ETTL on Digital Rebel (and on other DSLR)

[d_l1]

Since I rarely do flash photography, I have a very limited

understanding of ETTL and how flash works in general.

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Here's an interesting thread on dpreview regarding flash output and

ISO.

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<a href="http://forums.dpreview.com/forums/read.asp?

forum=1031&message=6662973">dpreview thread</a>

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The reason I post it here is that I think this community has some

more knowledgeable folks regarding this issue.

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*SHOULD* the change in ISO have an effect on flash output(the AMOUNT

of light the flash emits)? Does a relationship exist?

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Could it be simply explained by the change in ISO varies the

sensitivity of the sensor so that the sensor could detect more light

at higher ISO, thus giving an overexposed image?

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This was same theory transfered from film photography, right?

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But why would the camera give an underexposed image at ISO 100?

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Those of you that have other DSLR's, do you get the similar results?

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Any thoughts much appreciated.

[jim_larson1]

I would suggest you read this:

 

http://photonotes.org/articles/eos-flash/

 

 

A few comments:

 

1) E-TTL is not new to dSLRs. The canon film cameras use this system as well.

 

2) I have a 550Ex and a 10D. Flash exposure is fine for me at ISO 100 with both the 550EX and the internal flash. I have dialed FEC on a few odd lighting occasions, but in general I trust the meter.

 

3) I may have misread your question, but OF COURSE a change in ISO results in a change in flash output.

 

The concept behing the E-TTL flash is to put out only enough light to properly expose the subject. If you change ISO, the subject needs less light to be properly exposed. So if a camera can expose a subject at 60/F4 ISO 100, in theory the flash will only put out half as much light at 60/F4 ISO 200.

 

4) I used to read DPreview when I was shopping for my camera. What a scary forum!! So much heat. So little light! Crowd here is much more reasonable.