What determines the flash output on a speedlite?

What determines the flash output on a speedlite when using E-TTL?

For example, say it's very dark... and i've got my speedlite to go off remotely using E-TTL. Will the camera determine the flash output regardless of the flash's position to the subject? If i move that flash around, will the output be exactly the same?

Does the camera determine the flash output and send it to the flash, or does the flash determine the output on its own?


.... or do i not make sense because i'm a noob to off-camera flash?

E-TTL means that the camera determines the exposure. It talks to the flash and asks it for more or less light, depending on what the scene looks like and the position and distance of the flash relative to the scene. Basically, the camera is looking for a certain amount of light, it does not care where the flash is. The two sort it out between themselves (that's what the E-TTL pre-flash is for ).

E-TTL is known as a dedicated flash metering system. That means that the flash units are "dedicated" to compatible cameras and the camera tells the flash what to do.

With E-TTL, the camera measures light from the pre-flash reflecting back from the subject and the rest of the scene. Based on this measurement, it determines the amount of flash power needed and commands the master flash, which in turn commands the slaves in a wireless E-TTL setup.

When you move the slaves, or point them in a different direction, it not only changes the amount of light that bounces off the subject, but also changes the amount of light that hits the background and other things. So it would be impossible for any flash metering system to create an exposure that's exactly the same.

E-TTL involves complex algorithms that evaluate reflected light from the pre-flash in dozens of metering zones, attempt to identify the subject and expose it correctly. Sometimes it makes a good guess, sometimes not.

The EOS Flash Bible (link in my sig) would be a great resource if you want to learn more.

Curtis N wrote in post #6236154
E-TTL is known as a dedicated flash metering system. That means that the flash units are "dedicated" to compatible cameras and the camera tells the flash what to do.

With E-TTL, the camera measures light from the pre-flash reflecting back from the subject and the rest of the scene. Based on this measurement, it determines the amount of flash power needed and commands the master flash, which in turn commands the slaves in a wireless E-TTL setup.

When you move the slaves, or point them in a different direction, it not only changes the amount of light that bounces off the subject, but also changes the amount of light that hits the background and other things. So it would be impossible for any flash metering system to create an exposure that's exactly the same.

E-TTL involves complex algorithms that evaluate reflected light from the pre-flash in dozens of metering zones, attempt to identify the subject and expose it correctly. Sometimes it makes a good guess, sometimes not.

The EOS Flash Bible (link in my sig) would be a great resource if you want to learn more.

so is the flash output not relative to the aperture/iso/shutter speed? it just does its own thing?

and in a dark situation, how are u supposed to tell how your image is going to come out if you dont let the camera work on P for a few shots? won't the flash look different depending on what aperture/SS you choose?

The pre-flash is there to tell the camera what power to ask for. If the flash is far away, or having to bounce the light off a high ceiling, and the camera sees that it needs more light, it will ask for it! So you don't need several shots to determine exposure - one pre-flash will do it.

funk1196 wrote in post #6236052
What determines the flash output on a speedlite when using E-TTL?

Does the camera determine the flash output and send it to the flash, or does the flash determine the output on its own?

funk1196 wrote in post #6236229
so is the flash output not relative to the aperture/iso/shutter speed? it just does its own thing?

and in a dark situation, how are u supposed to tell how your image is going to come out if you dont let the camera work on P for a few shots? won't the flash look different depending on what aperture/SS you choose?

The camera, after talking with the flash(es), determines E-TTL exposure. It knows its own settings and those of the flashes and takes those settings into account. As long as the flash is on-camera, or the Infra-red communication link is working.

The camera instructs the flash (either the flash by itself or in Master Mode) what light level is required according to its calculations. The Master instructs any Slave flashes according to its settings, such as the Ratio facility.

The 580EX II flash is the only Canon flashlite at present that has a built-in exposure sensor - but forget that at this stage!

When you have E-TTL flashes set up, the camera makes the usual exposure evaluation, and then tells the flashes to fire a pre-flash (at low power) so that - the camera - can take another "with flash" exposure reading. It then compares the two in fine detail, and arrives at the required exposure after taking account of the Flash Exposure Compensation (FEC) setting. The flash is then told to fire, and at what lighting level. All this paragraph happens so fast we rarely notice the preflash - the camera's processor is very fast.

Trust the camera, more or less, and see what you get. Use FEC if needed; change settings, go full manual not only on the camera but on the flashes themselves.

Each shutter-press is evaluated from fresh data. But slight changes, say if you zoom in slightly, can produce a different evaluation and result in a different combined exposure.

By the way, shutter speed is always slower than the flash speed, so it has no effect on the flash lighting.

E-TTL communication cannot be performed using PC Synch cables, optical sensors or radio triggers where the only command is "Fire!".

CliveyBoy wrote in post #6236887
The camera, after talking with the flash(es), determines E-TTL exposure. It knows its own settings and those of the flashes and takes those settings into account. As long as the flash is on-camera, or the Infra-red communication link is working.

The camera instructs the flash (either the flash by itself or in Master Mode) what light level is required according to its calculations. The Master instructs any Slave flashes according to its settings, such as the Ratio facility.

The 580EX II flash is the only Canon flashlite at present that has a built-in exposure sensor - but forget that at this stage!

When you have E-TTL flashes set up, the camera makes the usual exposure evaluation, and then tells the flashes to fire a pre-flash (at low power) so that - the camera - can take another "with flash" exposure reading. It then compares the two in fine detail, and arrives at the required exposure after taking account of the Flash Exposure Compensation (FEC) setting. The flash is then told to fire, and at what lighting level. All this paragraph happens so fast we rarely notice the preflash - the camera's processor is very fast.

Trust the camera, more or less, and see what you get. Use FEC if needed; change settings, go full manual not only on the camera but on the flashes themselves.

Each shutter-press is evaluated from fresh data. But slight changes, say if you zoom in slightly, can produce a different evaluation and result in a different combined exposure.

By the way, shutter speed is always slower than the flash speed, so it has no effect on the flash lighting.

Okay so a few questions:

1. Does that mean that there is no preflash with anything that isn't ETTL?

2. You make it sound like there is more than 1 preflash, is that true? Sounds like there's two pre-flashes... do both preflashes go off no matter what the conditions are?

3. SS has NO effect on the how the flash is produced? whether its 1/4000 or 30 seconds, the flash will expose the same?

One thing to take into consideration is that if there is something in the foreground your camera/flash may adjust the exposure for that object, rather than you intended subject.

funk1196 wrote in post #6237570
Okay so a few questions:

1. Does that mean that there is no preflash with anything that isn't ETTL?

2. You make it sound like there is more than 1 preflash, is that true? Sounds like there's two pre-flashes... do both preflashes go off no matter what the conditions are?

3. SS has NO effect on the how the flash is produced? whether its 1/4000 or 30 seconds, the flash will expose the same?

1. Correct

2. Only 1 pre flash as far as I'm aware

3. Shutter speed upto max sync of your camera (1/200 I think on XSi) will have no effect on the flash output. Above that and the flash works differently (that's FP flash/high speed sync), but that's a whole different story. As long as the flash has enough power the subject SHOULD be correctly exposed regardless of shutter speed. Except of course when the shutter is so slow that the ambient will cause over-exposure.

funk1196 wrote in post #6237570
1. Does that mean that there is no preflash with anything that isn't ETTL?

2. You make it sound like there is more than 1 preflash, is that true? Sounds like there's two pre-flashes... do both preflashes go off no matter what the conditions are?

3. SS has NO effect on the how the flash is produced? whether its 1/4000 or 30 seconds, the flash will expose the same?

1. Preflash is required only under E-TTL.

2. There is only one preflash, and that is when the shutter is fully pressed. It occurs after the ambient-light exposure reading has been taken. (A focussing beam may be emitted beforehand, but that is separate.)

3. Correct.

What no one has mentioned yet is that after running thru all those complex calculations factoring in the reflected light from the subject, the ambient light, the shutter speed, the f-stop, and the distance to the subject, the Canon algorithm comes up with a correct amount of light for a proper exposure, transmits it if necessary to any slave flash units, and then over or underexposes the image based on some hidden, inner set of code that Canon refuses to share with anyone and is by all accounts amended by the day of the week and the latitude/longitude of the camera.

Not sure if anyone hit on this point, but suppose the E-TTL metering determines that the flash (assume just one flash on the hotshoe) needs to fire at 1/8 power for "proper" exposure with the ISO set to 100 and the f-stop set to f/4. If you were to up the ISO to 200, the flash only needs to fire at 1/16 for the same "exposure". Likewise, if you keep the ISO at 100, and open up to f/2.8, you'll again only need 1/16 power. In the other direction, ISO100, f/5.6 would need 1/4 power. Basically flash exposure is a combination of flash power (literally the duration of the flash burst), ISO, and f-stop.