Shooting at high ISOs and tweaking sharpness in an image can introduce excessive noise. Balancing these two aesthetic elements is an art.
In the beginning of photography, most people were unaware or oblivious to noise, but with higher-ISO films, image grain became an issue; I look upon grain as a form of image noise. Grain occurs from film's granular structure and appears throughout an image, but most notably within the darker and higher-contrast areas. Understanding is important for the next stage of photography—digital!
Noise found in a digital image is caused by various issues. Noise is created when light passes to the image sensor, when the analog light is converted to digital and at just about every step along the way to your computer, and in your computer, as well.
Compounding the complexity is the fact that not all noise is the same. Noise manifests differently, depending on what step in the process created it. Furthermore, some aspects of noise are akin to the myriad of normal details in an image and, as a result, there's no single solution to diminishing noise in digital images.
Having said that, I want to immediately assure you that there are solutions to help reduce the appearance of noise; the solutions aren't complicated. Your ability to “see the difference” between noise and image detail will take you on the path to better images.
Grain: Film To Digital
In a film-based environment, grain became creatively exploitable and could be an aesthetic element in a picture. Grain was anticipated, and pictures were made with that expectation. The push was for higher ISO, so pictures under dimmer lighting were possible as were faster shutter speeds for action. Films were developed and we said, “Damn the grain, full speed ahead!” It was acceptable, and then technology progressed and faster films eventually had less grain.
For digital, grain may appear in the printing process or with a special application. As with film grain, it, too, can be used creatively.
I recall the first digital images and their noise. Instantly noticed and rejected, everyone said “no” to noise! But digital has persevered and image noise has diminished. Images can look good, in spite of a smattering of noise, and noise is no longer the issue it was.
Less Or No Noise In Newer Cameras
For several new cameras, the appearance of noise has become a moot point. The improvements come from newly developed high-resolution, low-noise sensors; imaging engines with “cleaner” performance; postprocessing that incorporates highly developed tools; and printing technologies that reveal native digital image quality—all bringing digital to higher levels of quality and more pleasing appearance.
Nikon's new D3 with a 12.1-megapixel FX-format (23.9x36mm) is virtually equal to a 35mm film frame as is the sensor in the Canon EOS-1Ds Mark III. But it's not just the size of the sensor, it's the combination of size, image-processing technologies and the cleaning of noise-producing components that bring us today's levels of improved noise reduction. Imagine shooting from ISO 200 to 6400 and having a cushion at each extreme to adjust ISO lower and higher, still with exceptional image quality. Low-light and high-ISO performance among today's top-of-the-line camera models will vary, but overall, the newer cameras represent key steps in the progress of digital photography.
Detail And Sharpness
Recognize that whenever we adjust an image's noise, we simultaneously affect the appearance of the image's details—in essence, the sharpness. If we decide to diminish noise, the image loses sharpness, but with new tools, you can control it.