Monday, March 10, 2014

ISO and Exposure

Of the three components of exposure (Shutter Speed, Aperture & ISO), ISO is the one that feels the most ambiguous. Shutter Speed is the easiest to understand: how long the shutter is actually open. And even though the math used to figure out the f number to describe the aperture is beyond most of us, the concept of smaller number equals a bigger opening and double the amount of light let in makes enough sense to grasp.

ISO on the other hand describes how sensitive the camera's sensor is to the light that it is exposed to. Uh, say what? With no other real explanation of how this works we are simply left with the information that each value doubles (higher value) or halves (lower value) the sensitivity.

So to get a good visual of how the ISO setting effects the final image, I set my camera on manual (after meeting on Program AE) and took a series of photos at the same Shutter Speed/Aperture combination and simply changed the ISO after each shot. I had originally metered the scene @ ISO 400 so there was room on either side to see the differences. Below is a composite with all six of the images (sized down to fit in one image) set in two rows to see how the exposure changes with just the ISO being different.

1/5 second @ f/8

Looking back, I wish I would have taken one more on ISO 6400 to see the difference next to  ISO 3200. But with the ISO 3200 image already starting to overexpose I decided not too.



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