Microscope objectives are grouped under two conjugate types: finite and infinite (infinity
corrected).
In a finite conjugate optical design, light from a source (not at infinity) is focused down to a spot. For
a microscope, the image of the inspected object is magnified and projected onto the eyepiece (or sensor
if using a camera). The particular distance through the system is characterized by either the DIN
or JIS standard; all finite conjugate microscopes are either one of these two standards. These types
of objectives account for the majority of basic microscopes, such as general inspection or assembly
systems. Finite conjugate designs are used in applications where cost and ease of design are major
concerns. Additionally, these objectives are typically used for brightfield techniques only.
In an infinite conjugate, or infinity-corrected optical design, light from a source placed at infinity
is focused down to a small spot. In an objective, the spot is the object under inspection and infinity
points toward the eyepiece (or sensor if using a camera). This type of modern design utilizes an additional
tube lens between the object and eyepiece in order to produce an image. Though this design
is more complicated than the finite conjugate, it allows for the introduction of optical components
such as filters, polarizers, and beamsplitters into the optical path. Fluorescence microscopy applications
typically utilize this type of design. Infinity corrected objectives often incorporate multi-element
designs and correct for a number of optical errors such as flatness, chromatic aberration, spherical
aberration, and polarization.
The quality of an objective and eyepiece determine how well the system performs. Understanding
quality correction is extremely important when deciding on the type of objective to use. Quality corrections
are often denoted on the objective itself to allow the user to easily see the design of the objective
in question. There are typically three levels of chromatic and spherical aberration corrections:
achromatic, fluorite and apochromatic. Achromat objectives are among the simplest and least expensive
objectives. Color correction occurs at the red and blue wavelengths, while spherical aberrations
are accounted for at a green wavelength. The limited corrections make these objectives better suited
for general and monochromatic applications. Fluorite (or Fluor) objectives are constructed using
advanced glass types containing the mineral fluorite (or fluorspar), hence the name, or now often with
synthetic materials instead. The different glass types used in Fluorite objectives allow for improved
chromatic and spherical aberration corrections for at least 2 colors. They also often feature higher
numerical apertures and resolving powers than achromat objectives, delivering brighter images and
better contrast in color imaging applications. Apochromat objectives provide the highest level of
corrections. Their chromatic aberration corrections are for at least 3 colors (red, blue and green) and
the spherical aberration corrections are for at least 2 to 3 colors, making them ideal for white light
applications. They tend to have longer working distances and the highest numerical apertures. Note
that many of the newer high-performance Fluorite and Apochromat objectives often have even more
corrections, correcting for 4 or more colors chromatically and 4 colors spherically. All three objective
designs, however, suffer significantly from distortion and field curvature, which worsen as objective
magnification increases.
Field curvature is a type of aberration present when the off-axis image cannot be brought to focus in a
flat image plane, resulting in a blurred image as it deviates from the optical axis. There are two levels
of quality designation for field curvature correction: Plan (or Planar) and Semi-Plan (or Semi-
Planar and Microplan). Objectives with no field curvature correction have a flat field in the center
65% of the image. Semi-plan objectives feature 80% of the field appearing flat. Plan objectives achieve
the best overall correction and display better than 90% of the field flat and in focus.
Inside Microscopy
Eye Point
Eyepiece
Eye Lens
Field Lens
Real Image
Plane
OTL
(150mm for DIN)
(146.5mm for JIS)
PD
(45mm for DIN)
(36mm for JIS)
Objective Lens
Working
Distance
MTL
(160mm for
DIN/JIS)
Object
Key:
Total Magnification = x
Field of View =
NA=n sinθ
(air, n=1)
θ
Objective
Power
Eyepiece
Power
Eyepiece Field Stop Diameter
Objective Power
Figure 1
Infinity Corrected Objectives Optical Aberration Correction
Type of Objective Spherical Aberration Correction Chromatic Aberration Correction Field Curvature Correction Applications
Achromat 1 Color 2 Colors No Monochromatic applications
Fluorite 2+ Colors 2+ Colors No Polychromatic applications
Apochromat 3+ Colors 3+ Colors No White light applications
Plan Achromat 1 Color 2 Colors Yes Monochromatic applications
Plan Fluorite 2+ Colors 2+ Colors Yes Polychromatic applications
Plan Apochromat 3+ Colors 3+ Colors Yes White light applications
288 +44 (0) 1904 788600 | Edmund Optics® N NEW PRODUCT NEW LOW PRICE
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TECHNICAL NOTE
Infinity Corrected Objectives
Edmund Optics® Objectives........................289
Mitutoyo Objectives............................290-293
Olympus Objectives.............................294-296
ZEISS Objectives.........................................297
Nikon Objectives.................................298-299
Infinity Video Microscopes
and Objectives......................................299-301
Finite Conjugate Objectives.....................302-304
Eyepieces & Microscopes.........................305-310
Microscopy Camera.................................311, 393
Understanding Microscope Objectives
The total magnification of a compound microscope is calculated by multiplying the magnifications of two independent optical systems: the objective and
eyepiece. The objective, which is placed close to the object sample, provides a magnified, inverted real image that is oriented left to right. The eyepiece
then creates a magnified virtual image of the real image created by the objective. By combining the objective and eyepiece, the microscope creates a virtual
inverted image that is focused onto the observer’s retina (Figure 1). Unlike magnifiers, which use only a single optical system, microscopes allow the insertion
of a reticle between the objective and eyepiece.
Industry Standards of Infinity Corrected Objectives Note: Values are typical
Specification Edmund Optics® Mitutoyo Olympus ZEISS Nikon
Mounting Threads M26 x 36 TPI M26 x 36 TPI RMS M27 x 0,75 M25 x 0,75
Typical Tube Length 200 mm 200 mm 180 mm 165 mm 200 mm
Typical Parafocal Length 95 mm 95 mm 60 mm 45 mm 60 mm
Magnification Range 2X to 100X 1X to 200X 4X to 100X 2,5X to 100X 2,5X to 100X