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Photo 101 : ISO Speed < Previous | Next >
Final piece of the puzzle - ISO speed
The ISO (International Standards Organization) speed indicates how sensitive to light the sensor is. The more sensitive to light, the faster the shutter speed you can use for a given aperture.
For example, if you are in a bright room, you can be shooting at 1/60th second at f/2.8 with ISO 100 setting. When you draw the curtains, the light becomes dimmer and suddenly you can only shoot at 1/15th second at f/2.8. You are not able to handhold the camera steadily at such a slow speed, so what can do you?

You can increase the sensitivity of the sensor by increasing the ISO speed, so the sensor needs less light to achieve the same brightness. By increasing the ISO speed from 100 to 400, you can now shoot at 1/60th second at f/2.8 again, even though now the room is dimmer.

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With low ISO

With high ISO
When you adjust ISO speed, the sensor doesn’t really become more sensitive. But rather, the sensor amplifies the signal so the image seems brighter. But by amplifying the signal, it also amplifies the sensor noise at the same time. That is why people always say that high ISO setting leads to “noisy” images. The noise is the tiny speckles that appear in the dark parts (or the shadow areas) of the images.

To attain the highest image quality, set the ISO speeds to the lowest that you can still handhold the camera steadily or attain the depth-of-field that you need.
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