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Photography Question 

Uriah H. Carr
 

Higher ISO Setting Causes Color Cast


I'm familiarizing myself with my new Canon Rebel XT DSLR.

Last week, while shooting indoors with the built-in flash, I discovered that setting the white balance to "Auto" gave accurate color, whereas setting it to "Flash" gave color with an orange cast.

This discovery was made with the ISO setting at 200. This week, I shot pix under the same conditions with the same settings save for one change: inadvertantly the ISO setting was changed to 400. The pix have a red color cast in the neutral tones.

With regards to raising the ISO setting, I know about the increase of noise within the picture, but a crappy color cast? Can anyone explain what's going on?


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September 28, 2006

 
- Gregory LaGrange

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  Built-in flashes aren't very strong so they don't cover much range. At 200iso, you got very little of the exposure from the flash for the subject at whatever distance it was. You mostly got available light. So with auto white balance, it was corrected by the camera. But since flashes many time have a slight blue cast(daylight with a little extra blue), setting the white balance to flash you have available light plus the camera correcting for some extra blue.
With 400iso, you get more of the exposure coming from the flash since the higher iso increases the range that a low power built-in flash can affect a picture. That's my theory.


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September 28, 2006

 

Jon Close
  My theory, too. Except that at the higher ISO setting the flash is providing less of the light in the exposure and the ambient is more. With the WB set for FLASH, the light from the flash will be rendered correctly and the ambient light (incandescent/tungsten) will be have yellow/orange/red cast. At ISO 200, 1/90 and f/5.6, most of the light on the subject will be from the flash and the camera will record little of the background ambient light (orange cast). At ISO 400, 2x as much of the exposure will be from the ambient light, so the mismatch is more apparent.


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September 28, 2006

 
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