D3100: Why is Auto ISO messing things?

Started Feb 5, 2012 | Discussions thread
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learnerguy Contributing Member • Posts: 683
D3100: Why is Auto ISO messing things?

With overcast and cloudy skies early Saturday morning, I decided to click some pictures for learning D3100. Object was a single home visible from my patio.

M Mode, Auto ISO min 100 and max 1600, Auto WB, AF-S, AF-Servo

I set the FL, dialled the aperture at f5.6 and set the aperture such that meter indicated a correct exposure.


M Mode Auto ISO Off, Dialled ISO 100 all else same

What surprises me is that both pictures are taken in identical situations, then why did the Auto ISO logic go all the way to 1250 in the first image to allow me a higher shutter speed which I had not asked for nor the scene needed it?

At 1/500th of a second, the same camera is proving the same image scene that ISO 100 is possible? Then, why does the Auto ISO logic take precedence over Aperture and SS?
ISO Auto 1250, image size 7.7MB
ISO 100 manually dialled, image size 6.4MB

Even if a Auto ISO algorigthm wanted to help the camera with Auto ISO it should have gone with ISO 200 or ISO 400. Why 1250 on the higher side of 1600? Is Nikon not wrong in deciding such Auto ISO algorigthms for newer versions of its cameras when some of its older ones might not have done the same?

Please guide me. I am not a professional photographer hence I may not have understood something I should understand, except that Nikon is wrong in deciding a higher 1250 ISO in Auto ISO logic when the same shot is equally possible at lower ISOs though not the Min ISO.
D3100, 18-55mm VR, Canon S95

 learnerguy's gear list:learnerguy's gear list
Canon PowerShot S95 Nikon D3100 +2 more
Canon PowerShot S95 Nikon D3100 Panasonic Lumix DMC-F5
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