ETTL Power output

[admirable]

I'm not a frequent flash user and I've been asked this question but don't know the answer.
How do you tell how much light a flash puts out in ETTL mode?
eg: If a shot is taken in auto (P mode in Canon flash in ETTL) the exif will tell you the shutter and aperture setting but no details on flash output 1/60th 1/120 etc?

 

[Terrywoodenpic]

You can not. It is not dosed out in that way,it is a very complex algorithm that shuts off at the flash when a balance has been reached. However many godox flash guns have a button to switch between manual and ettl, which converts the last Ettl shot to a manual setting. This can be used as a sort of. Ettl exposure meter.

 

[admirable]

You can not. It is not dosed out in that way,it is a very complex algorithm that shuts off at the flash when a balance has been reached. However many godox flash guns have a button to switch between manual and ettl, which converts the last Ettl shot to a manual setting. This can be used as a sort of. Ettl exposure meter.

thank you

 

[Phil V]

Exactly what @Terrywoodenpic said.

Mostly impossible but both Godox and profoto have a CTM button to convert to manual.

Though I’m interested in why you’d want to know. Because the exif doesn’t store the manual flash settings either.

The whole ‘point’ of TTL flash for me, is that it’s miraculously adaptable. It copes in situations where I couldn’t possibly adapt ‘on the fly’, so any attempt to learn from it would still mean that I’m doing a worse job with loads more work.

 

[wiltw]

Today we cannot know how much power was sent via digital TTL, but it does not have to be this way!
The camera commands the flash to 'preflash', then it meters the light that comes back from the scene, then the camera commands the flash to 'outlut level X' when it fires when the shutter opens...so the camera knows how much it commanded from the flash, but it does not tell us...but a software change could embed the predetermined amount of light that the camera commanded from the flash, and embed that information 'level X' into the EXIF data attached in that image. Of course, the manufacturer would have to provide us with a translation table in order for us to understand what fractional power level corresponds to 'level X'...but, as Phil insinuates, what real value (beyond curiosity) could the user extract by having access to the commanded level of flash output?

 

[Phil V]

Today we cannot know how much power was sent via digital TTL,

Did you not bother to read the other responses that said precisely that it is possible today?

 

[Terrywoodenpic]

Today we cannot know how much power was sent via digital TTL, but it does not have to be this way!
The camera commands the flash to 'preflash', then it meters the light that comes back from the scene, then the camera commands the flash to 'outlut level X' when it fires when the shutter opens...so the camera knows how much it commanded from the flash, but it does not tell us...but a software change could embed the predetermined amount of light that the camera commanded from the flash, and embed that information 'level X' into the EXIF data attached in that image. Of course, the manufacturer would have to provide us with a translation table in order for us to understand what fractional power level corresponds to 'level X'...but, as Phil insinuates, what real value (beyond curiosity) could the user extract by having access to the commanded level of flash output?

Godox flashes do "remember" and use that data to set manual when you use the CTM button.
However the camera Exif protocol does and can not record it. As it was not designed to do so. Until the exit protocol is updated that will remain the case.

 

[admirable]

Though I’m interested in why you’d want to know. Because the exif doesn’t store the manual flash settings either.

I was asked by a friend who took images of young kids in fancy dress, some with sparkly reflective bits, at an event. The ETTL varied the flash output slightly from image to image and she wondered if it was possible to get a guesstimate from the camera and flash in ‘P’ and TTL mode and then convert them to manual settings on the camera and flash and have the exposure on the images looking similar.

 

[Phil V]

I was asked by a friend who took images of young kids in fancy dress, some with sparkly reflective bits, at an event. The ETTL varied the flash output slightly from image to image and she wondered if it was possible to get a guesstimate from the camera and flash in ‘P’ and TTL mode and then convert them to manual settings on the camera and flash and have the exposure on the images looking similar.

Well firstly I’d never use P mode when using flash, you’re just adding too many levels of guesswork, and M mode is quite an arbitrary thing to ‘lock in’. So point 1 would be don’t use P w flash (I appreciate a lot of people do though, and it works well >90% of the time).

Sequins are always going to be a crap shoot with TTL flash, in the same way they are with any auto exposure. You can mitigate a bit by using CW flash metering, but if you have real problems you’ll have to revert to manual flash settings. But as above, that’s easy for me cos my Godox flashes do that.

 

[Phil V]

I was asked by a friend who took images of young kids in fancy dress, some with sparkly reflective bits, at an event. The ETTL varied the flash output slightly from image to image and she wondered if it was possible to get a guesstimate from the camera and flash in ‘P’ and TTL mode and then convert them to manual settings on the camera and flash and have the exposure on the images looking similar.

The description here sounds like this is a ‘static’ shoot, in which case reliance on auto and ETTL are both ‘mistakes’ IMHO.

I always use M when shooting with flash as the primary light source because how would my camera know what ambient v flash mix I am looking for?

So let’s presume this is indoors and a static ‘studio’ type situation.

400 iso (to make the flash not work too hard) 1/100 SS (to remove the chance the trigger is slow) and an aperture of 5.6 to keep the DoF generous.

At that point, ETTL flash is going to make an ok job of the exposure, however if the subject to flash distance is consistent, why have the flash guess at the exposure when we can dial it in and adjust in 30 secs.

Start at 1/4 power, chimp, adjust, chimp again and your set. Bearing in mind that at this point your only remaining variable is the flash power.

Whereas with ETTL and P mode, every single variable is up for grabs by the camera for every single shot. And like a kid in a sweetshop it’ll choose quite randomly based on ambient changes, skin tone, outfit colours. Add in sequins and it’s like the kids been fed e numbers.

 

[Terrywoodenpic]

I was asked by a friend who took images of young kids in fancy dress, some with sparkly reflective bits, at an event. The ETTL varied the flash output slightly from image to image and she wondered if it was possible to get a guesstimate from the camera and flash in ‘P’ and TTL mode and then convert them to manual settings on the camera and flash and have the exposure on the images looking similar.

That is more a basic photographic exposure question.
Like the difference between reflected and incident light readings.
Modern exposure systems have been working for many years to better the way in which to get more consistent exposures.
This has always failed especially when continuity of subject exposure is needed

When ever you point the camera at anything, its basic exposure is influenced not only by the subject but also by anything surrounding it giving very variable results.

This is also true of Ettl results. As each shot, while mostly being perfectly usable, is nothing like consistent.

As @Phil V said, the best alternative,, and the one almost everyone uses in these situations, is to go manual. And Chimp and adjust the power.
It only takes a couple of shots to establish the correct exposure this way. And provides the consistency of subject tones you are looking for. It is very quickly done and with digital costless.

 

[sk66]

I was asked by a friend who took images of young kids in fancy dress, some with sparkly reflective bits, at an event. The ETTL varied the flash output slightly from image to image and she wondered if it was possible to get a guesstimate from the camera and flash in ‘P’ and TTL mode and then convert them to manual settings on the camera and flash and have the exposure on the images looking similar.

Well, you can see what exposure settings were used and lock those into manual mode... that only leaves the flash power variable, which is pretty easy to dial in.

 

[wiltw]

Did you not bother to read the other responses that said precisely that it is possible today?

Yes, I read it and I contributed MORE information on the topic: that it is today potentially possible because the camera sends a computed power request to the flash, we merely need the camera to embed that info in the image file.
And I described how digital TTL gets computed, to send a specific power request to the flash in advance of exposure.
Did YOU not understand the contribution of more information to the discussion?

 

[Phil V]

Yes, I read it and I contributed MORE information on the topic: that it is today potentially possible because the camera sends a computed power request to the flash, we merely need the camera to embed that info in the image file.
And I described how digital TTL gets computed, to send a specific power request to the flash in advance of exposure.
Did YOU not understand the contribution of more information to the discussion?

I reacted to the fact that you stated unequivocally

Today we cannot know how much power was sent via digital TTL,

After this.

both Godox and profoto have a CTM button to convert to manual.

Baffling. have you forgotten you’d said the above. Really it doesn’t matter that you’d added detail to show how it might be possible in the future when people had already stated it was possible now.

 

[wiltw]

I reacted to the fact that you stated unequivocally

After this.


Baffling. have you forgotten you’d said the above. Really it doesn’t matter that you’d added detail to show how it might be possible in the future when people had already stated it was possible now.

OP asked, "How do you tell how much light a flash puts out in ETTL mode?

1. One reply stated, "Godox flashes do "remember" and use that data to set manual when you use the CTM button. However the camera Exif protocol does and can not record it. "
2. You stated, "both Godox and profoto have a CTM button to convert to manual."
3. I replied (paraphrased), "Canon eTTL figures out power level to ask flash to output, but does not store that information."

Is there something wrong with corroboration of statements by multiple individuals, or addition of complementary information, particularly in view of the fact that the web often results in contradictory statements, further baffling the person asking the question?!
And is there some reason for you to be particularly abrasive in your commentary to me?