Definition: points per inch; pixels per inch: Measure of input resolution e.g. of scanning device, measured as the number of points on the object at which a sample or measure is taken or which are resolved by the device per linear inch on a given axis: e.g. 600ppi means six hundred points per inch, usually along the axis of the scanner sensor. Muddying the resolution waters While there is no doubt that, in general, a higher ppi figure indicates higher resolution, it is not necessarily the case that the image quality is higher. For one thing, the pitch at which a scanner steps may actually be smaller than the area sampled by each CCD. For another, resolution is not the only determinant of image quality -- dynamic range and linearity of tonal representation are vitally important too. Then there are software issues such as the bit depth at which the digitisation takes place. At the same time you have to be careful that a ppi rating is not an interpolated figure.

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