In an earlier article I discussed the basics of digital camera sensor size. Diffraction is a wave phenomenon that limits what can be achieved in capturing images through digital photography. At this point, digital camera sensor technology is close to the limits imposed by diffraction. Higher quality can be achieved with larger sensor size, not with more megapixels (MP).
Your DSLR's ISO setting is one of the three main exposure controls, the other two being shutter speed and aperture. In a dimly lit scene, increasing the ISO sensitivity helps you to get a good exposure. But there are limits above which noise starts to become apparent in your images.
ISO sensitivities above 3200 will always introduce some noise into the image. This is explained by the physics of light diffraction. Simon Crisp writes, "Once you get up to 6,400 you should expect photos to resemble camera-phone shots."
Below ISO 3200 the noise in the image depends to a great extent on the amount of light captured by the sensor. This is why a full-frame sensor will introduce less noise into an image than an APS-C, or cropped, sensor. In turn, the APS-C size sensor will introduce less noise than the tiny sensors found in point-and-shoot cameras and those used in cell phones. Cell phone camera sensors must be small so they can be used with small lenses. If the phone makers used APS-C or full-size sensors, their phones would be huge! So when you read about a high MP cell phone camera (41 MP is the highest I've seen), rest assured that your trusty 12+ MP DSLR can produce higher quality images.
Full-frame digital cameras are now at the point where they can outperform 35 mm slide film in terms of resolution and low light performance. In order to help take advantage of this, it is necessary to use faster shutter speeds and other techniques to reduce camera shake. A shutter speed of 1/60 sec. has been a rule of thumb for the slowest shutter speed to use when hand-holding the camera, but this will not produce the best quality images. A sturdy tripod combined with a remote shutter release or 2-second delay, mirror lock-up, lens vibration reduction, and other techniques are important tools in producing the highest quality images. Proper exposure, critical focus, a steady camera, and good image composition are all needed to make good photographs.
Ultimately, the intended use of your images could drive the image quality that you need to achieve. If your goal is to create Web sites, almost any digital point-and-shoot or DSLR will do. Producing reasonably large prints demands a higher quality image. I suggest that you should always strive for the highest image quality possible.
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