Wednesday, April 21, 2010

Exposure: Light, Aperture, Shutter Speed, and ISO - Part 3 - ISO

In the previous articles, we talked about aperture (f/stops) and shutter speed. We're now going to finish the basics of exposure by talking about ISO.

What we're really talking about is the sensor in the digital camera. It's make up of millions of microscopic pixels crowded into a couple of square centimeters of space, if that.

ISO (at least in a photography context) is the measurement of how sensitive the sensor is to light. The higher the number, the more sensitive.

And happily, that measurement actually goes in the right direction, as opposed to the confusing f/stops and shutter speeds that we talked about in the previous articles. The ISO actually increases with the sensor's sensitivity. You'll see digital cameras starting with an ISO as low as 50 and increasing from there.


As with f/stops and shutter speeds, the doubling of the number indicates one full f/stop change in the exposure. So going from ISO 400 to 800 would be the equivalent of increasing the exposure by one f/stop - just like going from f/8 to f/5.6

In other words, by lowering the f/stop, we let more light through the aperture. Or, we can simply double the sensitivity of the sensor by raising the ISO.

There are some problems though. As you increase the ISO and raise the sensitivity of the sensor, you get more "noise." Think of it as grain or visual static.

This image was taken at an ISO of 200


This was taken at 6400


Notice that the second one shows more noise, particularly in the center along the edge of the tape. If you want to see it even more clearly, click through the pictures and look at the larger versions on Flickr.
Is Everybody Happy?!?!
What causes noise? There are some technical explanations, but allow me to use a metaphor. When you raise the ISO of a sensor, you're making all of those individual little pixels work harder to capture the light. Just like any crowded group of workers, they're bumping into each other more, jostling, yelling, getting into fights, etc. In other words, as they work harder, they make more noise. As you make the sensor work harder by making it more sensitive to light, it makes more noise.

Noise means that you need to consider whether increasing the ISO and getting more noise is worth it for the picture you're trying to take. Happily with today's cameras, the manufacturers are getting better and better of controlling that noise problem.

So there you have it. There are basically three elements that control your exposure.


The aperture controls the amount of light passed through the lens. It's measured in f/stops which increase as the amount of light let through is decreased.

The shutter opens to allow the light from the aperture to pass to the sensor. It controls the exposure by only letting the light in for a specific amount of time. It's measured in fractions of a second.

And the sensor's sensitivity to light is measured by its ISO. As the ISO goes up, the sensor becomes more sensitive.

Correctly balancing these three elements leads to the right exposure for your picture.

By the way, ISO is the short name of the for the International Organization for Standardization, in case you're vaguely interested.

Jim Frazier

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