Camera Resolution, Print Resolution, Color Depth, File Size

These often confuse people.  It's good to nail down these basic terminology so that we can all speak the same language.

* Camera Resolution:  measured in megapixels, is how many pieces your camera's image sensor divides up the image (into picture elements, or pixels).  This has nothing to do with color depth, even though it affects file size.  For example, the Canon EOS 5D Mark II's image sensor has a physical dimension of 24mm x 36mm;  it divides the image up into 3744 rows and 5616 columns (each of the row/column unit is a pixel).  That's 3744p x 5616p = 21,026,304 pixels = 21 megapixels.

* Color Depth:  measured in bits per pixel or bits-per-channel per pixel, is how each pixel is represented digitally.  For sRGB Color Space, the color depth is 24 bits (per pixel), or 8 bits per channel (Red/Green/Blue) (per pixel).  8 bits per channel is typical for a lot of photo processing, where each pixel is represented digitally as a number from 0 to 255 for each channel (Red, Green, and Blue).  A 0 value means no color for that channel;  a 255 means a pure color, for that channel.  Thus, the total number of colors that can possibly be represented by the 8-bit-per-channel (or 24-bit total) scheme is therefore about 16 million.  Canon DIGIC image processor outputs images that are 14 bits per channel (total 42 bits per pixel).  (More on color spaces later.)

* File Size:  measured in bytes, is the size of the image file on the computer.  This depends on  1) camera resolution, 2) color depth, 3) image file format, and 4) image compression.  For example, for a BMP file format (an uncompressed format), an image of 3744 pixels by 5616 pixels with a color depth of 24 bits (3 bytes), the file size will be 3744p x 5616p x 3bytes = 63,078,912 bytes = 61,600.5 KB = 60.16 MB.  (1 KB = 1024 bytes;  1 MB = 1024 KB.)

* Print Resolution:  measured in pixels per inch, determines how large (the maximum size) your image can be in print, without interpolating.  For example, I use the Costco Photo Center at NE San Jose;  it uses the Noritsu QSS-3411 Photo Printer that has a print resolution of 320 pixels per inch.  If I sent my image file of 3744p x 5616p (full size file) to that printer, the maximum print size that I can get without interpolating would be (3744 / 320 =) 11.7 inches in width and (5616 / 320 =) 17.6 inches in length.  And yes, if you plan to print your photo at a printer, you should know its print resolution.  For most people, the final prints (if they ever print their images) are usually 4" x 6";  therefore, the camera resolution needed is only about 2.5 megapixels (4 x 320 x 6 x 320);  that's what I set my camera to, when I do most of my day-to-day photo-taking.