Canon Speedlite Questions

[Scotty72]

The flash system has neither the ability not the desire to measure distance, Scotty. It doesn't care how far away your subject is, only how bright it is.

The usual arrangement is that the flash system fires a very short pre-flash, measures the light coming back through the lens (this it the E-TTL part!), and from that, calculates how strong to make the main flash, which follows as soon as the flash system has finished doing its sums, i.e., a few milliseconds afterwards. The pre-flash can have unintended consequences in some cases - for example, when I'm working with small birds which get spooked by the pre-flash and take off before the main flash fires. (They have amazing reflexes!)

Ok.. Thanks.

I think I'm almost there.

I do get the pre-flash (the E in E-TTL). That tells the main flash 'how hard to go'.

Q1: During the exposure: is the sensor still telling the camera when to stop (like normal TTL)? (I would think yes)

and

Q2: and, if I have an off camera flash too. Is this off camera flash also told how hard to go or does it just fire off a standard burst until told when to stop?

[mikec]

Q1: E-TTL system uses a different sensor in the camera, not the main imaging sensor I believe. I think the sensor for E-TTL is located near the view finder somewhere as it's getting light reflected off the mirror.

Q2: No, unless you are using something that allows E-TTL transmission like the new PW system. If using E-TTL the camera needs to tell the flash what the flash burst needs to be. If you are using dumb triggers (non E-TTL) you need to use manual mode on the flash.

Pretty sure the link posted above explains E-TTL too, it's just a little further down the page.

[agb]

Q1: E-TTL system uses a different sensor in the camera, not the main imaging sensor I believe. I think the sensor for E-TTL is located near the view finder somewhere as it's getting light reflected off the mirror.

Q2: No, unless you are using something that allows E-TTL transmission like the new PW system. If using E-TTL the camera needs to tell the flash what the flash burst needs to be. If you are using dumb triggers (non E-TTL) you need to use manual mode on the flash.

Pretty sure the link posted above explains E-TTL too, it's just a little further down the page.

From the link I posted.

The reflected light from this preflash is analyzed by the same evaluative metering system that the camera uses for metering ambient light levels. The appropriate power output (ie: flash duration) of the flash is determined and stored in memory. The entire sensor area is evaluated and compared to the ambient metering, and the area around the active focus point is emphasized. If you are in manual focus mode then either the central focus point or averaged metering is used.

[Scotty72]

Sorry, maybe I wasn't clear enough...

If I have an E-TTL master (ex 580II) and a 2nd E-TTL flash (another ex 580II) off camera. Will the camera tell the 2nd flash how strong it needs to fire and if yes, how does the camera know how far the 2nd flash is from the subject and therefore compensate?

And. TTL senses how much light gets into the sensor then shut off the flash once the exposure is deemed correct. In E-TTL, after all the pre-flash calculations, will it still use a similar process to TTL to shut off the flashes (both of them) if (for whatever reason) that second flash pours in too much light. Or once the pre-calculations are done, that's it?

I hope my question is better phrased this time

Scotty

Scotty

[mikec]

Yep if you are using something that can transmit the ETTL then it will send the info to other flash. But it won't know how to adjust it suit your desired lighting, you'll need to use ratios.

All the ETTL calcs are completed before the shutter opens, no more are done once the shutter us exposed to my knowledge.

[Tannin]

Maybe I wasn't clear enough!

how does the camera know how far the 2nd flash is from the subject and therefore compensate?

It doesn't know. And it doesn't care.

It just fires a tiny pre-flash, measures the light coming back into the lens, and adjusts the power of the flash accordingly. It doesn't matter if there is one slave flash or 100 of them, if it is one metre from the subject or one kilometre - it just measures the exposure from the pre-flash and adjusts.

kilometer

[Scotty72]

Maybe I wasn't clear enough!



It doesn't know. And it doesn't care.

It just fires a tiny pre-flash, measures the light coming back into the lens, and adjusts the power of the flash accordingly. It doesn't matter if there is one slave flash or 100 of them, if it is one metre from the subject or one kilometre - it just measures the exposure from the pre-flash and adjusts.

kilometer

Ok, I think I get it.

Now, in the spirit of, 'It's only a dumb question if you don't ask it.'

What if you have a subject that is well lit on one side but dark on the other (eg a face half in shadow)? And you have two flashes off camera - one on either side.

Will the E-TTL be able to tell one flash to be stronger than the other to even out the face? Or do I have to dial up the flash compensation by a stop or two?

I ask because this is where I have had spectacular failures. I've given up and resorted to trial and error using manual

Scotty

[mikec]

I don't believe so because the flashes and camera do not know where they are relative to the subject and each other. Also lighting ratios are subjective. Just like camera settings, there is no "correct" exposure, same goes for lighting. So the flash / camera can't determine what is "correct"

You'll need to use the ratio function to get it correct. Since the same ETTL settings would be sent to both flashes to control the individual output you need to the ratio function.

[Tannin]

In one word, no.

In more words, you will get roughly the same result as you'd get if those flashes were spotlights or windows letting in daylight and you were using aperture priority or full auto - the exposure system (the flash system rather than the camera's exposure system, but they work much the same way) does its best to get a good overall exposure with an average figure.

(There are various different ways it can calculate that average - the camera uses centre-weighted, matrix/evaluative, plain all-over average, highlight tone priority, and so on - but they are all the same thing in broad, they differ only in the details. The Canon flash system has fewer variations on offer, and they probably work in ways slightly different from the in-camera equivalent, but the overall thrust is the same - try to average out the exposure with a medium, pleases-everyone number.)

[Jules]

Personally, I have never liked the results from leaving my strobes on ETTL, despite doing a lot of experimenting with various ratio and FEC settings (flash exposure compensation). I find it far simpler to leave the speedlights on Manual and adjust the power output myself after a couple of test shots. Of course, I work in fairly controlled conditions, so have the luxury of being able to employ this approach.

I'm just saying this because I nearly did my head in trying to understand ETTL. Once I switched to Manual, OCF became much easier to understand and implement.

[Roosta]

I don't know if its any help Roosta but i started off using the 580exii on e-ttl and was totally bamboozled. And in one instance i couldn't get enough light into the image - thought i had a dud flash. I've since gone over to using everything in manual mode cos that way i can understand what's happening. as i change one variable - ie power on the flash, aperture etc, i can see exactly what's happening in the image. I do plan that when i've got a better handle on flash photography i'll give e-ttl another go. I gather there are situations that its very good for.

Thanks JudyD, that,s what I had in mind, get use to the thing first, like using the camera in scary manual for the first week or so, that's the way to learn.

Cheers.

[Roosta]

Personally, I have never liked the results from leaving my strobes on ETTL, despite doing a lot of experimenting with various ratio and FEC settings (flash exposure compensation). I find it far simpler to leave the speedlights on Manual and adjust the power output myself after a couple of test shots. Of course, I work in fairly controlled conditions, so have the luxury of being able to employ this approach.

I'm just saying this because I nearly did my head in trying to understand ETTL. Once I switched to Manual, OCF became much easier to understand and implement.

Cheers to all above, very interesting conversation, I'm with Jules to the end so far, the shooting I'm going to be using the flash for is for backlighting a subject in low light situations, go figure, but with the manual setting and say a tree, that has plenty of time on it's hands, well for me, I think manual adjustment will be the way to go. (as in my mirrored tree series I posted in Land/sea)

Thanks you all for so great ideas/knowledege sharing. Now to get the 580 ex II.

[Michaela]

This has been very helpful and informative - thanks for asking the question, Roosta!

[Scotty72]

Thanks guys.

I think I am down with it now

Was good to clear up a few mis-conceptions I had.

[strictfunctor]

No-one's mentioned the STE2, a commander unit that's cheaper and less clumsy than putting 580 on top of the camera. Not needed any more with the 7D and 60D, of course. It makes it easy to control the ratio between 2 groups if you're using E-TTL.

+1 for using manual if you have a controlled situation and time to futz about, but otherwise E-TTL is what you want, and off-brand flashes aren't going to do that as well, if at all.

[andylo]

Hmm... Canon ST-E2 is expensive and it's crap!! It has a very limited range and use hard to find battery to power.

Get the Chinese made YN branded ST-E2! Double the range, use 2 x 2AA battery to power up! More importantly, 1/2 the price I got one and it's awwwwesome!

[GTP 290]

Maybe I wasn't clear enough!



It doesn't know. And it doesn't care.

It just fires a tiny pre-flash, measures the light coming back into the lens, and adjusts the power of the flash accordingly. It doesn't matter if there is one slave flash or 100 of them, if it is one metre from the subject or one kilometre - it just measures the exposure from the pre-flash and adjusts.

kilometer

I have a 50D with a 430 EX II and have never had a pre-flash come off my flash unit. I do however hear a small motor operation when a half push my shutter button down I also notice 3 red vertical bars one the subject before the motor sounds comes from the unit. I use it in E-TTL for most of my nightclub work and every images comes out as desired (well lit).

[Scotty72]

I have a 50D with a 430 EX II and have never had a pre-flash come off my flash unit. I do however hear a small motor operation when a half push my shutter button down I also notice 3 red vertical bars one the subject before the motor sounds comes from the unit. I use it in E-TTL for most of my nightclub work and every images comes out as desired (well lit).

The pre flash lasts a few milliseconds so, unless you have particularly sharp eyes, you wont notice a thing.

[GTP 290]

The pre flash lasts a few milliseconds so, unless you have particularly sharp eyes, you wont notice a thing.

I've just gone outside and tested this and it can't be correct. I took 2 photos, the first photo I covered the flash with my hand while half pushing the button down and letting the camera focus (effectively blocking out any possible pre-flash). I heard the motor inside the flash unit makes it's little noise. I then remove my hand and pushed the button the rest of the way down. The second photo I just took the photo normally and then compared the two images. Both had almost identicle tone curves.

From my understanding the flash communicates with the camera using the apature, shutter speed, ISO and current light meter reading to adjust the power of the flash unit to suit the condition (almost as if the flash can see Through The Lens and take the information it needs for the shot)

[Scotty72]

Didn't we just go through all this earlier in this thread?

There is no way that, after pressing the shutter release you could move your hand so quickly.

The half press flashes are not e-TTL flashes. They may be the focus assist flashes.