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CHDK Overrides & Camera Auto Exposure

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CHDK Overrides & Camera Auto Exposure
« on: 28 / April / 2009, 05:38:44 »
 Hi !
        I have SX10. I am a little confused about how the chdk overrides work. If I understand correctly, the overrides take effect only when we shoot. The native camera osd don't  show effect of the override. The exif values stored in the image are also values only within the camera's normal range irrespective of actual values.

     But what happens to camera's auto-exposure? Will the camera take into account the overridden value to calculate exposure? or do we have to manually set exposure compensation?




Offline fudgey

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Re: CHDK Overrides & Camera Auto Exposure
« Reply #1 on: 28 / April / 2009, 17:31:26 »
Camera's autoexposure happens before overrides. So if you have a fully automatic mode from your camera enabled AND you have CHDK Tv override, you will get Canon's ISO and Av/ND filter selection, and the overrided Tv will be used. Usually the exposure is not correct unless you by sheer luck happened to override to whatever Canon autoexposure had selected.

So yes, you must set exposure compensation to fix exposure if you want to or have to use autoexposure.

Canon's MakerNotes section of the JPEG EXIF tag actually has the correct overrided exposure settings recorded, btw. Most apps just don't know how to display those (no, CHDK doesn't write them there, the camera does... even without CHDK those values are more accurate than ones found in standard EXIF tags, but they don't look familiar because they are the camera's approximation of real APEX values instead of rounded/convenient/familiar "market" values displayed on Canon's OSD).


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