Setting the ISO determines the sensitivity of the image sensor to light. At low settings such as ISO 100, the sensor needs comparatively more light to make an exposure than at a high ISO setting such as 800.
However, as the sensitivity setting increases on a digital camera (a higher ISO number), the output of the sensor is also amplified. So while you have the option of increasing the ISO sensitivity at any point in shooting, the tradeoff in increased amplification or the accumulation of an excessive charge on the pixels represents an increase in digital noise. And the result of digital noise, depending on the appearance and severity, is an overall loss of resolution and image quality.
In practice, the most compelling benchmark in evaluating digital noise is the quality of the image at the final print size. If the digital noise is visible and objectionable in an 8-x-10-inch or ll-x-14-inch print when viewed at a standard viewing distance of a foot or more, then the digital noise degraded the quality to an unacceptable level. It is worthwhile to test the camera by using all the ISO settings, processing and printing enlargements at the size you typically print, and then evaluating how far and fast you want to take the XSi/450Ds ISO settings.
The XSi/450D offers two useful Custom Functions that help counteract noise: C.Fn-3: Long-exposure noise reduction and C.Fn-4: High ISO speed noise reduction.
Reference See Chapter 5 for details on each of these Custom Functions and how to set them.
Note The ISO sensitivity setting affects the effective range of the built-in flash, and the range depends on the lens that you're using. In general, the higher the ISO speed, the greater the effective flash range.
There really is no substitute for knowing how far and fast to push the ISO settings on the XSi/450D unless you test it at each of the ISO settings and compare the results. To compare the results, view the images at 100% enlargement in an image-editing program, and then compare the shadow areas. If you see grain and colorful pixels where the tones should be continuous and of the same color, then you're seeing digital noise. Having low levels of digital noise won't spoil prints from the image, but high levels of digital noise can be objectionable in prints of 8 x 10 inches and larger.
To change the ISO sensitivity setting on the XSi/450D, follow these steps:
1. Set the camera to a Creative Zone mode such as P, Tv, Av, and so on, and then press the ISO button on the top of the camera.
The ISO speed screen appears in the viewfinder. If the ISO screen doesn't also appear on the LCD, it's because the shooting information display on the LCD was not active when you began. Press the Display button to display the shooting information on the LCD, and then press the ISO button.
2. To change the ISO setting as you watch the viewfinder, turn the Main dial to the setting you want. Or to change the ISO setting on the LCD, press the down or up cross key to select the setting you want. Options include Auto (the camera automatically selects an ISO between 100 and 800) and individual settings from 100 to 1600. The ISO option you select remains in effect until you change it again. The current ISO sensitivity setting is displayed in the viewfinder and on the LCD.
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