>I am trying to figure out what will happen if I change my >aperture of 5.6 to 2.8 without changing my shutter speed. I >find all sorts of equivalent exposure discussion, but not one >that tells me how this change might affect my ISO. > >My example is a shot taken at 1600 ISO, 5.6 at 1/160, and want >to get the ISO down. > >What am I missing, other than a proper mathematical brain...
I'm going to start at basic, just to be thorough:
Shutter and Aperture determine how much light strikes the sensor, and ISO determines how sensitive that sensor is to light, and together they give you an exposure (hopefully, a proper one). One stop of any of those is a halving or doubling of the exposure.
So, if you go from f/5.6 to f/2.8, you are opening your aperture two stops. To obtain the same exposure in any given situation, you must then either speed up your shutter, slow down your ISO, or both, by a total of two stops.
Given your example of proper exposure, here's a quick list of other proper exposure solutions, grouped by ISO:
- ISO 1600, 1/160, f/5.6 - ISO 1600, 1/320, f/4 - ISO 1600, 1/640, f/2.8
- ISO 800, 1/80, f/5.6 - ISO 800, 1/160, f/4 - ISO 800, 1/320, f/2.8
- ISO 400, 1/40, f/5.6 - ISO 400, 1/80, f/4 - ISO 400, 1/160, f/2.8
- ISO 200, 1/20, f/5.6 - ISO 200, 1/40, f/4 - ISO 200, 1/80, f/2.8
- ISO 100, 1/10, f/5.6 - ISO 100, 1/20, f/4 - ISO 100, 1/40, f/2.8
You can see that as you go decrease the ISO by one stop, the shutter speed at any given aperture changes as well by one stop. Since you want to keep a 1/160th shutter speed and only change your aperture, we can see that opening the aperture to f/2.8 would mean you'd shoot at ISO 200.