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From what I understand, when in Av mode, the flash behaves largely as fill flash - the exposure is more or less set as if no flash is in use. However, in Tv mode, the manual suggests the shutter speed will be honored as along as it is less than flash sync speed.

However, in my test, this is not the case (Canon 6D + 600ex): In Tv mode, I am seeing similar behavior as in Av mode. The actual shutter speed of 1/6 or 1/10 is not what I set at all (1/100). I am confused ... what am I missing here?

  • the ambient light is a bit low: it shows either as 1/6 or 1/10. – python152 Oct 4 '15 at 3:47
  • As far as I know, your understanding is correct. Could you post a sample photo with EXIF data? (Note that photos uploaded to Stack Exchange have the EXIF data stripped off, so you'll need to include that separately). – Philip Kendall Oct 4 '15 at 7:48
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I bet you have enabled safety shift mode, an option which overrides your setting in Tv and Av modes if the result, in combination with ISO and the automatically adjusted parameter, would be severely underexposed or overexposed.

You should be able to disable this; check the camera's manual for how to do it on your particular model. On the Canon 6D, it's in the menu titled C.Fn I: Exposure. It defaults to Disabled, but can be set to Shutter speed / Aperture (as I think yours is) or to ISO speed (which would leave your dialed-in parameter alone but might crank up sensitivity/amplification to prevent all-black frames).

  • you are absolutely correct. It was set to "Shutter speed/Aperture". I must have changed it inadvertently. Many thanks. – python152 Oct 4 '15 at 10:02

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