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This paper gives preliminary results of a continuing experimental study of factors affecting the precision of centering black circular measuring marks in sharp, high-contrast targets with homogeneous backgrounds, subtending visual angles up to 45 minutes of arc, in photopic vision. The results support the proposition that adjacency effects at edges contain significant visual information, and this could appear to be important where visual settings are being made by bringing geo-metrical configurations into close relationship with one another. The maximum information content for the centering task investigated was contained in ribbons approximately 1 minute of arc wide around the light areas of target and measuring mark. The most precise pointings were made by selecting a measuring mark to give a minimum annulus width within these ribbons irrespective of the target size. The results support the concept of a retinal zone approximately 4 minutes of arc in diameter over which acuity is constant, but further suggest that this may be dependent on the type of task involved. The acuity in the horizontal retinal meridian was some 30% greater than that in the vertical meridian for annulus widths up to 4 minutes of arc. Whilst the results were obtained for a restricted set of circumstances, they will be discussed in the broader context of measurement to objects on photographs in an attempt to relate the accuracy attainable to the image quality parameters of the photographic system as it affects the rendition of edge gradients on photographs.
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