Quick question regarding ETTL and flash

Guitarman0 · 28 Aug 2012 at 21:55

Hi,

I have a quick question regarding ETTL using a flash (both on camera and off camera using the build in wireless trigger on the 60D). To determine the correct flash exposure does the camera evaluate the scene and decide how much flash light is required based on the ambient light for a correct exposure or is there a pre-flash before the proper exposure to determine how much light will be needed from the flash?

The reason I ask is to try and determine whether the camera will factor in the fact that I am bouncing the flash when using ETTL or whether the camera is assuming my flash is pointed directly at the subject.

Sorry if that doesn't make sense. If it doesn't I will try and reword it

Thanks!

James J · 28 Aug 2012 at 23:07

As far as I'm aware, the ETTL doesn't factor in how far the flash is away from the subject or which way it is facing, instead it evaluates how much ambient light is available and how much flash power is needed to fill the scene adequately. The flash exposure feature comes into its own when you are using off camera flash for creative photography though as the same still applies, but you need to dial in extra power if the flash(s) are further from the subject or if you are bouncing flash from a high ceiling.

Generally, I use +1 EV if the flash is twice as far as my camera away from the subject or -1 EV if its half the distance and go from there.

Beepcake · 29 Aug 2012 at 08:24

It will take it into account - the camera will tell the flashes to emit a preflash, determines how much flash power is required for a "correct" exposure, then tells the flash(es) to fire at the appropriate power. Sounds complicated, but the flash light travels pretty fast :-)

If you put the flash on camera and set it to a long shutter speed and rear curtain sync you should see the distinct pre-flash and actual flash happening. You can also use FEL to "pre-calculate" the flash power, although this also (on the 60D at least) exposes "correctly" only the centre part of the image so you may need to recompose after locking the flash exposure.

How it determines the "correctness" of the exposure and whether it's neither use nor ornament in a lot of situations is a different kettle of fish

Devrij · 29 Aug 2012 at 08:44

Beepcake said:
It will take it into account - the camera will tell the flashes to emit a preflash, determines how much flash power is required for a "correct" exposure, then tells the flash(es) to fire at the appropriate power. Sounds complicated, but the flash light travels pretty fast If you put the flash on camera and set it to a long shutter speed and rear curtain sync you should see the distinct pre-flash and actual flash happening. You can also use FEL to "pre-calculate" the flash power, although this also (on the 60D at least) exposes "correctly" only the centre part of the image so you may need to recompose after locking the flash exposure.

How it determines the "correctness" of the exposure and whether it's neither use nor ornament in a lot of situations is a different kettle of fish

This. Shoot in manual for your background exposure, then see how the flash decides to expose your subject and use flash compensation to get it how you want it.