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Message: <blockquote data-quote="jring" data-source="post: 3584113" data-attributes="member: 133897">Hi,in binoculars with their very fast and short focal length objectives field curvature is quite pronounced (because of short focal length) and depth of field is very shallow (because of fast focal ratio) so in order to get edge to edge sharpness over a wide true field of view, field flatteners are very much necessary.In slower instruments with longer focal ratio, field flatteners might not be necessary at least for visual use. For deep sky astro imaging, flatteners are being used most of the time.Joachim</blockquote>