BetterPhoto Q&A
Category: New Answers

Grain and Pixellation

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December 30, 2008

"Grain" is a term left over from the days when many of us still shot film.

Silver Halide grains existed in the paper we used to print with. Chemical grains also exist in the negative or positive celluloid of the film itself.
How tightly or loosely these grains were packed determined the final look.

As we enlargeged the image, these grains became apparent, making the photo look "grainy" or not as sharp as we might like it.

Today, in the digital world, the term "grain" is often used interchangably with "noise" and "pixellation". The two are not the same.

Pixellation from the root word Pixel; is visible evidence of the camera's sensor pixel sites.

A 12 Mb camera has 12 million pixel sites contained in some physical dimension. In a full frame DSLR, these 12 million photo sites "Pixels" are arranged on a chip measuring 36mm x 24mm.

The physical dimension of the sensor,
heat dissapation, number of pixels and firmware design will dictate when and how pixellation will occur.

Low light photography presents a special problem as these "photo Sites" are not being fully energized. (Voltage and current levels)

Like the human eye, low light yields low color rendition.

"How to achieve a low key, dark image without getting pixellation in the dark parts?"

There are two problems here with digital camera's.

1) A low light image requires longer shutter times. As the chips photo sites are being bombarded with photon energy (light) the chip is developing heat.(Noise) This heat leads to "thermal noise" The longer your shutter is open, the worse this gets.

2) Sensors primarily record RGB. Low light images posess little color.

With todays consumer available camera's, while handling noise better, they can not handle very long exposures well. Noise reduction sofware helps, but does not actually eliminate this noise.

Just for fun, if you look into visual camera's astronomers use, you will see temperture plays a critical role as they keep not only the electronics thermally stable, but the optical train is kept VERY cold.

Digital astronomical camera's are another story. Amateurs have been using them for a few years with dramatic results.

Hope that helps a little,

Pete

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December 31, 2008