Controlling ISO

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As explained at the start of this chapter, your camera's ISO setting controls how sensitive the image sensor is to light. At higher ISO values, you need less light to expose an image.

Remember the downside to raising ISO however: The higher the ISO, the greater the possibility of noisy images. See Figure 5-8 for a reminder of what that defect looks like.

In the fully automatic exposure modes, the camera selects an ISO of 100, 200, or 400, depending on the available light. You have no control over ISO in those exposure modes. In the advanced exposure modes, you can specify an ISO setting from 100 to 1600.

The current ISO is displayed in the upper-right corner of the Camera Settings display, as shown on the left in Figure 5-15. To adjust the setting, just press the top cross key — the one that sports the label ISO. You then see the screen shown on the right in the figure. Use the cross keys or rotate the Main dial to highlight your choice. Then press the Set button.

Figure 5-15: Press the top cross key to access the ISO setting.

Dampening long-exposure image noise

Even if you set your camera to a low ISO value, images may still suffer from noise when you use a very slow shutter speed. The longer the exposure, the greater the chances of this digital defect, which gives your pictures a mottled look.

The Rebel XTi/400D offers an in-camera filter that is designed to help eradicate the type of noise that occurs during long exposures. This feature is provided through a Custom Function, however, which means that you can access it only in the advanced exposure modes. To check it out, visit Setup Menu 2, select Custom Functions, press Set, and then use the cross keys or Main dial to select Custom Function 2, as shown in the figure here. Press Set again to activate the scrolling list of options in the middle of the screen. You can choose from these settings:

I Off:No noise reduction is applied. This is the default setting.

I Auto: Noise reduction is applied when you use a shutter speed of 1 second or longer, but only if the camera detects the type of noise that's caused by long exposures.

I On: Noise reduction is always applied at exposures of 1 second or longer.

Before you enable noise reduction, be aware that doing so has a couple of disadvantages. First, the filter is applied after you take the picture, as the camera processes the image data and records it to your memory card. The time needed to apply the filter is about the same as the original exposure time, which slows down your shooting speed.

Second, noise-reduction filters work primarily by applying a slight blur to the image. Don't expect this process to totally eliminate noise, and do expect some resulting image softness. You may be able to get better results by using the blur tools or noise-removal filters found in many photo editors because then you can blur just the parts of the image where noise is most noticeable — usually in areas of flat color or little detail, such as skies.

Language English Video system NTSC

Custom Functions(C.Fn)

Clear settings Sensor cleaning:Auto Sensor cleaning:Manual Firmware Ver. 1.1.1

Custom Function

Long exp. noise reduction

0102030405080708(191011 0 0 D 0 0 0 D 0 0 0 0

To see the current exposure settings, start by pressing the shutter button halfway. The following things then take place:

1 The camera locks focus, and the exposure meter comes to life.

1 The current aperture and shutter speed appear in the viewfinder. (If you take your eye away from the viewfinder, the Camera Settings display appears, and you can view aperture and shutter speed there instead.)

1 In Manual mode, the exposure meter also lets you know whether the current settings will expose the image properly. In the other advanced exposure modes — Tv, Av, P, and A-DEP — the camera indicates an exposure problem not with the meter, but by flashing either the shutter speed or f-stop value. (See the section "Monitoring Exposure Settings," earlier in this chapter, for details.)

Adjusting aperture and shutter speed

You can adjust aperture and shutter speed only in P, Tv, Av, and M exposure modes. In A-DEP mode, the camera forces you to use its selected exposure settings. (You can, however, tweak the exposure by using the exposure-compensation feature discussed in the next section.)

The technique you use to change the exposure settings depends on the exposure mode, as outlined in the following list:

1 P (programmed auto): In this mode, the camera initially displays its recommended combination of aperture and shutter speed. To select a different combination, rotate the Main dial.

• To select a lower f-stop number (larger aperture) and faster shutter speed, rotate the dial to the right.

• To select a higher f-stop number (smaller aperture) and slower shutter speed, rotate the dial to the left.

1 Tv (shutter-priority autoexposure): Rotate the Main dial to the right for a faster shutter speed; nudge it to the left for a slower speed. As you change the shutter speed, the camera automatically adjusts the aperture as needed to maintain the proper exposure.

Remember that as the aperture shifts, so does depth of field — so even though you're working in shutter-priority mode, keep an eye on the f-stop, too, if depth of field is important to your photo. Also note that in extreme lighting conditions, the camera may not be able to adjust the aperture enough to produce a good exposure at your current shutter speed — again, possible aperture settings depend on your lens. So you may need to compromise on shutter speed (or, in dim lighting, raise the ISO).

i Av (aperture-priority autoexposure): Rotate the Main dial to the right to stop down the aperture to a higher f-stop number. Rotate the dial to the left to open the aperture to a lower f-stop number. As you do, the camera automatically adjusts the shutter speed to maintain the exposure.

If you're handholding the camera, be careful that the shutter speed doesn't drop so low when you stop down the aperture that you run the risk of camera shake. And if your scene contains moving objects, make sure that when you dial in your preferred f-stop, the shutter speed that the camera selects is fast enough to stop action (or slow enough to blur it, if that's your creative goal).

i M (manual exposure): In this mode, you select both aperture and shutter speed, like so:

• To adjust shutter speed: Rotate the Main dial to the right for a faster shutter speed; rotate left for a slower shutter.

• To adjust aperture: Press and hold the Exposure Compensation button, shown in Figure 5-16, as you rotate the Main dial.

See the Av label under the button? That's your cue as to the aperture-related function of the button — Av stands for aperture value.

Exposure Compensation button

Exposure Compensation button

Canon Eos Rebel Xti Slow
Figure 5-16: To set aperture in M mode, press the Exposure Compensation button as you rotate the Main dial.

Rotate the dial to the right for a higher f-stop (smaller aperture); rotate left to select a lower f-stop. Don't let up on the button as you rotate the Main dial — if you do, you instead adjust the shutter speed.

Keep in mind that when you use P, Tv, Av, and A-DEP modes, the settings that the camera selects are based on what it thinks is the proper exposure. If you don't agree with the camera, you have two options: You can switch to Manual exposure mode and simply dial in the aperture and shutter speed that deliver the exposure you want; or if you want to stay in P, Tv, Av, or A-DEP mode, you can tweak the autoexposure settings by using the feature explained in the very next section.

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  • lavinia
    How to change iso auto to manual on eos rebel xti?
    5 months ago

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