The Joy of Pixel Density

Started Jul 13, 2008 | Discussions thread
Victor Engel Forum Pro • Posts: 18,609
A more theoretical visual example

http://www.pbase.com/victorengel/image/100405024/original

You may wish to open that link in a separate window and follow along as I describe it here.

The image shows the text "Hi, there!" written in a gradient on a gradient background, the foreground and background gradients going in opposite directions. It is divided into sections, which I will describe from top to bottom.

The top section is meant to represent individual photons. Pixel values are either 0 or 255 because photons are discrete.

The second section represents a sensor with a very high density of sensels. It was created from the top section by binning the values in 2's horizontally and vertically. The value of each pixel is sum of all the photons divided by the area of the pixel. Each pixel's area is 4.

In the third section, the pixels have an area of 16. Otherwise, the same procedure was used as for the previous section.

Each remaining section has pixels that are double in linear size compared to the previous section, or 4 times the area.

What we can see from this illustration is that the bigger the pixels, the more possible colors there are but the less detail is resolved.

It's true that smaller sensels have a lower well capacity, but this illustration shows that smaller sensels need a lower well capacity. The well capacity divided by the sensel area should be a constant for sensors of different pixel densities to have the same performance in terms of dynamic range resolution.

The other thing to notice is that although the lower sections have more tonal gradations, the previous levels can be converted into the subsequent levels by binning.

The per pixel noise is much greater for the higher density sensor. That's not so clear from my picture. However, if you resize the images so that the pixels are all the same size, it becomes evident. I've taken the upper left corner of each image and resized them so that their pixel sizes are the same.

http://www.pbase.com/victorengel/image/100408878/original

Here it's obvious the smaller sensels have a higher per pixel noise level.

The noise in this example is all shot noise. The results would be different if read noise were added.
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http://www.pbase.com/victorengel/

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