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04-08-2009, 09:25 AM   #1
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Auto ISO

I really hope this isn't a stupid question, but here goes...

I have my istD set at ISO 200 most of the time, just because I'm comfortable with fixed ISO and want the low noise. But given that auto ISO really only changes the ISO within a small range, if I switched to auto ISO (200-400) in Tv mode, how does the camera decide which to change (or which to change first) - ISO or aperture? Can't find anything about that in the manual.

04-08-2009, 09:51 AM   #2
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That's a good question - and I can't really answer it. Also I'm not personally familiar with the *istD (in fact why am I even replying )... On the Pentax K200D and K20D, the ISO range covered by auto ISO is adjustable, so I'd suggest that if yours only covers 200-400, you may be able to adjust it to cover 200-800 for instance if you so desire... I'm sure *istD owners will chime in on this aspect...

As to your question, the way my K20D works in Tv is that it will adjust the aperture until it's wide open before it starts to bump up ISO. I tend to use Av and M to the exclusion of all other modes because I like to choose the aperture specifically to suit my purpose.
04-08-2009, 02:53 PM   #3
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Not sure about Pentax. When I was still with Nikon, the camera always preferred to increase ISO to changing shutterspeed or aperture. After I got some nice shots ruined by too high ISO I have never used the automatic ISO setting with Nikon nor with Pentax. I like to have full control over what the camera does.
04-08-2009, 03:25 PM   #4
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On my K20D I have auto set to 100-400. I don't know exactly what algorithm it uses but it seems to favor low ISO setting. It seems like (in AV mode) it uses the lowest ISO it can without the shutter speed dropping below 1.5 times the focal length.

04-08-2009, 06:21 PM   #5
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I second Parallax's comment. I shoot primarily in Av mode and Auto ISO (100-800). ISO seems to change to preserve hand-hold shutter speed as a function of the focal length being used. Unfortunately I can consistently hand-hold steady shots with IS down to about 1/10 sec. My camera doesn't know this ;-)
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