# Aperture vs ISO for action shots

Started Jan 30, 2013 | Discussions thread
Re: Aperture vs ISO for action shots

jbart1 wrote:

Is there a mathematically formula I could apply to theoretical situations that would allow me to predetermine settings based on a certain number of lumens of available light? Is there a particular correlation or crossover point between ISO and F-number based on a constant shutter speed. Kind of like how horsepower and torque always have a crossover point at 5250rpm

So if I set shutter to 1/500 and my aperture is f4 and ISO is 800 what ISO will I need to set to get the same amount of light at f8, or what aperture would I need to get the same exposure at ISO 200? Is it a linear relationship, logarithmic, or some kind of nonlinear curve based on sensor design

If you double the shutter speed, you half the aperture size to get the same exposure.  So 1/500th at f11 becomes 1/250th at f16 or 1/1000th at f8.  Doubling or halving either value is a change of  one "stop"  ISO is not really part of exposure because it doesn't affect the amount of light gathered.  It amplifies the signal after it hits the sensor.  In terms of brightness(not exposure), doubling or having ISO is considered a one stop change.

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