Flash output power question

Please foregive my ignorance, but I am new to flash photography and there are a bunch of things I don't understand even after reading manuals, forum, etc.

I have a Rebel XT and a 420ex.

I understand the BASIC concept of ttl and all that stuff: The camera measures the light through the lens (ttl) and can even fire pre-flash to determine exactly the required flash output and correct exposure (I also understand that the flash tracks the zoom...).

I start by posting a simple question:

If I shoot in 'M', what determines the flash output (i.e. how powerful it flashes)? Assume e.g. I bounce the flash off the cieling, or that I stuff the flash in a small softbox or whatever. How does the flash know how much output to produce? Does it just revert to som standard power when I leave the safe realm of the basic zones?

Go easy on me will, ya?

Thanks

That's the beauty of TTL metering, in its various forms.

It fires a preflash and measures its own light output reflecting off the subject and coming back through the lens. Since it's zoomed, diffused, filtered, bounced or whatever when it fires the preflash, it can correctly determine the power required for the "real" flash.

It's a beautiful thing. Just keep in mind that the useful distance is MUCH shorter when you bounce the flash, and may require a wider aperture or higher ISO setting for proper exposure.

Thanks Curtis!

Does it work the same in both Basic and Creative zones?

Assume for example I work with the flash diffused through some on-flash softbox or something, and I am shooting in 'M' at ISO 400 at f/4 1/60s. Assume this looks OK. Then I change to f/8. What will happen? Does the system try to power up the flash to compensate for this?

Thanks,

It works the same any time you're using E-TTL II (which is all you can use with the 420 on the 350D). It sets the flash output to be "right" for the aperture you (or the camera's exposure meter) set for the available light. If you were shooting at f/4 and decide to stop down to f/8, it'll have to pump up the flash 2 stops more (if it's got that much left).

Wow, thank you so much! Can anyone remember what we did before there were Internet forums?

Two simple follow-ups:

1. If I understand you correctly, the E-TTL as you describe it will work even if I bounce the flash in an umbrella. Will it?

2. How does then FEC work in 'M'? As I understand it, FEC is exactly what it sounds like, i.e. flash EXPOSURE compensation. That would mean that it adjusts the exposure (i.e. time and/or aperutre) to get a brighter or darker image. By this definition it should not mess with the flash power output. Now, assume i shoot in 'M' at f/4 and 1/60s and this seems to get the correct exposure (the E-TTL works). Now, I set FEC at +2. What happens? It can't really change my exposure, because in 'M' I have control over that. So what does it do? Power up the flash 2 stops? If that is what it does, i believe the term is wrong (should then be FOP for "flash output compensation").

Can you guys keep on setting me straight, please

Thanks so much,

Yes. But the flash needs to be connected to the hotshoe with a Canon Offshoe Cord II or similar product that makes all the connections.

Think of every flash exposure as two exposures in one. There is the ambient light exposure, which you or the camera adjust via aperture, shutter speed, and ISO. Then there is the flash exposure, which the E-TTL II system adjusts via flash output, calculating the affects of the aperture and ISO settings already determined.

FEC will vary the amount of flash output to attain whatever exposure you dial in, regardless of other camera settings.

So.. the FEC is really... FOC then???

Thanks,

Exposure is controlled by 4 things. The aperture, the shutter, the ISO, and the light. In normal conditions when you're not using flash, you can't change the light. So you change aperture or shutter speed (or, in some cameras, or modes, ISO). With a flash, you can change the light, without messing with other settings the photographer presumably had very good reasons for selecting. So you vary the flash output. It's still affecting exposure. Just a different wheel to ratchet up.

Bear in mind that the flash is reading an "average" exposure. Try this: take a pic of a person in front of a white wall & a dark wall at the same distance & settings. You'll find that you have to compensate.

It's called Flash Exposure Compensation (FEC), because it adjusts the exposure by changing the flash output.

As opposed to the regular Exposure Compensation (EC), which adjusts exposure by changing the aperture, shutter speed, or both (depending on camera mode).

It's important to understand the difference when using flash, because with E-TTL II or any other type of auto flash metering, EC won't do you any good if flash is the primary light source.