Help me understand TTL flash...!

[landwomble]

Hi Folks
I've always used manual flash, which is fine for posed stuff, and works generally OK for me when I've been shooting live music gigs once I've got my settings dialled in.
Over the weekend I shot a birthday party which was pretty much like a wedding style setup. Lots of family members around, some shots in front of a balloon arch, some shot on dancefloor, etc and doing this with a manual flash did work, but was quite a bit of work given the vastly changing lighting.

I'm using YN 560 III manual flashes and I have a bunch of triggers for these so I was thinking of investing in the YN685 TTL flash for these kind of events, that way I have a system that's compatible for multiple light stuff but also have a new auto flash for times like this.

What I'm struggling to get my head around is how to use ETTL and balance ambient and flash exposure. What I'm wanting to achieve is the sort of glossy flash nightclub style effect where the background is subdued, the subject is brightly lit and do this with as little effort as possible during an event where the lighting changes a lot.

How do folks do this? I shoot with a Canon 6D if it makes a difference.

Do I manually set aperture/shutter/iso on camera to under expose by a couple of stops ambient light, then let the flash in ETTL mode do it's own thing to expose the image correctly?
If I leave the camera in AV, and just use FEC on the flash, how does the camera decide what the "right" mix of ambient and flash light is?
Can I leave it in auto, use exposure comp on the camera to dial down and underexpose the ambient and then expect the auto flash to make up the difference?

I'm sure there's a simple way of doing this but I think I'm missing something about how camera metering/flash metering/camera exposure comp and flash exposure comp interact!

Any pointers appreciated!

 

[Sharps]

I would like to listen to the answers too

 

[troutfisher]

I can't find the references now but basically when I was shooting indoors using a Canon 7d the advice was -Camera to manual , SS to sync speed , ISO and Aperture to suit, ignore the meter it will show under exposure , Flash to ETTL and shoot away.
From memory when in other than M the flash acts as fill light but in M it acts as the main light.

Do I manually set aperture/shutter/iso on camera to under expose by a couple of stops ambient light, then let the flash in ETTL mode do it's own thing to expose the image correctly?

In a word -Yes

 

[landwomble]

Fill is another question! If camera is in auto, its going to expose subject correctly, so how does the flash not over expose?

 

[mike weeks]

I would set the camera on manual and just experiment with flash exposure comp on ETTL

It’s the way I do it

Mike

 

[troutfisher]

Fill is another question! If camera is in auto, its going to expose subject correctly, so how does the flash not over expose?

That's what TTL is for BUT in auto if the camera decides the correct SS is 0.5 secs that's what you will get which is fine if you are into camera shake .

 

[Snapsh0t]

Fill is another question! If camera is in auto, its going to expose subject correctly, so how does the flash not over expose?

With many cameras and TTL flash the camera will use a very weak, often unnoticeable, pre-flash to determine the exposure. The camera then controls the flash, telling it when to start and stop during the actual exposure.

 

[Phil V]

Simple answer first:
When shooting with flash as your primary light source, always set your camera to manual. ETTL will take care of the flash exposure, and if it gets confused, there’s always FEC.

The key is to remember you’re balancing 2 separate exposures; the ambient and the flash.

So if I wanted a slightly underexposed background, I’d probably use a general aperture around 5.6 and then pick a hand holdable shutter speed and ISO combo to get the background I need, maybe 1/60 and 800. ETTL flash will just handle the flash exposure (but how you use it is another story)

Fill flash has a relationship to this; you can leave your camera to TTL meter the ambient, (which is largely measuring the ‘lit’ parts of your scene), then use ETTL flash to ‘fill’ the shadows on the unlit parts, ie ‘filling the shadows’. Doing this successfully means you’re not matching the flash to the ambient, just lifting the shadows a bit.

Once you have these recipes nailed, you can experiment with slow speed sync, 2nd curtain and HSS. But they all depend on understanding that you’re balancing your 2 light sources.