Resolution
Resolution is determined by the number of pixels used to describe the image. It depends on how the image is constructed, be it by scanning or directly generated by an artist, or how it is edited or scaled. It is usually measured in ppi (pixels per inch) or dpi (dots per inch).
Higher resolution maintains definition and sharpness.
Photographic systems can resolve detail down to about .01 mm (.0004 in.) The eye may be able to resolve detail down to .001 mm.
At very high spatial frequencies the eye is unable to resolve color information. Thus, there is a trade-off between the spatial frequency and the number of bits needed to specify color.
Thus, the smaller the pixel the harder it is to see its color. A 300-dpi pixel requires 24 bits to describe it while only 2-bits are required for a 3000 dpi pixel (i.e. the eye only recognizes its existence).
Resolution of an image can be changed with image editing software like Photoshop. It may be resized with fixed number of pixels (this doesnt change the inherent resolution of the image only the units with which to measure the pixels) or scaled up or down (increasing or decreasing the number of pixels).
When decreasing the number of pixels an averaging is performed, while new pixels must be calculated by an interpolation method when increasing the number of pixels.
The resolution of the output device should be considered when choosing the resolution for the image file. A PostScript imagesetter or printer generates a matrix of spots (device pixels) in order to represent the image.
When discussing resolution, scan resolution is usually reported in ppi (points per inch), output device resolution in dpi (alternatively elements per inch or spots per inch) and halftone screen frequency (see below) in lpi (lines per inch).