In the days before digital photography, seemingly every corner store had rack upon rack of film on display. Each roll of film was marked with a speed--measured in ISO--such as 100, 200, or 400. Higher-speed film was handy for low-light photography, but it had a serious disadvantage: grain.
Film grain was every photographer's nemesis. Instead of smooth, natural textures, grain put ugly blotches all over a photo. And though the days of grainy photos are far behind us, digital photos have a similar problem: digital noise.
You've undoubtedly seen noise in your own photos. On the plus side, noise tends to be very small; and when you view a many-megapixel photo on a computer screen, pixel-size noise is so small that it usually disappears into the background. You might look at a very noisy photo and not even know it. Noise becomes apparent, though, when you zoom in--if you crop it down to a small detail, for example, or if you attempt to make a large print. Let's learn how to control noise.
The science of noise
Noise is an appropriate term, because it describes what happens visually in a photo: Tiny, fairly regular but unintended splotches of color mar the photo. The source of the problem is "noisy data"--unreliable or contaminated data. And that's what is going on here.
Millions of photosites crowd your camera's sensor, and each one registers incoming light as voltage. Unfortunately, photosites can get confused. If the light is too weak or the sensor's sensitivity is set too high, photosites may interpret random voltages as light. The result is a photo with noise.
Various factors can contribute to noise in photos. The one you have the least control over is the size of your camera's sensor. The smaller the sensor is, the more noise will appear in your photos, because the tightly packed photosites tend to overlap and interfere with each other. That's why smartphones, which have tiny sensors, tend to generate very noisy photos, regardless of any other factor. Compact digital cameras are somewhat less noisy, and full-frame digital SLRs--which have comparatively big sensors--tend to have the least noise. All other factors being equal, a DSLR will take a cleaner photo than a smartphone every time.
Controlling noise with ISO
Unlike sensor size, your camera's ISO setting--which controls the camera's sensitivity to light--is largely within your control. For example, you can increase the ISO setting to take sharper photos in low light. However, a higher ISO also means more noise. Of course, sometimes the trade-off makes sense. If you're trying to take photos late in the day without a flash, cranking the ISO is your best option. But when you're done, dial the ISO back down as low as it will go, so you can avoid taking needlessly noisy photos the next day when you're shooting in bright sunlight.
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