5D III: ISO changes in aperture priority mode

[mflahertyphoto]

Hi,

I started noticing not long after I bought the 5D III that when I'm in aperture priority mode, and set the ISO manually, that when I start shooting near 30 seconds long exposure the ISO starts automatically changing before I trip shutter.  It rises and the exposure time usually goes down to 25 or even 20 seconds.  I have had to switch to manual mode.  

With the 5D II, no such thing happened.  The shutter speed (30 seconds) would just flash to let me know I was underexposing.  I don't understand how the ISO could automatically change unless you set it on auto ISO (which I never use).  Please tell me this is a problem with the camera, or is resettable somehow.  Would probably sell the camera if this was a standard thing that you can't change.

Thanks a bunch,

Michael

[diverhank]

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@mflahertyphoto wrote:

  The shutter speed (30 seconds) would just flash to let me know I was underexposing.  I don't understand how the ISO could automatically change unless you set it on auto ISO (which I never use).  Please tell me this is a problem with the camera, or is resettable somehow.  Would probably sell the camera if this was a standard thing that you can't change.

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There is a setting option for exposure protection to have the camera shifts either speed/ aperture or ISO.  It is most likely that yours is set to Shift ISO.  You can either turn this off or to some either speed or aperture. Edit: See page 323 of the manual.

[mflahertyphoto]

Thanks.  Looked in the manual but somehow missed that.