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The pixel output of the WFC can correspond to the full diameter field of view (ZOOM off), or can be ``zoomed'' so as to correspond to only the central region of the field (ZOOM on). The angular size of each pixel is thus halved when the ZOOM is on. Observations should only be made with ZOOM on when there is a scientific reason for requiring the best possible spatial sampling in the WFC data (e.g. for extended sources with high surface brightness). All other observations, including those made with the pointed mode filters (see table 5.3), should be performed with ZOOM off. Note that different advice was given in the AO-1 documentation.