The size of a file saved to a camera’s memory or transferred to a computer is related to the amount of information captured in the image by the camera., The camera image information capacity is usually called the resolution of the camera.
Most current cameras have resolution ratings measured in terms of the picture element, or pixel. For cameras available in the astronomical market, resolutions are in the range from 0.3 megapixels (MP) up to about 30 MP. Scientific astroimaging cameras have resolutions up into the gigapixel range.
A higher resolution in general means that the camera’s chip delivers information subdivided into more pixels, each of which effectively acts as an individual light sensor, detecting levels of illumination and colors. Size does matter and you should choose a camera that has high resolution and therefore captures more information, all else being equal.
The resulting file size in megabytes in digital memory can be larger or smaller than the resolution of a digital image in megapixels. So a 12 MP camera might produce an 18 MP raw image file or a 4 MP jpg image file. The differences result from: (1) the image is processed either by the camera or the chip itself to eliminate spurious signals, noise and the like; (2) because the image is compressed for easy use for viewing on computers, sending in e-mails, etc.
A simple way to think of resolution and a camera’s recording ability is in terms of the array size of pixels delivered for a given resolution. For example, a camera that records a 640x480 (equals 307,200) array of pixels is usually described as a 0.3MP resolution camera. Resolution is also often related to display on computer monitors and their widths and heights in pixels. Common abbreviations for resolutions are VGA (0.3MP and 640x480). SVGA (0.5MP and 800x600), XGA (0.8MP, 1024x768) and SXGA (1.3MP 1280x1024).
Video cameras frame recording abilities are also rated in terms of resolution. For example, HD video is most commonly 1920x1080 pixels or individual frames of 2.1MP resolution.
(NOTE: Resolution for a digital camera is not the same as the angular resolution for an optical system. They can be related through the physical pixel size and the image scale of the camera-lens system you are using.)
Most cameras have the ability to combine or “bin” information from pixels at lower resolution as well, limiting the information they record and resulting in smaller file sizes. This might be done for image processing reasons, to match camera resolution with the imaging system’s angular resolution or image scale or to permit more images to be saved by the camera memory.