Macro TTL off camera flash exposure using diffuser question

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Bexter Contributing Member • Posts: 506
Macro TTL off camera flash exposure using diffuser question

Hi,

I am using a wireless Metz 15 MS-1 ring flash inside a diffuser box.  This is triggered by the on camera flash on my Olympus EM1 Mk ii.

This setup is using TTL and my understanding of TTL is that is should compensate and put out a brighter flash when inside the diffuser.  However I find I need to add +1.7 or +2 to the TTL value to get a correct exposure.  This is for macro.

If I just hand hold the Ring flash in the same position but out of the diffuser the exposure is always correct and I don't have to add anything to the TTL value.

Is this expected behaviour or does anyone have any ideas why this is happening?

Thank you!

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Sony SLT-A77
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Olympus E-M1
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