ISO refers to how sensitive your DSLR camera's digital sensor is to light. As the ISO increases, so does the sensor's sensitivity to light. The more sensitive the sensor is, the more light it will capture in a shorter amount of time (keeping aperture and shutter speed constant). Lowering the ISO results in darker photographs (keeping aperture and shutter speed constant).

The ISO should be kept as low as possible at all times. The reason for this is to reduce digital noise in your DSLR photography. Digital noise is the the grainy or somewhat pixelated appearance in a digital photograph. You'll see the difference below in the digital SLR photography image examples.  

There are two situations when the ISO should be raised above the minimum. The first is when you've found a perfect aperture and shutter speed for the image you're creating, but can't get enough light to the sensor for a proper exposure. You'll have to raise the ISO to make the sensor more sensitive to light which results in a brighter exposure. The second situation is when motion is a problem. If you can't use a tripod and you can't keep the camera steady in your hands for the length of the exposure, compensate with the ISO. Increasing the ISO will allow you to have a shorter shutter speed and take hand held camera DSLR photographs without as much blurring. 

The two DSLR photography images below show the difference between high and low ISO settings. The aperture was kept at f/5.6 for consistency. The only changes between the two are shutter speed and ISO which were balanced to create two images with a similar level of brightness. 

 

The image below was taken at a low ISO. It has vivid colors and is very sharp.

  • ISO 200
  • Shutter Speed 1/4s
  • f/5.6 

 

The photograph below was taken with a high ISO. Notice all the noise caused by the high ISO.  

  • ISO 3200
  • Shutter Speed: 1/100s
  • Aperture: f/5.6 
Although the image above has a lot of noise, it's still a sharp image, and unless you're a photographer you wont know enough to realize it's a noisy image. If you have to make a decision it's always better to get a sharp noisy photo rather than a blurry low noise photo.
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