Crazy underexposure set at specific flash setting combination

[Dermit]

I was not sure where to put this so I decided to put it here since it only happens with flash. At first I thought it might just be my camera, or flash. But I confirmed with a frined of mine who uses the same camera and flash as I do. I am not sure if it happens with other models/combos. Here's the situation...

On my 5D set to Av mode on any aperture. On my 580EXII set to manual mode, any power, with high speed sync on. The camera thinks it's on the sun or something because it picks a shutter speed of 1/8000 and blinks like that's not fast enough. (ISO was at 200) If I switch to Tv mode, no matter the speed, it picks my tightest aperture (f/32 on my 70-200) and blinks like it might go blind even with this tiny aperture.

If I do indeed take a shot at this setting it's a black imgae, as expected.

Now i must say that this was stumbled on as an accident. I never intended to be on HSS on the flash, but HSS was on from a previous shoot under different circumstance but in ETTL mode and that combo is fine, everything works great. So that being said this was not a show stopper, I simply turned HSS off, which is what I wanted anyway, and finished out my shoot. And often times i shoot in full manual, which is fine as well since I pick my own ss and aperture. So i never see this as a show stopper for me because there is always another (probably better) way to shoot it. But i wanted to bring it up in case anyone else encounters this and it throws them for a loop. I thought my camera meter was going whack-o for a bit.

So. My question is this... does anyone else see this with other camera/flash combos? And if so is this 'normal'/'intentional'? And if so, why? Or is it a Canon bug?

[egordon99]

hmmmm, strange. I'll try to give it a go tonight with my 40D/580EXII. I'll be inside at night, so I'd EXPECT slow shutter speeds.

[tim]

Weird. Never tried that, i'm in manual when I use a flash.

[Lotto]

The flashing 1/8000 SS means the camera can not control shutter speed with that particular combo. If you look at the EXIF, Av mode actually use max 1/8000. Same with the Tv mode, as the camera warning that it can not control the exposure automatically.

You can use HSS in manual mode, but if you want the camera to control the flash exposure too, don't set the flash to manual mode.

[PacAce]

Yes, this is a complaint or observation (depending on how you want to look at it) many have reported over the years. Other brand cameras don't behave this way so it's not clear why Canon chose to implement it this way whenever the flash is set to manual mode and the camera is set to anything other than manual. :|

[Dermit]

Yes, this is a complaint or observation (depending on how you want to look at it) many have reported over the years. Other brand cameras don't behave this way so it's not clear why Canon chose to implement it this way whenever the flash is set to manual mode and the camera is set to anything other than manual. :|

...but i can set my flash to manual mode, and my camera to Av or Tv with the HSS off and it works fine. It is the combination of HSS and manaul mode flash with the camera in non-manual mode that throws it off.

You can use HSS in manual mode, but if you want the camera to control the flash exposure too, don't set the flash to manual mode.

Lotto, I always was lead to believe that the only thing that controls the flash exposure is the flash itself. I've never heard that the camera ever directly controls the flash exposure. I always thought that the flash read the camera settings, sent out a preflash (in ETTL) and determined the amount of power needed to expose properly based on how much light reflected back and given the current camera settings. Or in Manual mode it will just deliver the amount you have dialed in. I always thought that the camera itself totally ignored the flash in Tv or Av and ONLY determined exposure based on ambient light... and this is absolutely true when HSS is off. With HSS off it acts exactly how I expect it. So for what reason would HSS on change how the camera reacts when it should still only care about ambient light?

[Dermit]

Weird. Never tried that, i'm in manual when I use a flash.

I am in manual mode (camera) when using flash in low light situations... but when outdoors in daylight I am usually in Av or sometimes Tv. But on the day this happened I switched to manual mode and got it dialed in just fine. I just had to watch things close as my ambient light was changing fast as it was just before sunset.

[PacAce]

...but i can set my flash to manual mode, and my camera to Av or Tv with the HSS off and it works fine. It is the combination of HSS and manaul mode flash with the camera in non-manual mode that throws it off.

But when you set the camera to Av mode, non-HSS mode, isn't your shutter speed fixed at 1/250 or whatever your max sync speed is. In true Av mode, the camera should set the SS according to whatever is required for a proper ambient light exposure, no?

So, by the same token, when you set the flash to HSS mode, the camera will pick the highest available shutter speed.

In either case, the camera isn't behaving as you would expect it to in Av mode (unless you have a custom function set which sets the SS to max sync speed whenever a flash is used in Av mode).

[Wilt]

Flash on manual says (depending upon CFn) shutter defaults to 1/250 or to the speed which registers sufficient light with ambient lighting. Put the flash on manual+HSS, and what does that supposed to mean?!?! What does the photographer want the camera to do?! Flashing indicators means a signal to the photographer, "ambigous circumstances, this does not compute! Need more data!"

Same for TV mode...you set the shutter, camera picks the aperture, but the flash is on manual (likely even full power) and the camera is smart enough to know "you probably are going to have a bad exposure, fool! Check your settings!"

On manual flash and manual camera, the camera is thinking "I will simply let this fool do what he/she asks of me!"

:rolleyes: ;)

[Jim M]

Lotto, I always was lead to believe that the only thing that controls the flash exposure is the flash itself. I've never heard that the camera ever directly controls the flash exposure. I always thought that the flash read the camera settings, sent out a preflash (in ETTL) and determined the amount of power needed to expose properly based on how much light reflected back and given the current camera settings. Or in Manual mode it will just deliver the amount you have dialed in. I always thought that the camera itself totally ignored the flash in Tv or Av and ONLY determined exposure based on ambient light... and this is absolutely true when HSS is off. With HSS off it acts exactly how I expect it. So for what reason would HSS on change how the camera reacts when it should still only care about ambient light?
The camera interprets the preflash and the camera determines the amount of light that is needed. If I understand it correctly, in ETTL the flash doesn't do any thinking. It only responds to what the camera tells it to do. So, even though the camera and flash do what you expect them to do without the HSS, the camera is still the brains behind the operation.

[Jim M]

Oh, and how can HSS work with the flash set on manual? I don't think it can. If it can, then please explain how it works.

[PacAce]

Oh, and how can HSS work with the flash set on manual? I don't think it can. If it can, then please explain how it works.

There's no issue with the flash being in HSS mode as long as the camera is in manual mode, too.

[Lotto]

This is what I could understand, I could be wrong on some details..

In ETTL mode, the camera sensors read the preflash reflecting back from what they think is the main foreground subject, and determine what is the right amount of power for flash exposure on the subject. After the shutter opens and flash capacitors start to discharge, the camera will tell the flash shut off the discharge at that set amount. The flash control parts are pretty much the same in M, Av, and Tv modes, just that the ambient exposure adjustments are different in 3 camera modes.

In HSS mode, instead of shutting off single discharge, the camera let the flash fires a series of short and low power bursts. Each burst frequency is lot shorter than the 1/250. The camera control the HSS flash exposure by limit how many bursts or pulses each scene needs. (It might be able to control burst strength too, but I am not sure about that). So in HSS, not only the aperture affects the flash range, but shutter speed does too, like let say the pulse width is 1/10000, @ 1/8000, the camera could only record very few number of bursts, where @ 1/500 can record many times more.

Now if you put the flash into HSS and want to set the manual power too, that would mess up the way Canon wants to control the world, and they don't like that :)

OK, time for someone to correct my misunderstanding.

[PacAce]

This is what I could understand, I could be wrong on some details..

In ETTL mode, the camera sensors read the preflash reflecting back from what they think is the main foreground subject, and determine what is the right amount of power for flash exposure on the subject. After the shutter opens and flash capacitors start to discharge, the camera will tell the flash shut off the discharge at that set amount. The flash control parts are pretty much the same in M, Av, and Tv modes, just that the ambient exposure adjustments are different in 3 camera modes.

In HSS mode, instead of shutting off single discharge, the camera let the flash fires a series of short and low power bursts. Each burst frequency is lot shorter than the 1/250. The camera control the HSS flash exposure by limit how many bursts or pulses each scene needs. (It might be able to control burst strength too, but I am not sure about that). So in HSS, not only the aperture affects the flash range, but shutter speed does too, like let say the pulse width is 1/10000, @ 1/8000, the camera could only record very few number of bursts, where @ 1/500 can record many times more.

Now if you put the flash into HSS and want to set the manual power too, that would mess up the way Canon wants to control the world, and they don't like that :)

OK, time for someone to correct my misunderstanding.
Lotto, there is no issue with the flash being in HSS mode and in manual mode. That works just fine as long as the camera is also in manual mode. The problem occurs when the camera is set to one of the (semi) auto mode like Av or Tv or even P.

[Dermit]

Oh, and how can HSS work with the flash set on manual? I don't think it can. If it can, then please explain how it works.

It can, and does, do it all the time in camera manual mode. If I want a faster shutter speed than the max sync, but want to manually control the flash output power, why wouldn't it work? I realize there will be a power limit with HSS for the flash as it does multi bursts of light, but that does not explain why the camera does not even attempt a proper ambient exposure when in Tv or Av.

PacAce: There's no issue with the flash being in HSS mode as long as the camera is in manual mode, too.

Exactly. No prob if the camera is in manual as well. Which is why it confuses me as to why it can't figure things out in Av or Tv. I mean if I can dial in a proper setting in camera manual mode and lets say the shutter ends up being 1/500, faster than max flash sync., and this camera setting yields me a good exposure for ambient light and my flash, in manual and HSS mode, fires and exposes fine. Then why can't the camera figure out the proper aperture when I am in Tv mode or the proper Shutter speed when I am in Av mode for the ambient light? I still don't get it.

Again, this thread is more of a curiosity post than any 'must know, must get it' post for me because I would normally either shoot in full manual, or not use HSS and just control my shutter to be at or below max sync. This encounter was purely accidental so I figured I would post my experience here so if anybody encountered this in the future they wouldn't freak out and think their camera was not working. It's a Canon thing, not your specific camera.

[Jim M]

There's no issue with the flash being in HSS mode as long as the camera is in manual mode, too.
I guess that's what I get for never using Canon flashes in manual mode and HSS. So, the camera can figure out how intense to make each light burst to get consistent exposures with a given length of shutter speed and also have the entire charge dumped at the end or whatever portion of the charge you have set. That is very clever.

I just checked my 30D and HSS doesn't work in P, Tv, or Av. I had a flash of genius a moment ago about why this might be so, but I've forgotten it and I my lunch break is over. It's probably gone forever, but it was probably wrong anyhow.

[Wilt]

This is what I could understand, I could be wrong on some details..

In ETTL mode, the camera sensors read the preflash reflecting back from what they think is the main foreground subject, and determine what is the right amount of power for flash exposure on the subject. After the shutter opens and flash capacitors start to discharge, the camera will tell the flash shut off the discharge at that set amount. The flash control parts are pretty much the same in M, Av, and Tv modes, just that the ambient exposure adjustments are different in 3 camera modes..


It is unclear that the camera, in reading the pre-flash, does
A) tells the flash in advance how much light to output, or if it does
B) tells the flash at the time of exposure, when to quench its output as 'enough'.

I tend to think the way Canon ETTL does it is A, not B. B is how film TTL works, because it would read light from the surface of the film. But digital sensor is too reflective so it cannot read in real time from the surface.

[PacAce]

It is unclear that the camera, in reading the pre-flash, does
A) tells the flash in advance how much light to output, or if it does
B) tells the flash at the time of exposure, when to quench its output as 'enough'.

I tend to think the way Canon ETTL does it is A, not B. B is how film TTL works, because it would read light from the surface of the film. But digital sensor is too reflective so it cannot read in real time from the surface.

Wouldn't it be both? The camera needs to determine how much light is required ahead of time and when it actually fires the flash, it has to quench the flash at the appropriate time but this is the part I'm not sure about on how it accomplishes that (maybe through some sort of timer circuit?).

[Wilt]

Wouldn't it be both? The camera needs to determine how much light is required ahead of time and when it actually fires the flash, it has to quench the flash at the appropriate time but this is the part I'm not sure about on how it accomplishes that (maybe through some sort of timer circuit?).

But the sensor is too reflective for the in-body photosensor to read the light striking the focal plane at the actual time of exposure. I think it reads the pre-flash, then tells the camera to output a particular level of power via a code...for example it sends a hexadecimal number right after the preflash, like 'FF' to output full power, or a number like 'FA' to be a bit less power or '02' to be near minimum power level. This would be fully compatible with FEL to a level determined well before actual exposure time and permitting reframing before firing.

[PacAce]

But the sensor is too reflective for the in-body photosensor to read the light striking the focal plane at the actual time of exposure. I think it reads the pre-flash, then tells the camera to output a particular level of power via a code...for example it sends a hexadecimal number right after the preflash, like 'FF' to output full power, or a number like 'FA' to be a bit less power or '02' to be near minimum power level. This would be fully compatible with FEL to a level determined well before actual exposure time and permitting reframing before firing.
Yes, I agree with needing to know ahead of time how much power to shoot at. But the question is, how does the flash determine when that power has been reached? I suggested that a timer is possibly used for this purpose to quench the flash at the appropriate time.

[Wilt]

Yes, I agree with needing to know ahead of time how much power to shoot at. But the question is, how does the flash determine when that power has been reached? I suggested that a timer is possibly used for this purpose to quench the flash at the appropriate time.


The flash has self contained power levels settable thru the back panel for manual, so it would be easy enough to provide those and additional fractional levels (which we already know the flash can do) via digital command commanding it to output the fractional power...1/1, 1/2, 1/4 and 1/6 and 3/4 and 2/3 and 1/3 and...on and on.

[PacAce]

The flash has self contained power levels settable thru the back panel for manual, so it would be easy enough to provide those and additional fractional levels (which we already know the flash can do) via digital command commanding it to output the fractional power...1/1, 1/2, 1/4 and 1/6 and 3/4 and 2/3 and 1/3 and...on and on.
Yes, but I was looking deeper than that. I guess it was just the electronics engineer in me posting. The general ordered the hill to be taken (this is your A) and you're reporting back that it was. I, on the other hand, am reporting back that not only was the hill taken (A) but also on how I think the soldiers took that hill (B). :)

[Lotto]

Ah Ha! Now I know the brain behind that dark mask :)

Leo, do you know if Canon use separate flash sensor(s) or the use the same ones for the ambient exposure?

[PacAce]

Ah Ha! Now I know the brain behind that dark mask :)

Leo, do you know if Canon use separate flash sensor(s) or the use the same ones for the ambient exposure?
Lotto, for ETTL metering, there is no sensor built into the flash. (I'm sure you already know this but just wanted to mention it for the benefit of others who may not.) The sensors used for metering ETTL and ambient lighting are the same ones (for the most part, depending on the EOS model) and are built into the penta-prism housing of the camera. What's different is the actual algorithm used to the determine ambient exposure and flash exposure.

[Jim M]

I just remembered my point. It seems to me that the amount of light available for illuminating the slit that the shutter becomes at high speeds is a set amount, but light required factored with the amount of time it takes the slit to travel from one side to the other is variable. This means that the guide number would be a constantly moving target as shutter speeds change. We know that the shutter doesn't actually travel from one side to the other in 1/8000 second. It probably travels from side to side in more or less 1/250 second with the curtains close enough together that the amount of light over time is the equivalent of 1/8000 second. What that means is the amount of light the flash needs to put out must illuminate each segment of the slit with a full charge of light. The flash/camera could probably be designed to do it, but it would drive photographers nuts with a constantly changing guide number. The exposure at 1/2 power at 1/500 would not be the same as the exposure at 1/2 power at 1/8000 of a second. I'm going for another cup of coffee while you guys shoot this full of holes.

[Wilt]

Lotto, for ETTL metering, there is no sensor built into the flash. (I'm sure you already know this but just wanted to mention it for the benefit of others who may not.) The sensors used for metering ETTL and ambient lighting are the same ones (for the most part, depending on the EOS model) and are built into the penta-prism housing of the camera. What's different is the actual algorithm used to the determine ambient exposure and flash exposure.

Lon, the fact that the sensors are in the pentaprism is strong indication that the camera could NEVER send a message to the flash 'enough!' unless it were based solely on some 'clocking' circuitry. The problem is that the camera would also have to be able to recognize the fact that some flashes output more, or less, than others....430EX, 540EX, 580EX, Metz 54MZ, Metz 58AF, for example. If the pre-flash were a command 'output 1/256 of power', for example, it could then scale the observed result into 'output 1/2' or 'output 3/8' as needed for that flash unit, since it is blind during exposure.

[PacAce]

I just remembered my point. It seems to me that the amount of light available for illuminating the slit that the shutter becomes at high speeds is a set amount, but light required factored with the amount of time it takes the slit to travel from one side to the other is variable. This means that the guide number would be a constantly moving target as shutter speeds change. We know that the shutter doesn't actually travel from one side to the other in 1/8000 second. It probably travels from side to side in more or less 1/250 second with the curtains close enough together that the amount of light over time is the equivalent of 1/8000 second. What that means is the amount of light the flash needs to put out must illuminate each segment of the slit with a full charge of light. The flash/camera could probably be designed to do it, but it would drive photographers nuts with a constantly changing guide number. The exposure at 1/2 power at 1/500 would not be the same as the exposure at 1/2 power at 1/8000 of a second. I'm going for another cup of coffee while you guys shoot this full of holes.
You got the gist of it and that is exactly how it is. The GN is different for each shutter speed! :shock: The GN drops one stop for each stop increase in shutter speed! That's much the way ambient light behaves with change in shutter speed. So it's no surprise that the same applies for flash lighting in HSS mode because basically the flash is trying to emulate continuous ambient lighting in order to work at the higher shutter speeds above max sync speed.

[PacAce]

Lon, the fact that the sensors are in the pentaprism is strong indication that the camera could NEVER send a message to the flash 'enough!' unless it were based solely on some 'clocking' circuitry. The problem is that the camera would also have to be able to recognize the fact that some flashes output more, or less, than others....430EX, 540EX, 580EX, Metz 54MZ, Metz 58AF, for example. If the pre-flash were a command 'output 1/256 of power', for example, it could then scale the observed result into 'output 1/2' or 'output 3/8' as needed for that flash unit, since it is blind during exposure.

Bingo!!! Yes, the camera would never see the main flash and hence wouldn't be able to measure it in real time. And it doesn't have to. The camera is the general. It just gives the orders to the the soldiers (the flash) of the objective to be achieved. The flash itself takes care of carrying out the order and achieving the objective. :)

[Jim M]

You got the gist of it and that is exactly how it is. The GN is different for each shutter speed! :shock: The GN drops one stop for each stop increase in shutter speed! That's much the way ambient light behaves with change in shutter speed. So it's no surprise that the same applies for flash lighting in HSS mode because basically the flash is trying to emulate continuous ambient lighting in order to work at the higher shutter speeds above max sync speed.
So the conclusion I draw from this is that Canon probably figured if you were using one of the automatic modes already, you would only be confused by the apparent inconsistency of the flash. But if you were thinking hard enough to use manual on the camera, you could probably figure out the flash as well. Guess they didn't want thousands of people returning "broken" flash units.