Depth of Field
Definition
Axial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound systems.
- High in
- Stereoscopic microscopes
- Very low in
- Oil-immersion compound microscope systems
- Relationship to NA
- Decreases sharply as numerical aperture increases
Common questions
What exactly is depth of field in a microscope?+
Depth of field is how thick a slice of the specimen appears in sharp focus at any given moment. It measures the axial (top-to-bottom) thickness of the specimen that looks acceptably sharp. As numerical aperture increases, this thickness drops sharply.
Why does depth of field matter in forensic microscopy?+
In forensic work like fiber or glass analysis, shallow depth of field forces you to focus plane-by-plane to examine all features of a specimen. Stereoscopic microscopes keep more of the sample in focus at once, which is why they are preferred for detailed visual inspection. Oil-immersion compound systems have very shallow depth of field and demand careful technique.
How is depth of field calculated?+
Depth of field is approximately equal to n times the wavelength of light, divided by the numerical aperture squared. In plain terms, as the numerical aperture (NA) goes up, the depth of field drops sharply. This is why high-power objectives let you see fine detail but require more careful focusing.
Related terms
- 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...
- 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...
- Comparison Microscope
- Two compound microscope bodies connected by an optical bridge that presents one half of each body's image in a single split eyepiece...
- EDX / EDXRF
- Energy-dispersive X-ray detector attached to an SEM. Identifies elements from characteristic X-rays emitted when the beam strikes the sample. Backbone of GSR...
- Empty Magnification
- Magnification beyond approximately 1000 times the objective NA; enlarges the image without adding resolvable structural information, producing blurred detail that can be...
- Fluorescence Microscope
- Uses UV or short-wavelength excitation to make fluorophores emit longer-wavelength light. Used for semen, saliva, fibres and ink discrimination.
- 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...
- Parfocality
- The property of an objective nosepiece turret such that switching objectives keeps the specimen approximately in focus without re-focusing; defined by ISO...
- Plan Objective
- An objective corrected for field curvature so that the entire image field is simultaneously in focus; required for photomicrographic evidence documentation to...
Explained in these topics
- Microscopy Fundamentals: Magnification, Resolution and NAThe axial range over which the specimen appears acceptably sharp, approximately equal to n lambda / NA squared. Decreases sharply as NA increases.
- Microscopy for Forensic Science: Polarizing, Comparison, Stereoscopic, Fluorescent and ElectronAxial thickness of the specimen that appears acceptably in focus at one time. High in stereoscopic microscopes, very low in oil-immersion compound systems.