Before the rush

Before the rush
by evan-pak

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When I am using a Canon flash in the E-TTL mode, how can I tell which flash power the E-TTL magic chose?

For example, if I got a correct exposure in the E-TTL mode, can I easily choose the same flash power in the manual mode (without a lot of trial & error)?

Is this information available anywhere (camera, flash, EXIF data)? Does it depend on the Speedlite model?

In particular, I would like to easily find out how close to the limits of my flash I am shooting. If I noticed that E-TTL is using almost full power, then I might want to increase ISO to play safe and avoid underexposure if the conditions change a bit, and if I noticed that it is using very little flash power, I might want to decrease ISO.

Of course I know that there is a green "flash exposure confirmation" LED. This way I can check that I had enough power in my flash – but by how large a margin?

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up vote 3 down vote accepted

I don't think that data is available.

However, I've found that you get a feel for good starting point just by using your gear. Once you get to know your gear you can look at a situation and say to yourself, "OK, I'm going to have my flash about 6' away, ISO 400 and f/5.6... I'll start with the flash at 1/8th power and adjust from there".

That kind of approximation is likely more useful than trying to use E-TTL to meter it for you - especially since it can be tricky to know that the TTL is metering the right area of your composition. If you really need an exact reading you'd be better off picking up a proper light meter that can fire your strobes.

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One way to see this is to use manual (M) mode when using a flash.

As you adjust the aperture and shutter speed, the exposure metering will be displayed in the viewfinder. If you are using ETTL, then the flash will make up for the difference between the metered exposure and the "Correct" exposure.

If you know what the difference was you can calculate when settings you should use in manual mode using the flash's Guide Number and the distance to the subject.

I know this isn't truly what you had in mind, but the flash power is not included in the exif data.

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You can get a rough gut feeling by judging how long it takes your flash to restore charge after the pop. For calibrating, set your flash into manual mode, full power, pop a shot and see how long it takes to become ready for next pop. All my flashes have an indicator light to tell when they are "ready", so I assume other flashes have some indication also; some flashes whistle during recharge. Whenever it takes about as long to recover you were running close to full power (assuming the batteries aren't close to empty). If the flash is immediately ready after a shot, it was working on low power.

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You need to use a flash meter. They usually measure incident or reflected flash and you can pick up a simple one for around $70.00 from B&H photo. Possibly eBay will be able to turn up a much cheaper deal. I suggest something like the Interfit Flash Meter which B&H sell.

Many exposure meters include a flash meter facility and if measuring incident light (the light falling on the subject) is something you feel you may want to try, then an exposure meter such as the Sekonic L358 will measure flash exposures too.

Hope this helps

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Interesting: The answer was obviously not considered to be an answer... even though a flash meter is the only accurate method of answering the questions put by the OP. I am curious to know why the only known (and widely accepted by professional photographers) method of accurately measuring a flash from a speedlight or a studio flash light; was dismissed by the downvoter. – Jeff Cable Apr 14 at 11:52

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