Plan Objective
Definition
An objective corrected for field curvature so that the entire image field is simultaneously in focus; required for photomicrographic evidence documentation to avoid misleading sharpness gradients across the image.
- Correction applied
- Field curvature
- Effect
- Entire image field in focus simultaneously
- Forensic use
- Photomicrographic documentation of trace and fibre evidence
- Contrast with
- Standard objectives, which are sharp only at the field centre
Common questions
Why does field curvature matter specifically for photographic documentation?+
A visual observer can refocus by eye as they scan across the field, but a single photograph freezes one focus plane, so an uncorrected objective would show the centre sharp and the edges blurred in a way that could misrepresent the evidence.
Does using a plan objective change the magnification or resolution of the image?+
No, the field-flattening correction addresses focus uniformity across the field, not the magnification power or resolving ability of the objective, which are set by its numerical aperture and focal length as with any other objective.
Related terms
- Abbe Diffraction Limit
- The minimum period of a periodic grating structure that can be resolved: d = lambda / (2 NA). Sets the hard optical...
- ASTM E766
- The ASTM International standard practice for image-scale calibration in microscopy, extended by OSAC to govern optical microscope calibration in US forensic trace-evidence...
- Depth of Field
- Axial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound...
- Empty Magnification
- Magnification beyond approximately 1000 times the objective NA; enlarges the image without adding resolvable structural information, producing blurred detail that can be...
- Infinity-Corrected Optics
- An objective design in which the back focal plane produces a collimated (afocal) beam rather than a converging beam, allowing optical elements...
- ISO 8576
- The ISO standard governing the accuracy of stage micrometers and reticle eyepieces, specifying Class I and Class II tolerance levels traceable to...
- Koehler Illumination
- The standard illumination method for research microscopy in which the light source is focused at the condenser back focal plane, producing uniform...
- Numerical Aperture (NA)
- The product of the refractive index of the medium between objective and specimen and the sine of the half-angle of the maximum...
- Parfocality
- The property of an objective nosepiece turret such that switching objectives keeps the specimen approximately in focus without re-focusing; defined by ISO...
- Rayleigh Criterion
- The condition that two point sources are just resolved when the central maximum of one Airy disc coincides with the first minimum...
- Stage Micrometer
- A calibrated glass slide with a ruled scale of known spacing (typically 0.01 mm divisions), used to establish the pixels-per-micrometre calibration factor...