How does eTTL work?

[arith]

How does eTTL work? Does it take exposure or work on distance and aperture? If I put an ND filter on will it adjust the length of flash.

It is a long time since I've used a flash, back in the days of Guide Numbers and I used a suck it and see method for bounce.

I haven't got a flash yet but if I set up a flash on a tripod say in a triangle 60 degrees from the line of view of the lens and a white card the other side of the object in line with the flash; (artificial scenario) how do I work out exposure?

Maybe I made that a bit complicated but stopped at adding a ND filter and strong backlight; all I want to know is how it works.

[Colin Southern]

Hi Arith,

In short ...

E-TTL examines the ambient light levels in (typically) 35 zones (63 for 1D series cameras) then emits a short "pre-flash" and examines how much the levels in those zones change, and from that work out what the foreground is and how much light to throw at it when the main flash fires. Which means that you don't need to compensate for using filters, although having said that, if you DO use the likes of an ND fiter it wouldn't be too hard to get to the point where even a full dump wouldn't be enough to properly illuminate the scene.

E-TTL doesn't take distance into account, but E-TTL II does, although it doesn't appear to bias the result very much (and of course for that part to work you need to have a lens with a buit in focusing distance encoder, and a camera-mounted flast pointing straight ahead, not bounced).

N K Guy wrote an online "book" if you're interested to read about all of this and more (it's generally regarded as the "bible" of EOS flash photography).

Hope this helps

[arith]

Cheers; I was thinking of getting a Nissin Di622 for taking Christmas shots by a Christmas tree, it could even be used for fill in. D

[Colin Southern]

No worries

My suggestion would be to stick with Canon flashes though; I have serious doubts about 3rd party units being 100% compatable.

[arith]

Your link even answers my later question about speed of X-sinc; 1/125 fastest.

[Colin Southern]

Your link even answers my later question about speed of X-sinc; 1/125 fastest.

Aww - not fair ...

... we could have answered that for you too!

[arith]

No worries

My suggestion would be to stick with Canon flashes though; I have serious doubts about 3rd party units being 100% compatable.

Got it; it's a doddle. Just a quick test and I noticed I had to shoot the flash to see it working in slave mode.

Now I think there is a way to set the camera body flash so that it only emits a preflash.

Do that later.

[Colin Southern]

Hi Arith,

I'm not quite sure what you're meaning I'm afraid

One "gotcha" that confuses people though ... If you set an on-camera flash to master mode - but tell it not to fire - it still fires the pre-flash (which may look like it's firing normally). This is because the master programs the slaves via the preflash.

[arith]

Hi Arith,

I'm not quite sure what you're meaning I'm afraid

One "gotcha" that confuses people though ... If you set an on-camera flash to master mode - but tell it not to fire - it still fires the pre-flash (which may look like it's firing normally). This is because the master programs the slaves via the preflash.

You are right; but I wanted the pre flash without the main flash; doesn't work but I have to set to external only to stop AF assist, because this fires the slave.

Worth a try though.

Another thing is I'm stuck on 1/200 fastest; I thought it should be able to go faster than that but I can get round it with ND filters.

Edit: Set to not fire at all fires the pre flash? I will try that.

[Colin Southern]

You are right; but I wanted the pre flash without the main flash; doesn't work but I have to set to external only to stop AF assist, because this fires the slave.

If you want a pre-flash with out the main flash, set the on-camera unit as master controller, but tell it not to fire. AF assist doesn't have anything to do with firing slaves (slave control is done by the main flash head) (ie "white light").

Another thing is I'm stuck on 1/200 fastest; I thought it should be able to go faster than that but I can get round it with ND filters.

Press the HSS (High Speed Sync) button on the flash (2nd from right if your using a 580 series).

[arith]

If you want a pre-flash with out the main flash, set the on-camera unit as master controller, but tell it not to fire. AF assist doesn't have anything to do with firing slaves (slave control is done by the main flash head) (ie "white light").



Press the HSS (High Speed Sync) button on the flash (2nd from right if your using a 580 series).

cheers;

but I have only got C.Fn-05 AF assist/Flash

select:

0 Emits/Fires
1 Does not emit/Fires
2 Only ext.Flash emits/Fires
3 Emits/Does not Fire

and I chose 2 because the AF assist sets off the slave.

Unfortunately I can't afford a 580 series so I've gone for the Nissin which is about the same; does modelling ect but does not have a high speed synch button.

But I can get round that; it is not as if I'm going to be using it all that much because I like to do landscape.

cheers I put a couple of examples in another thread because thought the questions were unrelated. More to do with the flash than eTTL but I could be wronge; I usually am.

[Colin Southern]

I've gone for the Nissin which is about the same; does modelling ect but does not have a high speed synch button.

Oh dear

[arith]

Oh dear

Well I'm quite pleased with it; but if I win the lottery which is unlikely since I don't do it, but still could win, then I'm going to get a 5D with a 24mm TS lens and all the panoramic gizmo's it is possible to get.

Until then I will dodge and burn with compromise.

Thanks Colin; you are a great help and I really appreciate it.