DPP Home Technique Camera Technique Breaking The ISO Barrier

Tuesday, June 9, 2009

Breaking The ISO Barrier

Forget the megapixel race. The real digital revolution is in low-noise/high-ISO digital capture.

This Article Features Photo Zoom

The Technology Behind Improving ISO
What’s the tech that goes into improving high-ISO digital capture?
Sensor: CMOS sensors consume less power and dissipate heat better than CCDs, making them the ideal starting point for eliminating noise.

Pixel Size: Larger pixels increase signal without increasing noise. Nikon’s D3, for example, uses an 8.49-micron pixel—considerably larger than the 5.49-micron pixel in the D300.

Sensor Size: Larger sensors make room for larger pixels and for spacing pixels apart—an ideal way to dissipate the heat that creates visible noise in an image.

Fill Factor: Gaps between pixels decrease the light-gathering ability of a sensor. Microlenses fill in those voids to increase coverage and further improve signal.

Sensor Filters: A more transmissive RGB filter over the sensor of Canon’s EOS 5D Mark II further increases the signal-to-noise ratio by delivering more light through the filter to the sensor.

Signal Amplification: Preamplification and noise cancellation occur on the sensor before the analog electric signal is converted to digital.

Digital Processing: Improvements to the algorithms used to process digital signals help reduce noise and clean up the signal. The faster processing while shooting also minimizes heat—a prime culprit of noise.

More Megapixels: All other elements being equal, a camera with a higher resolution will produce an image that, when print sizes are equalized, has the appearance of lower noise due to the increased signal—i.e., higher dpi—relative to the print size.

Simulating Film Grain Add texture to your photos for a fast-film look
If what you want to achieve with your digital images is an old-school, fast-film effect, you want to add grain. Grain and noise aren’t the same thing, so you can’t just crank up the ISO on your digital camera and expect the noise to look like grain. It just doesn’t work that way. Grain is grainy; noise is noisy. See the difference?

Seriously, noise really is noisy and distracting because of how and where it appears in an image. It typically detracts from an image rather than adding to it. Grain, however, can enhance the mood—say, giving an image a bit of grit or strength—and it’s uniformly seen across the spectrum from light to dark.

So how do you add grain in the computer? Easy. You could just select Photoshop’s Grain filter, but that’s not a very hands-on approach. For customizable controls, choose the ironically named Add Noise filter, which can be applied in various ways. The simplest approach is to apply the filter to an image, scaling the amount for a more or less intense effect. Gaussian and Monochromatic settings in the Add Noise dialog box help keep the effect film-like.

For additional control, apply the noise to a separate layer—either a duplicate of the background or a neutral-gray layer. With the noise kept independent of the image, it can be blurred to soften it or sharpened to make it edgier. Experiment with layer modes such as Overlay, Darken or Lighten to see dramatic changes in how and where the grain is enhanced. If the effect is too strong, dial back the layer’s opacity to let more of the original image show through. If it’s a little weak, duplicate the layer and continue adding grain in any of these simple and effective ways.


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