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I'm very confused about the Pentax auto-iso implementation. What does "fast/normal/slow" mean for the Auto-iso in the menu settings? The camera in question is a K-3, but I think it applies pretty equally to Pentax's with this option.

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Those settings apply to the way Auto ISO works in Program or Tv/Av modes. When set to "Slow", it tries to keep the ISO setting as low as possible (withing the range) until it really has no choice but to boost ISO. It will change something else instead (either the shutter speed or aperture or both). "Fast" works the opposite way; it will prefer to boost ISO before making any radical adjustments to shutter speed or aperture. (Exactly what the preferences are, I'm not sure. There's usually a bias in program modes towards higher shutter speeds, slightly above the rule of thumb for handheld imaging for recognised lenses, but if there's a bias for apertures other than the max/min for Pentax, I don't know.) "Normal" treats shutter speed, aperture and ISO as essentially equal partners in the exposure triangle (with ISO being slightly less equal, at least the way I understand it, changing at a lower rate throughout the program range). The setting shouldn't make any difference when you're in TAv (or "automatic manual mode"), since the ISO is the only thing the camera can change automatically.

  • I guess I'm looking for something more specific. When you say "Exactly what the preferences are, I'm not sure." - that's what I'm wondering. – rfusca Mar 16 '14 at 5:48
  • @rfusca - It would take, at most, a half-dozen shots to see what it's doing with the aperture, and a person with a K3 in their hands is in a much better position to do that than I am. Go to Tv and see what happens as the aperture should be opening up beyond f/4 without "fast" selected. – user2719 Mar 16 '14 at 5:52
  • Hmm I guess so. It's just very non-obvious. It obviously depends on focal length, but there's some kind of limits. For example, at 70mm, slow/normal/fast is 50/100/200 shutter speed. Its that at 50mm as well. At 25, its like down to 25/50/100. At 18 its also like 25/50/100. – rfusca Mar 16 '14 at 5:58
  • Ok, its more complicated than that even. At 70mm f/2.4 its 50/100/200 at 2.8 its 60/120/250. Its just confusing. – rfusca Mar 16 '14 at 7:12
  • For your testing, keep 2 things in mind: 1. Set the Program line to Normal if you're P mode. 2. I do not believe the aperture affects the behavior directly. Instead, it decides on whether to raise ISO or instead lower exposure time or open the aperture when it has still room to work with the ISO value. So you need to adjust the lighting every time you switch a setting, then see at which point it rather changes ISO over the other parameters. – Thomas Tempelmann Mar 16 '14 at 11:09

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