Plan designates an objective that projects, at the eyepiece diaphragm plane, an image which is flat from edge to edge of the field of view. Some objectives, e.g. the W-PLAN / U-PLAN and W-PLAN F / U-PLAN F, will give flat images even with eyepieces of field numbers (F.N. see below) up to 25; others up to F.N. 22.

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