Pixels versus Photosites


Today’s Question: It is my understanding that pixels are not the same as photosites where each photosite records one color [on a digital camera sensor]. The digital to analog converter creates pixels. What’s the real story?

Tim’s Quick Answer: This is mostly a matter of semantics. While the term photosite refers to a physical component of an image sensor, it is extremely common to refer to photosites as pixels even though pixels are elements of a digital image.

More Detail: The term pixel was derived from “picture element” and is used to describe the smallest element of a raster-based image (as opposed to a vector-based image). A pixel is therefore a component of a digital image, and a digital image often contains many (often millions) of pixels.

Technically speaking the individual elements on an image sensor are photosites, not pixels. Those photosites contain hardware that converts light energy into a discreet digital value that then gets processed into the values that define pixels in the resulting digital image.

It is also true that digital cameras only record a single color for each photosite, because those photosites aren’t actually recording color but rather light intensity based on light that has been filtered for color. Even sensors that capture full color for each pixel, such as the Foveon X3 sensor, only capture one color per photosite. In the case of the Foveon sensor there are simply three photosites on the image sensor corresponding to each pixel in the final image.

In common usage, the photosites on an image sensor are often referred to as pixels. For example, most digital camera marketing materials describe resolution in megapixels (millions of pixels) since the images captured with that sensor will contain the number of pixels referenced. But we tend to think of the image sensor as having a particular number of megapixels, which contributes to the photosites being referred to as pixels.

In my earlier days I was more pedantic about this issue, and would use the term “photosite” when referring to an image sensor and “pixel” to refer to a digital image. I’ve since decided to conform with the more common use of the term pixel when talking about photosites. I similarly stopped using all capital letters when talking about raw captures, since raw in this context is not an acronym, even though I liked using all caps for raw so that it stood out like the other acronyms and initialisms used for other file formats. And yes, I do differentiate between acronyms and initialisms, even though I no longer tend to differentiate in the same way between photosites and pixels.

By the way, I also find it amusing that photosite is the technical name for what amounts to a pixel on an image sensor, but my spell checker doesn’t think photosite is really a word.