Some objectives have the inscription Plan. What does that mean?
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 new planapochromats and new planfluorites, will give flat images even with eyepieces of field numbers (F.N. see below) up to 26.5; others up to F.N. 22.
