One of the most common issues discussed in evaluating and comparing digital camera models are these concerning pixel count and file generated by the same camera.

This is just a small technical note on the subject. T've been maintaining and updating it for the last 15 years, and one day it may evolve into a full article, but don't hold your breath.

What really is a pixel?

Strictly speaking, the camera sensor does not have pixels (as an RGB pixel is a point with information on all three color components). In the most common implementation, the Bayer matrix (perhaps 99% of cameras made), it has a rectangular grid of photosites, which actually are monochrome pixels (i.e. providing just one, not three values). Each photosite has a tiny R(ed), G(reen), or B(lue) filter on top, which means that it carries information on just one component of the image at that point. Therefore, to be exact, a "20-megapixel" camera (note the quotes) actually collects about 20/3 = 6.7 megapixels of information, with each layer somewhat shifted from the others (actually, for most images this may be closer to 20/4 or 5 megapixels)>

One of the teasons for that difference is that in a Bayer matrix 50% of photosites are G, 25% R, and 25%; another — that the amount of information carried by a single RGB component depends on participation of that component in the total. I'm also putting aside the question of color depth of the recorded information, i.e., number of bits per photosite, which defines how many different light values per each photosite can your camera capture. Who said this has to be simple?

After a picture is taken, a translation process is applied to the original photosite information, translating it into pixels, i.e., points with full RGB information. This is usually done before the image is saved to a memory card, unless you're using the raw file format, in which case it is postponed to the stage of off-camera raw development.

A 20-megapixel camera using the "native" resolution creates a 20 MP file, placing a pixel at each photosite location (or rather "logical location" in the file). In our simplified model, to create an RGB pixel where, say, an R photosite is, the camera will use the actual R information from that place, while recreating the approximate values of G and B by interpolation and averaging (smoothing) from the appropriate neighbors collecting those colors.

This means that regardless of resolution, the captured image is always interpolated, period, and sensors are providing just one-third of the pixel count used to store images. You thought you had a 20-megapixel camera? How does "five megapixels" sound?

In-camera resolution downsizing

Having the built-in math to do the interpolation, the camera does not have to place your logical pixels where the original photosites are. If your 8 MP camera is saving your images as 2MP files, each of those two million pixels will be interpolated from the nearby original photosites as needed. Actually, you may have more pixels in file than photosites in the camera. This is what Olympus wasb doing in some of the C-series cameras, in their "ENLARGE SIZE" mode (in addition to slaughtering the English language).

in a number of early Fujifilm models, the pixel grid not only had more points than the photosite grid, but also was rotated at 45 degrees with respect to the latter.

In a good implementation (and I believe all major makers use that), the in-camera image resizing (like saving, say, 12-MP files) still uses the information from all photosites, but converts it into smaller number of pixels. If you are sure you will never, ever need more (and probably you won't), feel free to do that,, saving on storage space and ptocessing time, but I would, for a number of reasons, recommend against that: nowadays processors are fast, storage cheap (down by 10,000 times since 2002, if I remember right), and one day you may actually want to use a small crop from a frame with something you never expected, worth a closer look.

if you need to speed up loading of your images on the Web, adjust the size and compression of their Web version in postprovessing; many programs (including the FastStone, which I'm usually recommending as your first image manager) have an option to do that for multiple files at one time.

Sigma SA: three or ten megapixels?

The only cameras which use 1:1 pixel-to-photosite ratio are the SA-series SLRs made by Sigma. They are being made, abandoned, and revived periodically, so that I already lost count; therefore treat this just as a conversation item.

While they are using (for example) a 3MP grid, each node has three photosites placed on top of each other. This is 9 million photosites, and if the color layers were shifted, the market would recognize the Sigma as a 9MP camera. In reality, the amount of information captured in both cases is identical. Taking a Sigma image into your postprocessing program and enlarging it to 9MP should provide images of quality comparable to "nine-megapixel" cameras (quotes intentional!).

Well, that was the time when Sigma switched from three to ten megapixels in their promotional materials, without changing the sensor; what an inexpensive way to change the specs! On the other hand, this levels the playing field, so I don't think it is a misrepresentation.

The megapixel rat race

Unfortunately, the market is specs-driven, and people are used to "larger number means better" paradigm. More (of whatever it is) must be better than less! Recently I resd sbout a phone camera with 110 MP!

Pixel count might have been a limiting factor of image quality in the late 90's, when cameras were evolving from 0.3 MP to 1 MP and beyond. This is, however, history, except that the mass market never became aware of the fact.

Once you reach five or six MP of chip resolution (or rather that count of photosites), other factors are becoming more important than megapixels. That's why some professional-class, expensive cameras claim "only" five MP of resolution, while some very simple ones, aimed at an ignorant audience, sport 30 MP or more.

Increasing the pixel count may actually be the cheapest way to improve the on-paper specs of a camera, less expensive than better optics, or even better color handling (not limited to providing more bits per color).

The mass market is, however, largely ignorant, and manufacturers are using that to their advantage.

This, however, is another story. Here I wanted just to tell you that a megapixel is not really a megapixel, like camera makers would like us to believe.