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a650 needs twice the light of other cameras.

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Offline drb

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a650 needs twice the light of other cameras.
« on: 29 / December / 2008, 21:07:45 »
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I have just tested four Canons P&S's. I can set up a scene - and at ISO 400, AV mode, f 2.8, all the cameras decide this should be shot at 1/30th.

If I try the exact shot with an a650 with CHDK, it wants to take this same shot at *1/15th*.

In every case, it wants the shutter open for twice as long twice on the a650 compared to these others. If I double the ISO on the a650, then all shutter speeds match.

Either the a650 reports all ISOs far from actual value, or something else is up that is confusing me.

In every case I tested, it looks like the older cameras need half the light that an a650 seems to need. The a650 shots look great, but this still seems rather odd.

If I enable the CHDK "show real iso" it shows Canon 800 as 523 - have I screwed up a setting? I don't think I have any overrides set anywhere..

Is this not a CHDK thing? Any info would be a big help.
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Offline dzsemx

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Re: a650 needs twice the light of other cameras.
« Reply #1 on: 07 / January / 2009, 21:56:39 »
i own two cameras, my girlfiend has A720 and i tried many other powershots and there is no such big difference,
they meter almost the same values

the real ISO is the actual sensitivity of the sensor.
the raw data looks underexposed and generally greenish.
the RGB channels are gamma corrected, and amplified with different values for white balance
after all these corrections(brightening, WB correction), the resulted sensitivity is that one,
that you see on the screen by the original canon OSD
it's not a lie
some camera models lie about the sensitivity, generally the powershots does not do this
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