ISO Speed

As was discussed in previous articles; exposure is controlled by the shutter speed, aperture, and ISO speed.  I've already discussed the basics of shutter speed and the aperture.  In this article, I will be discussing what the ISO speed is and how to use it when trying to expose your image.

For digital cameras, the ISO speed equivalent indicates the sensitivity of the image sensor to light.  By increasing the ISO speed, you increase the sensitivity of the image sensor.  This will allow you to shoot at low-light while maintaining a high/higher shutter speed.  However, this doesn't come without a cost.  By increasing the ISO speed, you also increase the amount of digital noise in an image.  The good thing is that most newer cameras, especially full-frame DSLRs, handle high ISO noise very well.  This means that you can increase your ISO speed to 800 or 1600 and the image would still be usable.  On my camera, there is an option to turn on the HIGH ISO SPEED NOISE REDUCTION.  I'm sure other current DSLRs also have this feature.  Try to turn it on and notice a reduction in image noise.

Here's what you can try:

1. Set for the lowest ISO to minimize noise.  Set your camera to ISO 100.
   
2. Be creative by controlling the Depth Of Field (DOF). Set your camera to aperture priority (Av for Canon). This will allow you to control the DOF to create creative shots.
   
3. Meter. Try to meter (pressing the shutter button half way) a shot and you're camera should display the shutter speed it selected to get a proper exposure.  Also consider the focal length of the lens you are using; a rule of thumb is that the shutter speed should not be lower than 1 / (focal length).  Meaning, if you're using a 50mm lens, you should not go lower than 1/50" (or 1/60") shutter speed. Another thing to consider is also whether the subject is moving.
4. Adjust. If the shutter speed is too low for you to be able to compose a shot properly (e.g. lower than 1/(focal length) or if it is too slow for your moving target), you can then increase your ISO speed to the next setting which is ISO 200.
5. Repeat. Compose the shot again, check the shutter speed selected by your cam, and increase the ISO speed again until you're contented with the shutter speed selected by your camera.

Now, go out and practice shooting with your camera using those steps.