The most common types of noise are produced at higher sensitivity settings. That’s because cameras achieve the loftier ISO numbers by amplifying the original electronic signal, and any background noise present in the signal is multiplied along with the image information.

One reason why point-and-shoot digicams often don’t have ISO settings beyond ISO 800 is that the noise becomes excessive at higher ratings. However, you can boost the information that the bigger dSLR sensors capture to higher ISO settings with relatively lower overall noise. I’ve used digital SLRs that had less noise at ISO 800 than some poor-performing point-and-shoots displayed at ISO 100. Obviously, the larger sensors found in dSLRs score another slamdunk in the noise department and make high ISO ratings feasible when you
really, really need them.

Noise doesn’t always result simply from using high ISO settings: Long exposures can cause another kind of noise. Although some techniques can reduce the amount of noise present in a photo, by and large, digital SLR cameras are far superior to their non-SLR counterparts when it comes to smooth, noise-free images. Thanks to the disparity in size alone, all sensors of a particular resolution are not created equal, and sensors with fewer megapixels might actually be superior to higher-resolution pixel-grabbers.

For example, most older 8-megapixel dSLRs produce superior results to the newest 10-megapixel non-SLR digicams. I’ve seen results from one $3,000 4.3-megapixel pro-level dSLR that runs rings around the best images possible from an $800 EVF model with an 8-megapixel sensor. So no matter how many megapixels a point-and-shoot camera’s sensor can hoard, that sensor isn’t as big as a dSLR’s. And when it comes to reducing noise, the size of the sensor is one of the most important factors.