My Pentax K-5 comes with a "regular" low ISO setting of 100, but if you enable "expanded ISO" you can set it as low as 80 (plus go up to an absurd 51200!). I know that in general to reduce noise, lower ISO settings are better, but I'm wondering if that holds true for this "expanded" setting as well. If I'm outside and there's plenty of light and I can use 80 ISO without ruining my shot with inappropriate shutter and aperture settings, should I?
(I know there may be certain specific circumstances where the super-low ISO would be helpful, e.g. if you are shooting right into the sun and don't want the shutter at 1/8000 s. That's not the situation I'm talking about.)
Answer
True, lower ISO does typically bring lower noise, but all digital cameras adjust ISO by altering internal exposure sensitivity, and any change from the 'native' ISO typically introduces some noise, or other trade-off, such as dynamic range. This includes setting ISO below the 'native' ISO.
This 'native' ISO is that for which the sensor and electronics are optimized. Other ISOs are achieved by pushing (or pulling) exposure internally.
Since your camera offers this 'expanded ISO" that provides for 80 ISO, you can assume that this ISO is sub-optimal, and could exhibit more noise or a loss of dynamic range than the 'native' ISO. Unfortunately, few camera manufactures publish what the 'native" ISO is, but for the most part it is assumed to be the lowest ISO in normal settings. Some suggest Canon cameras are 'native' for ISO 100, and full stop ISO are best (100,200,400 etc.), but I haven't seen such results for Pentax.
Google for "native ISO" and you likely will find some interesting discussion and even sample images.
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