So why when most of the sensors are far beyond 8 bit depth, we still have image files with size very close to the number of mega pixels we have on the camera?
To start with, the sensor doesn't output any color. Each pixel only records a single value: how much light struck the sensor. The number of bits determines how fine the steps between each brightness level can be. That's why a 12-bit or 14-bit file can record much finer gradations of lightness than an 8-bit file.
But raw files are also compressed, just normally in a lossless manner. If there are fewer unique values from all of a sensor's pixel wells the data can be compressed smaller than if there are more of the 2^12 or 2^14 possible tonal values for each pixel. Raw files from my 24MP camera generally run anywhere from 22MB to 29MB each depending on the content. Some cameras even use lossy compression to store raw files.
The way color is derived is by filtering each pixel for one of three colors: Red, Green, and Blue. But all that is measured on the other side of the filter by that pixel well is how much (i.e. how bright) light was allowed to pass through the filter. The filters still each let some light through that are colors other than the exact color of the filter. The further a color is from the color of the filter, though, the less amount of that color falling on the filter will make it through and be recorded by the pixel well. Some green gets past both the red and blue filters. Some red and blue get past the green filter. By comparing the difference in brightness of adjacent and surrounding pixels filtered for different colors the process known as debayering or demosaicing can interpolate an R, G, and B value for each pixel. Only after the color has been interpolated will the value of each color for each pixel be stated using 8-bits per color for 24-bits per pixel. In the case of JPEG this data will also be compressed. Basically JPEG designates which pixels are all the same exact combination of all of the different combinations of R,G, & B contained in the image. That is why images that are mostly the same uniform colors can be compressed smaller than images that have almost every possible combination of colors.
If you output a 28-30MB raw file from a 24MP camera after debayering it into a 16-bit TIFF the file will very likely be over 100MB in size because it is recording 16-bits for each of three colors for each pixel.
Nearly all image file formats worth speaking of are compressed in some way. The mechanism for doing this varies depending on the file format you're using - but the files which you see will have been compressed. This is also why not all the files are the same size, despite having the same number of pixels in them.
Sensors are typically 12 bits of raw data per pixel, which is 1.5 bytes per pixel. (Some offer 14 bits, 1.75 bytes per pixel.) However, the data is typically compressed smaller in the file, which is STILL 12 or 14 bit data, but the compression result conceivably perhaps might crudely approximate one byte per pixel then (smaller than 1.5 bytes anyway, in the file).
But MB is NOT 1*10^6*byte. Million is, and mega is properly a million, however MB is treated special (not per mega definition), and MB is 2^20 bytes, or about 1,048,5576 bytes (so size in MB is about 5% less than size in millions). Megapixels properly use millions, but bytes usually use 2^20, which is 1024x1024.
When raw is interpolated into RGB, for example to JPG, then JPG is 8 bits per color, so then three bytes per pixel (when RGB and JPG).
Nop, your initial math is incomplete. Let me explain the math of uncompressed files.
1 Mpx square photo. Ok.
8 bits = 1 byte. Ok.
If the image were only grayscale yeap, the image would measure arround 1Mb. But an RGB file it is arround 3Mb because it has 3 channels. Prepare one image 1000x1000 and save it as 24 bit BMP format. 2.93Mb.
But that is for an uncompressed image.
Save the exact blank image in some other formats. In PNG will measure... 23k. 1/123 of the size of the uncompressed BMP file.
Compressed image's file weight has nothing to do with that math. It has to do with:
The file size (width and height)
The bit depth
The compression algorithm
The content of the image itself. A flat white square compresses very efficiently, but a photo of a landscape with a lot of trees and a house wil not do as eficient.
If the file supports it, other layers, or channels.
