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Monday, November 12, 2007

Trick Shots: Low Light

Discover how high ISOs can improve the quality of your photographs

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Trick Shots: Low LightWhile recently critiquing the work of one of my students, I noticed that several of his images lacked sharpness, which I immediately attributed to camera shake. We looked at the images' EXIF data to find out at what shutter speed he was shooting while using a 200mm lens. He had been shooting at 1⁄30 sec.—far too slow a shutter speed to use with a telephoto, particularly without the camera being mounted on a tripod. When I asked him why he didn't increase the camera's ISO for a more reasonable shutter speed, his response was an all too common one.

"I don't want noise in my pictures," he said.

So, while the resulting images definitely lacked noise, the photographs weren't sharp enough to enlarge to any significant size. By avoiding the possibility of increasing noise, this photographer created another problem for himself, one that couldn't be remedied by any effort in Photoshop.

The idea that noise should always be avoided is rooted in the early days of digital photography when cameras produced "noisy" images at even moderate ISO settings. Compared with images produced with film at equivalent ISOs, digital images paled in comparison.

This has changed greatly, however, with improvements in sensor designs, camera processing and noise-reduction features. Amazing results can be had at ISOs of 400, 800 and even higher. The key question is when to choose higher ISOs and what to do with noise that's introduced as a result.

Trick Shots: Low LightUnderstanding Noise
It's important to understand what noise is. Each camera sensor, whether a CCD or CMOS, is made of light-sensitive diodes or pixels, which convert light (photons) into a digital signal that's then saved by the camera as a photographic image. Each of these diodes, when electrically charged, has a certain base level of noise called the "noise floor." The greater the amount of light that a sensor receives above the noise floor, the less noise appears in the photograph.

Pixel size also plays an important role in the presence of noise, which is why D-SLRs, with their larger pixels, provide less noise than compact cameras, even those sharing the same resolution. Larger pixels result in better light-gathering capability and, thus, reduced noise.

So, when light levels are low, the risk of noise increases. Taking on the appearance of multicolored specks, noise is easily seen in areas of uniform color like a blue sky. Noise also can be viewed in the red and blue channels of a digital image.

As you raise the ISO, noise increases in a way similar to the noise (static) you hear when raising the volume on your car stereo. Other factors, including heat and the length of time a sensor remains charged, impact the presence of noise. There are justifiable reasons to use a high ISO despite this increase, however,

By raising the camera's ISO setting, you're increasing its sensitivity to light, which enables you to use a faster shutter speed and make a sharp picture. This is one of the biggest reasons I regularly use high-ISO settings.



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