not uncovered by the shutter at once, and a slit moves over the sensor. The entire sensor may actually be exposed at one or two shutter speeds higher than the flash sync, but the camera system may not try to do it because the protocol allows some room for error. For example, if you use a simple, non-communicating flash, it might illuminate the whole frame evenly at 1/320, and only fail to illuminate a small horizontal strip at the edge of the frame at 1/400.

Many communicating flashes offer "high speed sync" which allows flash by strobing which illuminates different parts of the frame at different times. The downside is that the flash time is so long that it does not stop motion like normal flash sync does; it is simply extra light for the full duration of the shutter slit moving across the sensor.

What will be the result if I use slower or faster speed than what's stated?

If it's a system-communicating flash, then the camera probably won't allow faster shutter speeds, unless it has high-speed sync and it is enabled. Slower is always usable - you get a very long ambient exposure, and a very fast burst of flash.

And how abt EV adjustment on the flash. If we adjust lower or higher EV on the flash, does it means it will not sync at 1/250?

Exposure compensation has no effect on timing limitations.

 

[Lee Jay]

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

Everything you said was right, I'm just adding test data to illustrate. Make sure you look at original to see the text.

--
Lee Jay

 

[John Sheehy]

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

Everything you said was right, I'm just adding test data to illustrate. Make sure you look at original to see the text.

What is the official sync speed with that system? That looks just like one camera I tested with a manual flash years ago, which was officially 1/250. I really wished that the camera would do 1/400 with a communicating system flash, anyway, as I was almost always focal-length-limited anyway, and needed to crop. I could have just set the camera to 1/400, and remembered that the center was a little higher than what I saw in the OVF. HSS (high speed sync) is extra light, but it doesn't stop action any more than the ambient exposure does. Also, I've seen HSS illuminate the frame unevenly, in horizontal strips with slightly different exposures.

 

[Mike_PEAT]

When is states flash sync at 1/250, what does it means?

Basically put, it's the fastest shutter speed where the shutter is fully open with neither of the shutter curtains is blocking the image. You can see this in the top row.

Any faster the shutter starts closing with the 2nd curtain before the first curtain is fully open. You can see this in the lower row:

What will be the result if I use slower or faster speed than what's stated?

If you are using a TTL flash it shouldn't let you exceed sync speed UNLESS you have the flash in high-sync (HSS) mode. In HSS mode the flash pulses during the fast sync to expose the entire image as the slit moves.

However if you are using a non-TTL flash only part of your image gets exposed by the flash (the rest will be black).

And how abt EV adjustment on the flash. If we adjust lower or higher EV on the flash, does it means it will not sync at 1/250?

EV flash adjustment is usually done by adjusting the intensity of the flash.

One point no one has made yet...flash sync is only a maximum under best conditions. For instance your maximum sync speed may be 1/250 but if you have a lower-end TTL flash you may be limited to 1/180 (you can't select faster than 1/180).

As well, when using non-TTL flashes you may have to limit yourself to 1/120 due to the time your trigger takes to trigger the flash. As well if you are using optical slaves again there's a slow down.

 

[Lee Jay]

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

Everything you said was right, I'm just adding test data to illustrate. Make sure you look at original to see the text.

What is the official sync speed with that system

1/250th - Canon 20D.

 

[John Sheehy]

During a slow shutter speed, like 1 second, the first curtain will drop, and the sensor will stay completely exposed for 1 second, and then the second curtain will drop.

1 second is the amount of time that each row of pixels is exposed, so the time in which all the pixels are exposed is actually slightly less than 1 second, and the ratio of the time from when the first line is exposed until the last line's exposure ends, to the time each line is exposed, increases with shutter speed, and is tremendous at 1/8000.

 

[John Sheehy]

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

Everything you said was right, I'm just adding test data to illustrate. Make sure you look at original to see the text.

What is the official sync speed with that system

1/250th - Canon 20D.

I believe that's the camera I used when I tested it.

A little off topic for your post, but to save the 150 posts for others, I just thought I'd mention the fact that some cameras may use HSS if it is enabled, at the top shutter speed which would be allowed for normal flash sync, if HSS were not enabled. User beware.

 

[novetan5]

Hey guys, tks so much for all the information. Very informative and well explained.

Cheers

 

[Bob Janes]

I suspect this may hv asked before. But a search on flash sync turns out many threads but surprisingly most of it not quite related.

Here's my question.

When is states flash sync at 1/250, what does it means? What will be the result if I use slower or faster speed than what's stated? And how abt EV adjustment on the flash. If we adjust lower or higher EV on the flash, does it means it will not sync at 1/250?

This is sort of complicated. You're almost certainly referring to a camera with a focal plane shutter, which is typically found on cameras with interchangeable lenses. A focal plane shutter is near the sensor, so you don't have to put a shutter on each lens.

The way the shutter works is that it has a pair of curtains that run across the sensor. This video shows what happens in great detail:

In your case, the curtain takes about 1/250 second to go across the sensor. If you have a shutter speed slower than 1/250, there's a delay before the second curtain starts moving. If your shutter speed is faster than 1/250, the second curtain starts before the first one is completely across the sensor.

Now lets imagine you have a flash. Electronic flashes are pretty fast, being on from about 1/1000 second to as short as 1/25,000 at low power. If you want the whole image to be exposed to the light from the flash, the whole sensor has to be exposed at some instant in time. That can't happen if the shutter speed is faster than 1/250 because then there won't be any point in time where the entire sensor is exposed.

If you shoot flash at a shutter speed over 1/250, only part of your image will see the light from the flash. This will be painfully evident because part of your picture will be dark.

If your shutter speed is slower than 1/250, then you can fire the flash any time between the first curtain finishing its path and the second curtain starting. Some cameras allow you to select between first curtain and second curtain triggering to get different motion effects.

Flash EV has no effect here; it just changes how much light the flash puts out.

 

[PhotoTeach2]

It has been explained pretty well and I assume you understand now.

But I just want to add one more thing ...

The term "shutter-speed" is a misnomer because the shutter ALWAYS MOVES AT THE SAME "SPEED". (whether 1/8000sec or 8 full-seconds)

It simply refers to the "timing" between when the first curtain starts to open and the 2nd curtain starts to follow it.