580 EX II and flash metering

[coastalceltic]

I've been using Guy's "Mastering Canon EOS..." and it's been really helpful in learning how the camera and flash make the decisions they do, in various situations. I've been having a really frustrating time though figuring out why I've been getting such bad results in a particular situation.

The subjects (2 people) are standing in shade, and the background is a very bright scene at mid-day. I'm standing just a few feet away (580 EXII, 5D, evaluative metering, shutter priority). I expected to be able to underexpose the ambient a bit using exposure compensation, and have the flash light the subjects nicely. They were in shade but of course there was some ambient light on them.

The problem was, they were pretty well lit by the flash when I didn't underexpose the background, but when I did (moving, say, from F 7.1 to F8, staying at 1/200), the subjects would be underexposed too. It's my understanding that exposure compensation doesn't affect the output of the flash, so why did the flash output seem to be reduced? Obviously a smaller aperture means the flash output needs to be raised to maintain the same light levels on the subject, but this should be automatic, right? I could see how the subjects might come out dark if the flash was already at full blast and it couldn't go higher, but I was quite close to the subjects and I can't imagine the flash was at full power.

Was the 580 fooled? Did it see the bright ambient background and assume the subjects were also already brightly lit, and therefore dial the flash way down? But the flash is supposed to sense the light bouncing off the subject and therefore not make this mistake, yes? What am I missing here?

Thanks!

[Tiberius]

Honestly, in a situation like that, I'd use the camera and the flash on manual mode. I've yet to develop a trust of my camera's idea of how to integrate flash...

[belgske]

according to canon auto fill flash only works a)if distance between dark subject and light background is big enough
b) if subject is within max2-4 m away from camera

c)I wonder what lens you were using ? I guess you are using a canon lens ?
Roger
eos 50d+580exII

[coastalceltic]

[belgske]

I also struggle with these intelligent possibilities of the 580exII in combination with eos camera. It looks like you'll need also a compatible lens and even then I 'm looking for someone who can confirm it works as published.
Here are some resources I found :
-----------------------------------------
EOS flash ETTL II
lens distance information, automatic flash reduction


1. http://web.canon.jp/imaging/flashwork/functions/reduction/index.html

automatic flash reduction prevents overexposed "blowout" by automatically reducing the flash level. This function, available when using EX Series Speedlites on EOS cameras, essentially determines whether the flash should be used as the main or auxiliary light source.

2. http://web.canon.jp/imaging/flashwork/ettl2/technology/index.html

E-TTL II is Canon's latest proprietary automatic flash exposure control system. By employing multiple metering zones to measure both ambient light and preflash, then comparing the two and taking metering distances into account, this sophisticated system automatically adjusts the flash level to achieve natural reproduction with ideal exposure of both the background and subject.

In various shooting situations, even when the background is highly reflective, E-TTL II can use distance information from the lens to eliminate underexposure and achieve optimal automatic flash exposure control.

3. You are here: Home > Canon White Papers: Beyond the Manual
canon 50D/5DmarkII whitepaper:
http://www.usa.canon.com/uploadedimages/FCK/Image/White%20Papers/EOS%2050D%20and%205D%20Mark%20II%20WP2.pdf

P57 bottom
Flash metering NOT linked to active AF point Distance from compatible EF lenses now used

4. http://cpn.canon-europe.com/content/education/masterclass/canon_flash.do
* Home
* / Education
* / Masterclasses
* / Canon flash Masterclass

between min 1:00 and 1:10 again lens distance data is the 3th factor needed by the camera/flash for correct exposure

5.this source is in Dutch :
http://www.eoszine.nl/339/Laatste-nummer.html#ep179363=5
look at the example on page 11.
they use P-mode but get the best result by lowering BC (ambient) AND at the same time increasing FC (flash)
to do the same experiment yourself they advise to put someone in front of light window letting the subject cover 50% of view.
They do not mention the intelligent possibility of the ETTLII mentioned in other resources to detect foreground/background by itself.
----------------------------------------------

I understand you'll need at least one of the supported lenses from canon that deliver distance information, these are all EF USM lenses.
I do not have such lens, hope someone can confirm things work as expected when you have a compatible (USM) lens.
I guess in order for these 'intelligent' algorithms to work some 'ideal' conditions must be met :

- subject in front must cover x% of picture ?
- subject must be close to camera
- subject must be in center (?) of picture
- EV must be between certain values ?
- how important is this lens distance data ?

I am not sure if all the info presented on photonotes.org is accurate,
is it up to date with ETTL II and recent camera models ?

Hope to read some interesting reactions on the subject here from experienced pro's.

Roger

[belgske]

[coastalceltic]

Belgske,

Thanks for all those resources, I will check them out.

[belgske]

Check the instruction manual from your camera. On page 109 of eos 50D instruction manual ,the pages about flash exposure compensation there is a warning you have to disable Auto Light Optimizer (ALO CFn II-4 on 50D) for optimal result with Flash Exposure Compensation (FEC) and Exposure Compensation (EC) .
Even if you never changed this Cfn it will be on: default value = 0:standard. you must set value= 3: disable.

Roger

[belgske]

the instruction manual of the 580exII flash explains the different flash measuring programs on page 2.

It is automatically compatible with the camera's flash metering mode (E-TTL II, E-TTL, and TTL). The camera controls the Speedlite automatically in the following flash metering modes:
1. E-TTL II autoflash (evaluative flash metering with preflash reading/lens distance information)
2. E-TTL autoflash (evaluative flash metering with preflash reading) 3. TTL autoflash (off-the-film metering for real-time flash metering)
Regarding the camera's available flash metering modes, refer to the camera's instruction manual.

On my 50D I can only choose between ETTLII, Manual or Multi.
This means that for correct flash metering I need lens distance information.
I do not see this lens distance information in exif data of any picture I take (RAW or Jpeg).
The only place you can actualy view this information is in Digital Photo Professional software (DPP) . Read page 63,64 and 66 of DPP instruction manual (chapter about correcting lens abberations). It lists all compatible camera's and lenses . For EOS 5D in combination with some lenses you'll need the latest firmware of 5D (1.1.1 or higher). Most of the compatible lenses are USM type but not all of them.
Page 66, 'about shooting distance information' in DPP manual says : 'If no shooting distance information is saved to the image , the slider is set at infinity and a <!> appears above the slider.'

I wonder how flash exposure is calculated if no distance information from lens is available in case of non-compatible Canon lens or lens from other brand than Canon. Unfortunately you cannot select ETTL in place of ETTL II in such case.. so only manual correction or complete manual flash settings are the only way to get correct flash exposure .
I hope to find someone who can test/compare auto flash exposure results between compatibel and non-compatible lenses.

Roger

[jol3na]