Overview
ISO 19012-1:2013 - "Microscopes - Designation of microscope objectives - Part 1: Flatness of field/Plan" defines how the “Plan” designation is applied to microscope objectives and how to quantify the flat region of the primary image of a flat object surface. The standard applies to visual observation using the objective, tube lens and eyepiece combination as specified by the manufacturer, and is consistent with ISO 8578.
Key topics and technical requirements
- Plan marking and indication
- Objectives branded Plan (or described as “flat field”) must also display the objective field number (OFN) on the objective body (exemption for lenses sold before 2014).
- OFN examples: 18, 19, 20 … 30 etc. (e.g., OFN25).
- Definition of Plan objectives
- A Plan objective must have a plan field ratio (PFR) ≥ 0.85, where PFR = PFN / OFN.
- Plan field number (PFN)
- PFN specifies the diameter (mm) of the sharp region of the primary image of a flat object surface.
- PFN is determined as the maximum field of view of the primary image that meets flatness tolerances defined by the standard.
- Flatness criteria
- The standard uses the distances of the tangential and sagittal image surfaces from the image plane to compute average image surface distance (Δ) and astigmatic difference. Both |Δ| and astigmatic difference must be ≤ the depth-of-focus δ calculated by Berek’s formula.
- Determination assumes a 10× eyepiece for the visual assessment.
- Optical aberrations
- The Plan designation addresses field flatness and does not imply correction for other aberrations (see ISO 10934-1). The 2013 edition explicitly includes Petzval curvature in its considerations.
- Berek’s formula and depth of field
- Annex A provides Berek’s formula to compute δ (depth of focus/field) from parameters such as total visual magnification, numerical aperture (NA) and wavelength.
Applications and who uses this standard
- Microscope manufacturers - labeling objectives and specifying field performance.
- Optical designers - designing and verifying flat-field objectives (Plan).
- Quality assurance and calibration labs - testing PFN/PFR and compliance.
- Purchasing/specification engineers and lab managers - selecting objectives for photomicrography, pathology, materials inspection or any application requiring a uniformly sharp field.
- Standards bodies and educators - harmonizing terminology and training on objective designation.
Related standards
- ISO 8578 - marking of objectives and eyepieces
- ISO 10934-1 - vocabulary for microscopy (light microscopy)
- ISO 19012-2 - Chromatic correction (companion part)
Keywords: ISO 19012-1:2013, Plan objectives, flatness of field, microscope objectives, plan field number (PFN), objective field number (OFN), plan field ratio (PFR), Berek’s formula, depth of field, Petzval curvature, astigmatism.