In the course of some preliminary experiments on a new type of resolving power target the phenomenon of spurious resolution has been encountered. It is shown that this false resolution can be explained by a geometrical analysis without recourse to diffraction theory. The intensity distribution in the out-of-focus image of a point source is calculated from spherical aberration data and is integrated over the target by the method of Schade. Representative exposures and microphotometer traces are included. It is pointed out that the plane of best contrast rendition is not necessarily the plane of best resolution. It is demonstrated that counting the number of lines in the image of a multiline target will not always detect spurious resolution.
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