e-TTL wireless flash bounce

joescrivens · 31 Jul 2010

I was wondering if someone can help.

I have a 7d and a 430 ex II, i've used the wireless flash mode quite a lot but alys in manual. I always bounce the flash off a ceiling.

I want to use e-TTL flash but I dont understand how it works, if I am bouncing the flash how does the camera know thats what I am doing? Is there a way to make the flash shoot and thats how the camera knows its being bounced and adjust accordingly?

help

tdodd · 31 Jul 2010

ETTL - Evaluative Through The Lens. When you press the shutter button the camera will measure ambient, then fire a pre-flash and measure how the light from the flash is returned. It will then calculate how much power is required from the flash to illuminate the subject. In theory bouncing the flash should not matter, and if you bounce, neither should using a modifier such as a Stofen. The camera should sort it out - it knows whether the flash is in a bounce position or not.

However, automatic flash exposure calculations are as prone to being fooled by tricky subjects/scenes as regular ambient metering systems. Point the camera at a bride in white and the camera will probably underexpose to give a grey(ish) dress. Do the same for a groom in a black tux and it will probably overexpose to give a light black suit. With experience (something I lack) you will be able to make an assessment about the scene and make a pretty fair estimate of whether you need some FEC and how much.

While we're on the subject of flash exposures, if you do use the flash in a direct position, rather than bounced, the camera will take into consideration the focused distance as well as the light reflected and read back through the lens. That works well for a bare flash, but if you fit a modifier the camera/flash is unaware and may not give sufficient flash power. As soon as you raise the flash into a bounce configuration that distance information is ignored and everything is metered through the lens only. But don't be afraid to use FEC. It is there to be used.

EDIT : Oops - I missed the bit that you were using wireless flash triggering. To be honest it shouldn't matter. In that case the camera will make no assumptions about distance between flash and subject. It will rely solely on Through The Lens metering of the light from the flash that bounces off the subject and back to the camera. It will not care what route the flash light takes, only how brightly it reflects back to the camera.

J4MIE_P · 31 Jul 2010

tdodd said:
ETTL - Electronic Through The Lens. When you press the shutter ......................................................................................................................................It will not care what route the flash light takes, only how brightly it reflects back to the camera.

Great post (Don't want to quote it all as it'll spam the page :lol:

But great advice there and exactly what you needed to know

joescrivens · 31 Jul 2010

Ok thanks Tim.

I use back button focus. When I press it the flash does nothing, it doesn't send a pre flash. It's only when the shutter is down that the flash fires and the shot is taken.

I was expecting a pulse or something is it working right?

J4MIE_P · 31 Jul 2010

joescrivens said:
Ok thanks Tim.

I use back button focus. When I press it the flash does nothing, it doesn't send a pre flash. It's only when the shutter is down that the flash fires and the shot is taken.

I was expecting a pulse or something is it working right?

That's exactly what you will experience

If you are using One Shot focussing then when you focus with the back button it will emit it's AF assist lamp to aid focussing.
Then when you half press the shutter it will measure the ambient metering/ exposure.
Then when you fully press it will fire the pre-flash instantaniously followed by the flash for the subject.

Hope that helps

tdodd · 31 Jul 2010

The flash exposure calculation all happens in a split second as part of the exposure - you have to take a picture.

I'm guessing here, but with off camera flash there is no point in the Flash Exposure Lock function, but if you were using a single on camera flash then you would press the "*" button to fire the pre-flash in advance, recompose and then take the shot.

BTW, the pre-flash is the reason why you can sometimes get blinkers (I'm talking about human beings blinking) when using the flash. If there was no pre-flash then no human would have the reaction times required to blink when a flash went off. Also, with wireless flash control, light pulses are emitted as a means of communication between the camera and the slave flash units. So these things can certainly lose you some shots through blinking. My girlfriend is notorious for it. It's like she's psychic.

Manual flash operation with RF or IR flash triggers should mean no blinking, but with visible light used for comms, and the exposure preflash when using ETTL, be warned of the potential for lost shots.

joescrivens · 31 Jul 2010

ok, so this is the sequence of events:

Im in Av mode, I back focus which sets the exposure - I assume it knows I am using flash here when it sets this exposure?

I then press the shutter fully, the popup flash sends a signal which tells the flashgun, sned a preflash, this is read by the lens and calculated what the strength of the flash should be then the popup flash sends a second pulse to make the flash gun do it's flash?

does it really do it that quick because all I see is one flash in a split second

J4MIE_P · 31 Jul 2010

joescrivens said:
ok, so this is the sequence of events:

Im in Av mode, I back focus which sets the exposure - I assume it knows I am using flash here when it sets this exposure?

I then press the shutter fully, the popup flash sends a signal which tells the flashgun, sned a preflash, this is read by the lens and calculated what the strength of the flash should be then the popup flash sends a second pulse to make the flash gun do it's flash?

does it really do it that quick because all I see is one flash in a split second

Back focus ONLY focusses and DOES NOT lock the exposure
By HALF pressing the shutter you will set the exposure and the FULL press of the shutter does all the flash business

joescrivens · 31 Jul 2010

J4MIE_P said:
Back focus ONLY focusses and DOES NOT lock the exposure
By HALF pressing the shutter you will set the exposure and the FULL press of the shutter does all the flash business

hmmm I have back focus set to fcus and exposure though, half shutter does nothing on my camera

tdodd · 31 Jul 2010

You might find some useful information amongst this little lot - http://photonotes.org/articles/eos-flash/

tdodd · 31 Jul 2010

I haven't tried this, but I think this is a method by which you can see the pre-flash, even without using wireless flash....

Set 2nd curtain sync and a manual or Tv exposure of a couple of seconds. Take a flash photo. You will see the pre-flash emit, then a couple of seconds will elapse and then the 2nd curtain flash will fire. If you then do this with off camera wireless flash control via the popup then maybe (I doubt it) just maybe you will see a bit more of a burst of light at the beginning of the exposure. In a more typical flash exposure, with first curtain sync you just can't see all this happening.

joescrivens · 31 Jul 2010

tdodd said:
I haven't tried this, but I think this is a method by which you can see the pre-flash, even without using wireless flash....

Set 2nd curtain sync and a manual or Tv exposure of a couple of seconds. Take a flash photo. You will see the pre-flash emit, then a couple of seconds will elapse and then the 2nd curtain flash will fire. If you then do this with off camera wireless flash control via the popup then maybe (I doubt it) just maybe you will see a bit more of a burst of light at the beginning of the exposure. In a more typical flash exposure, with first curtain sync you just can't see all this happening.

i see, thanks tim i'll try that! just for my own curiosity, if i am using 1t curtain ettl am i ok in av mode, or should i be in manual with shutter between 160-250?

J4MIE_P · 31 Jul 2010

joescrivens said:
hmmm I have back focus set to fcus and exposure though, half shutter does nothing on my camera

Ahh my appologies then your first analogy was correct

I use back button solely for focussing and the half button for exposure lock

tdodd · 31 Jul 2010

J4MIE_P said:
Ahh my appologies then your first analogy was correct
I use back button solely for focussing and the half button for exposure lock

Either way, that only covers ambient light metering. There are only two ways I know of to get the flash metering to operate - either take a photo or use Flash Exposure Lock. I'm not sure that FEL is applicable when shooting with wireless off camera flash. I guess the easiest way to find out is to try it. According to the manual (page 114) the M.Fn button is used to fire the FEL pre-flash.

HoppyUK · 1 Aug 2010

joescrivens said:
ok, so this is the sequence of events:

Im in Av mode, I back focus which sets the exposure - I assume it knows I am using flash here when it sets this exposure?

I then press the shutter fully, the popup flash sends a signal which tells the flashgun, sned a preflash, this is read by the lens and calculated what the strength of the flash should be then the popup flash sends a second pulse to make the flash gun do it's flash?

does it really do it that quick because all I see is one flash in a split second

Yes, it does! The flash pulses are so close together they appear as one.

tdodd said:
You might find some useful information amongst this little lot - http://photonotes.org/articles/eos-flash/

There's a lot of good stuff in that link. The same guy, NK Guy, has just produced an excellent book on Canon flash. Really very good http://www.amazon.co.uk/Mastering-Canon-EOS-Flash-Photography/dp/193395244X

tdodd said:
I haven't tried this, but I think this is a method by which you can see the pre-flash, even without using wireless flash....

Set 2nd curtain sync and a manual or Tv exposure of a couple of seconds. Take a flash photo. You will see the pre-flash emit, then a couple of seconds will elapse and then the 2nd curtain flash will fire. If you then do this with off camera wireless flash control via the popup then maybe (I doubt it) just maybe you will see a bit more of a burst of light at the beginning of the exposure. In a more typical flash exposure, with first curtain sync you just can't see all this happening.

You can sometimes see the pre-flash through the viewfinder as you shoot. Since it goes out before the mirror rises, if you see a flash while looking through the viewfinder, that can only be the pre-flash element of the firing sequence.

tdodd said:
Either way, that only covers ambient light metering. There are only two ways I know of to get the flash metering to operate - either take a photo or use Flash Exposure Lock. I'm not sure that FEL is applicable when shooting with wireless off camera flash. I guess the easiest way to find out is to try it. According to the manual (page 114) the M.Fn button is used to fire the FEL pre-flash.

Flash Exposure Lock is equally applicable to off-camera flash though in practise you don't use it because you've probably fired off a few test shots and adjusted for correct exposure that way, with +/- compensation.

Basically FEL is to cover you for focus-recompose technique which can fool auto flash metering because flash is so sensitive to distance.