Image quality depends on a number of factors.
There are a number of factors that will have an effect on image quality in DSLR cameras. Some factors can be controlled by the photographer, including exposure, ISO and lens selection. Others are specific to the camera being used, such as sensor size and processors.
ISO & Noise
ISO is the camera's sensitivity to light. In bright settings a low ISO number (100-200) is typically used. In low light the camera requires a longer shutter speed, or increased aperture size, allowing more light to hit the sensor. However, some pictures may require a fast shutter speed---photos of indoor sports, for example. In those cases the photographer can increase the camera's ISO setting to compensate for the low light. A higher ISO results in the creation of digital noise, those blue---pink and purple specks that appear in pictures---generally viewed as an undesirable by-product.
Bigger is Better
We spend a lot of time thinking about the pixel count in digital cameras. And it is true that pixel count will have an impact on image quality. You should also consider the actual physical size of the sensor in your camera. Most DSLRs have one of two sensor sizes in them: a full frame sensor---equivalent to a 35mm negative---or a smaller sensor, often referred to as a cropped sensor. The effect these sensor sizes have on the image quality relates to image noise and dynamic range. Larger sensors also have larger pixels, giving them the potential to produce lower image noise and increase the dynamic range of the picture. Dynamic range refers to the range of color tones the sensor can capture.
Exposure
Exposure is another factor that will affect the image quality in DSLR cameras. Exposure is the amount of light that travels through the lens to the digital sensor. The amount of light is controlled in two ways, shutter speed and aperture. The shutter speed determines how long the shutter stays open, allowing light to hit the sensor. The lens aperture is the size of the opening that light travels through, from small to large. If you have a slow shutter speed then you need a larger opening, and vice versa. While many cameras have sophisticated auto-exposure programs, under certain conditions those programs will not always provide the correct setting. If an image is under- or over-exposed too much, the picture will not be usable.
Lens Quality
Whether you are using a film camera or digital camera, the lens you select will affect image quality as well. Long before light from your picture appears on the digital sensor, it has to travel through a number of glass or plastic elements inside the lens. Using cheaper lenses can result in a permanent effect on the quality of your photos. Cheaper lenses may result in poor autofocus or have chromatic aberrations---a purple fringing in your photos. They may also have difficulty with maintaining perspective on walls.
Digital Processors
DSLRs are as much computers as they are cameras. And, like computers, the digital processors contained in them can have a significant impact on image quality. The current Canon processor is the DIGIC 4 (Digital Imaging Core). It boasts quicker processing time compared to the DIGIC 3, but more importantly it has improved noise reduction in high-ISO images. The most recent DSLR to use the DIGIG 4 is the Canon 7D, which has a maximum ISO setting of 12,800.
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