High speed synch - when and why not set all the time?

Hello,
Apologies for this newbie question, (which may have been answered before), but I have searched through the board and still have a question to ask about synchronisation speeds.

The gear - a Canon 10D and a new 550EX flash gun. (Not had chance to try it out yet but have read the manual - well most of it!).

Without going to high speed synch mode, the 550EX will synchronise up to 1/200 sec automatically - great, I can go out and shoot Tv up to 1/200 sec or Av with an aperture of whatever I like provided it ain't so big that I will need anything faster than 1/200 sec. No problem so far.

However ... if I want to shoot faster than 1/200sec, I have to switch it into High Speed synch mode. The question I have is why didn't CANON make the 550EX synch automatically with speeds over 1/200 sec if that is what the shot demands? In short - why bother having to set high synch at all - let the camera tell the flash what speed it is going to set the shutter to and let the flash synch to it.

The 550EX is capable of synching to any speed - right? (within limits).

I don't think that I fully understand how synchronising works - and therefore am unclear as to what setting I need for fill-in flash in outdoor daylight - am I best always setting to high speed - so that the 550EX will synch to whatever Tv I have?

Thank you in advance - hope I explained my question OK.
Alan

You have a legitimate question there and I usually tend to keep my flash on HS sync mode a lot. But I can think of a couple of reasons why this may not be a good setup for everybody or for use all the time.

When you are shooting in normal sync mode (not high speed sync), the camera will automatically set the shutter speed to no more than 1/200 as you stated earlier. But, if the flash is set to HS sync, your shutter is left where it's at. So, let's say that your shutter is set to 1/2000 after a day at the beach. Then you go to a party later that evening and neglect to set the shutter speed back down to a reasonable speed, say 1/60 or even 1/200. So what happens when you take pictures at the party using the flash. Chances are most, if not all, shots will be badly underexposed because in HS sync mode the flash output is reduced. The amount of reduction depends on the shutter speed used above 1/200. I would say that at 1/2000, the flash output would be drastically reduced. So, the next thing you know, you are will be complaining why Canon doesn't warn you that you're in high speed sync mode so stuff like this doesn't happen. Well, they actually do. That's what the lightning with an "H" beside it in the viwefinder is telling. But, because you're so used to seeing that lightning with the "H" (since the flash is always in high sync mode), it never occurs to you to switch the flash out of HS sync mode.

When the 550EX is in high speed sync mode, it puts out a burst of flashes and each one is of somewhat limited intensity. As a result, you can shoot faster than 1/200th shutter but your flash range is limited.

---Bob Gross---

A "shutter trip" commences with the opening of the first shutter curtain to begin exposing the sensor, and concludes with the closing of the second shutter curtain to cover the sensor again. These shutter curtains can only move so fast, the time it takes them to go from top to bottom is roughly on the order of the camera's sync speed (e.g. 1/200). For shutter speeds faster than this, the second curtain begins closing the shutter before the first curtain has fully opened; in other words, the sensor is exposed by an open "slit" between the first and second curtains traveling vertically along the frame. The shorter the exposure, the narrower this slit is; for instance at 1/2000 sec you'd expect this slit to be about 1/10 the height of the sensor, moving vertically along the frame to expose the sensor along its full height.

In normal flash photography, the shutter must be fully open for a time, and the flash is "fired" during that time, either just after the first curtain has fully opened (i.e. near the beginning of the exposure) or else just before the second curtain begins to close (i.e. near the end of the exposure). If the flash was fired when the shutter was not fully open, e.g. when the first curtain was only half way open, then part of the sensor would be blocked for the duration of flash, which is extremely short. So normal flash doesn't work at speeds faster than the sync speed, which necessarily imply the sensor being exposed by a partial "slit" opening between the shutter curtains, with the shutter never fully open.

In high speed sync, the flash releases its stored-up energy gradually over a longer time, instead of all at once, e.g. by pulsing the flash in little bursts. So in a very fast exposure, where the sensor is exposed by a small "slit" between the first and second curtains, the flash lasts for the entire duration (roughly 1/200 second), continually flashing and "exposing" the sensor as the slit moves along the height of the sensor.

The downside is that some of the flash's energy is now uselessly bounced off the closed shutter. For instance, for a 1/400 shutter speed, you'd expect the "slit" to be about half the height of the sensor, and thus any flash energy released at any point in time can reach no more than half the sensor, the other half being wasted, blocked by the closed curtains. The faster the shutter speed, the smaller the slit, and the lower the percentage of flash energy that gets "used".

So high-speed sync uses flash energy less efficiently than normal flash, and that efficiency drops rapidly as shutter speeds get shorter. This reduces flash range, increases battery drain, and increases the time needed to allow the flash to recharge, relative to normal flash. This downside is why a camera/flash typically requires the photographer to intentionally enable this functionality, to say "yeah, I know, but I really want HS any way".
-harry

FWIW, I bookmarked this page a while back.. It provides an excellent explanation of high speed vs normal sync and provides sample photos showing what each does and what to expect:

http://webs.lanset.com​/rcochran/flash/hss.ht​ml

Good explanation Harry.

Thanks guys - makes sense now. All replies V-E-R-Y much appreciated - may need you again!

Alan

Just sat down with a print out of all of the above and done some studying - it all makes sense but please can you confirm that I have understood correctly :

With normal flash, the gun emits a VERY short burst of light once the sensor is fully exposed. Exposing the sensor fully takes 1/200 second. So it is ... open the shutter, fire, close the shutter.
The flash DOES NOT last for 1/200 second but occurs 1/200 second after the first curtain starts to move - and this is the time it takes to fully reveal the sensor.

With high speed flash, the gun has to emit light for the full duraton of the curtain movement - 1/200 second ensuring that all of the sensor gets some flash light. This is because curtain 2 is hot on the heels of curtain 1. This happens so that each part of the sensor is only exposed for whatever exposure time we have set.

So ... high speed synch emits stored energy for longer than normal synch - reducing the "throw" or guide number.

I guess that this is a "tell me in your own words" exercise. Thanks again for your indulgence. Maybe basic stuff but you gotta know the basics before getting to grips with the not so basic.

Alan

Also, check this out:

http://photonotes.org/​articles/eos-flash/

Everything you need to know about flash on Canon EOS bodies, but didn't even know you needed to know...

Yes, for "first curtain sync".

For "second curtain sync", the flash is triggered just before the second curtain begins moving, which occurs after the first curtain starts to move by an amount of time equal to the shutter speed.

Yes. The total amount of energy is the same, but for a normal flash it is all spit out in a single, quick, high power burst, whereas for HS it is emitted in a longer duration, lower power burst (this is either a long steady burst, or a series of quick, little pulses, or something in between).

These two types of flashes (high power and fast vs low power and slow) are still capable of emitting the same total energy, and would yield the same guide number (in theory) if the sensor could be exposed for the entire duration of the long, slow HS flash, but the sensor is necessarily covered up by the shutter curtains during part of the HS flash, which has the net effect of reducing the GN.

In any case, yes, you've got it!
-harry

hmhm wrote in post #216562
A "shutter trip" commences with the opening of the first shutter curtain to begin exposing the sensor, and concludes with the closing of the second shutter curtain to cover the sensor again. These shutter curtains can only move so fast, the time it takes them to go from top to bottom is roughly on the order of the camera's sync speed (e.g. 1/200). For shutter speeds faster than this, the second curtain begins closing the shutter before the first curtain has fully opened; in other words, the sensor is exposed by an open "slit" between the first and second curtains traveling vertically along the frame. The shorter the exposure, the narrower this slit is; for instance at 1/2000 sec you'd expect this slit to be about 1/10 the height of the sensor, moving vertically along the frame to expose the sensor along its full height.

In normal flash photography, the shutter must be fully open for a time, and the flash is "fired" during that time, either just after the first curtain has fully opened (i.e. near the beginning of the exposure) or else just before the second curtain begins to close (i.e. near the end of the exposure). If the flash was fired when the shutter was not fully open, e.g. when the first curtain was only half way open, then part of the sensor would be blocked for the duration of flash, which is extremely short. So normal flash doesn't work at speeds faster than the sync speed, which necessarily imply the sensor being exposed by a partial "slit" opening between the shutter curtains, with the shutter never fully open.

In high speed sync, the flash releases its stored-up energy gradually over a longer time, instead of all at once, e.g. by pulsing the flash in little bursts. So in a very fast exposure, where the sensor is exposed by a small "slit" between the first and second curtains, the flash lasts for the entire duration (roughly 1/200 second), continually flashing and "exposing" the sensor as the slit moves along the height of the sensor.

The downside is that some of the flash's energy is now uselessly bounced off the closed shutter. For instance, for a 1/400 shutter speed, you'd expect the "slit" to be about half the height of the sensor, and thus any flash energy released at any point in time can reach no more than half the sensor, the other half being wasted, blocked by the closed curtains. The faster the shutter speed, the smaller the slit, and the lower the percentage of flash energy that gets "used".

So high-speed sync uses flash energy less efficiently than normal flash, and that efficiency drops rapidly as shutter speeds get shorter. This reduces flash range, increases battery drain, and increases the time needed to allow the flash to recharge, relative to normal flash. This downside is why a camera/flash typically requires the photographer to intentionally enable this functionality, to say "yeah, I know, but I really want HS any way".
-harry

hmhm wrote in post #217895
Yes, for "first curtain sync".

For "second curtain sync", the flash is triggered just before the second curtain begins moving, which occurs after the first curtain starts to move by an amount of time equal to the shutter speed.

Yes. The total amount of energy is the same, but for a normal flash it is all spit out in a single, quick, high power burst, whereas for HS it is emitted in a longer duration, lower power burst (this is either a long steady burst, or a series of quick, little pulses, or something in between).

These two types of flashes (high power and fast vs low power and slow) are still capable of emitting the same total energy, and would yield the same guide number (in theory) if the sensor could be exposed for the entire duration of the long, slow HS flash, but the sensor is necessarily covered up by the shutter curtains during part of the HS flash, which has the net effect of reducing the GN.

In any case, yes, you've got it!
-harry

Hate to bump such an old thread but i just had to say EXCELLENT explanation of this! This is worth bumping just for those out there who are new and wondering what the heck high speed sync is for! Great job!

ben_r_ wrote in post #4548150
Hate to bump such an old thread but i just had to say EXCELLENT explanation of this! This is worth bumping just for those out there who are new and wondering what the heck high speed sync is for! Great job!

Someone directed me to this thread for me so that I understand the difference between Normal flash and High Speed Sync, and the explanation above were excellent. But I stil have some questions since I am very much a newbie to flash photography. All my questions are from practical points of views.

Question 1. Most EOS cameras have a custom function to set flash sync speed to "Auto" or "1/300 second" in AV mode. What is the difference between these choices. When I set it as 1/300, the exif showed that the each photograph was taken with1/300 second shutter speed, but that would not be true. As a result, I would not know until I saw the photos on a monitor if I chose the right ISO to stop the motion. But if I switch to "Auto", it showed the "real" shutter speed.

Question 2.
Since the duration of flash is much faster than "shutter trip", what are the best choices in order to stop motion with a flash in dim light. Bumping up ISO is a must, but what about flash? Should I choose 2nd curtain sync from camra's custom function, since flash occurs just before the second curtain closes?

Henry

Henry S wrote in post #5491762
Question 1. Most EOS cameras have a custom function to set flash sync speed to "Auto" or "1/300 second" in AV mode. What is the difference between these choices. When I set it as 1/300, the exif showed that the each photograph was taken with1/300 second shutter speed, but that would not be true. As a result, I would not know until I saw the photos on a monitor if I chose the right ISO to stop the motion. But if I switch to "Auto", it showed the "real" shutter speed.

IMO that CFn is quite useless.
If you want a fixed shutterspeed, manual is more flexible (you can set any shutterspeed then).

If the CFn is set, the shutterspeed is 1/300s for every exposure taken on Av, where the flash was charged AFAIK.
This will probably result in a lot 'amateurish, deer in the headlights / bat in a cave' type flashed images.

Henry S wrote in post #5491762
Question 2.
Since the duration of flash is much faster than "shutter trip", what are the best choices in order to stop motion with a flash in dim light. Bumping up ISO is a must, but what about flash? Should I choose 2nd curtain sync from camra's custom function, since flash occurs just before the second curtain closes?

Flash duration is shorter at lower power output.
You'd need some pretty fast motion to show blur in a flash image.
If you use a slower shutterspeed to let more ambient in, the ambient lighting may cause blur...

2nd curtain is useful for light trials (since they appear to go the right way then), but can screw up your timing, and make for a lot of images (depending on shutterspeed) with the subjects eyes closed or turned away, since the subject blinks because of the ETTL preflash, or think the preflash was the actual picture being taken.


Search POTN for "club photography" or something like that, quite a few threads.

You confused me, are you talking about setting camera in manual mode, then how should I set flash? Or are you talking about setting flash in manual mode, or setting both in manual mode? Please clarify and explain in more detail. What does AFAIK means by the way?


In theory, 2nd curtain is useful in getting light trails, I know that....then in dim light condition, just use 1st curtain, is that what you are saying?

AFAIK = As Far As I Know

Set the camera in M, leave the flash in ETTL.
I leave it on first curtain, unless I need light trials.

You can set a shutterspeed to capture the amount of ambient light you want, while not getting camera shake, ETTL will expose the subject.

Night Club photography:
Search finds lots of threads:
1
2
3
4
5
6
7
8

Should keep you in reading for a while