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Norsk språk| Optical | Name | Parfocal | Tube Length | Screw | Application |
| Apo | M Plan Apo | 95 | 200 | M26x0.706 | Industrial/Visible Light |
| Apo | M Plan Apo NUV | 95 | 200 | M26x0.706 | Industrial/Near UV |
| Apo | M Plan Apo NIR | 95 | 200 | M26x0.706 | Industrial/Near Infrared |
| Apo | TU Plan Apo | 60 | 200 | M25x0.750 | Industrial |
| Semi-Apo | TU Plan Fluor | 60 | 200 | M25x0.750 | Industrial |
| Apo | TU Plan Apo BD | 60 | 200 | M32x0.750 | Industrial/Dark Field |
| Semi-Apo | TU Plan Fluor BD | 60 | 200 | M32x0.750 | Industrial/Dark Field |
| Semi-Apo | L Plan Fluor BD | 60 | 200 | M27x0.750 | Industrial/Dark Field |
| Plan | M Plan | 45 | 180 | M20.32x0.706 | Industrial |
Besides the marks on the objective or specification table of microscope, there are several important "hidden" parameters of microscope objective, which you need to know: Parfocal, Tube Length, and Screw
Parfocal Distance
The Parfocal Distance refers to the distance from the threaded mounting end face of the objective lens (or the lens body's positioning surface) to the top surface of the specimen coverslip or the specimen surface itself. Its core function: when switching between objective lenses of different magnifications, the specimen's focal position remains essentially unchanged, requiring only minor focusing adjustments to obtain a sharp image.
Objectives from the same brand and product series typically maintain a consistent parfocal distance to ensure interchangeability.
International Standard Specifications
According to national standards and industry conventions, parfocal distance primarily follows these standard values:
|
Standard
|
Application Scenario | Description |
| 35 mm | Some legacy or specialized microscopes | Early standard, now rarely used, only in small toy level microscope use |
| 45 mm | Biological microscopes, mainstream research-grade microscopes | Most widely adopted international standard; used by major brands like Olympus, Nikon, Leica |
| 60 mm | Industrial inspection, long-working-distance objectives | Suitable for scenarios requiring greater operational clearance |
| 95 mm | Specialized industrial applications, large-scale equipment | Less common; used for specific custom systems |
-
Chinese National Standard: GB/T 2609-2006 Microscopes – Objectives specifies parfocal distances of 35 mm, 45 mm, or 60 mm
-
International Standards: ISO 9345 and RMS (Royal Microscopical Society) thread standards, commonly paired with a 45 mm parfocal distance
Why Is Parfocal Design Important?
-
Improved operational efficiency: Minimal refocusing needed when switching objectives—ideal for frequent lens changes in pathology screening, teaching demonstrations, etc.
-
Imaging continuity: Prevents loss of the observation target due to focal shift
-
Compatibility requirement: Mixing objectives from different brands with mismatched parfocal distances may cause focal offset; use with caution
Frequently Asked Questions
Q: Parfocal Distance ≠ Working Distance
A: Correct! Working distance is the actual physical distance from the front lens element to the specimen surface, which varies with magnification; parfocal distance is a mechanical mounting standard that remains constant within a given objective series.
Q: How can I determine the parfocal distance of my microscope?
A: Check the markings on the objective barrel, consult the product manual, or measure the distance from the 40x objective mounting flange to the top of objective lens, see if it is 35mm, 45mm, 60mm, or 95mm.
Tube Length
Tube Length in microscopy, which encompasses two distinct concepts: Mechanical Tube Length and Optical Tube Length.
Core Definitions
|
Type
|
English Term | Definition |
| Mechanical Tube Length | MTL | The physical distance from the objective mounting flange (nosepiece flange) to the top of the eyepiece tube (eyepiece seat) |
| Optical Tube Length | OTL | The optical distance from the rear focal plane of the objective to the front focal plane of the eyepiece (intermediate image plane) |
Simple analogy: Mechanical tube length is the "hardware dimension"，used for finite optical systems, while optical tube length is the "optical path design", used for infinity optical systems.
Finite-Corrected System & MTL
This is the design used in traditional microscopes, where the objective directly focuses light onto the intermediate image plane, using Mechanical Tube Length (MTL) to design..
|
MTL
|
Application Region/Brand | Description |
| 160 mm | International standard (RMS standard) | Recommended by the Royal Microscopical Society (RMS); adopted by the vast majority of biological microscopes |
| 170 mm | Early Leitz (predecessor of Leica) | Historical standard, now largely obsolete; not compatible with 160 mm systems |
| 180 mm | Some legacy American brands (e.g., Bausch & Lomb) | Used in the late 19th century |
| 250 mm | Metallurgical microscopes (reflected light systems) | Special standard for industrial inspection applications |
Infinity-Corrected System & OTL
The mainstream design for modern high-end microscopes. Objectives output parallel light rays, which require a Tube Lens to form an image. It will use Optical Tubel length (OTL) in design.
|
Brand
|
OTL | Parfocal Distance | Objective Thread |
| Nikon (CFI60) | 200 mm | 60 mm | M25 |
| Leica | 200 mm | 45 mm | M25 |
| Olympus/Evident | 180 mm | 45 mm | RMS |
| Zeiss (ICS) | 165 mm | 45 mm | RMS |
Magnification Calculation
-
Finite system: Objective Magnification = Mechanical Tube Length ÷ Objective Focal Length, e.g. 160mm ÷ 4mm =40x
-
Infinity system: Objective Magnification = Tube Lens Focal Length ÷ Objective Focal Length (e.g., 200 mm tube lens ÷ 10 mm objective = 20×)
Frequently Asked Questions
Q: How can I tell if my microscope uses a finite or infinity-corrected system?
Check the markings on the objective barrel:
-
Marked with "160" or "170" → Finite-corrected system
-
Marked with "∞" symbol → Infinity-corrected system
Q: Can finite and infinity-corrected objectives be used interchangeably?
Not recommended:
-
Using an infinity objective in a finite tube: Lacks optical tube length design, resulting in no image formation or severe spherical aberration.
-
Using a finite objective in an infinity system: Loss of numerical aperture, reduced resolution, and loss of parfocality.
Thread
The microscope objectives from different suppliers may have different install thread and screw pitch to install the objective to nosepiece.
RMS - Most Popular Thread
Up to 1858, to resolve the incompatibility between objectives from different manufacturers and microscope bodies, the Royal Microscopical Society introduced a mechanical standard called the RMS objective thread. It was once the universal specification for the interfaces of optical microscope objectives and remains widely used to today.
This thread standard was commonly written as RMS (0.800"-36) or (WJ4/5"x1/36"). The thread size is 20.32mm (0.8 inches) and it has the following specifications:
-
Install Diameter: WJ4/5"=20.32 mm
-
Screw Pitch: 1/36"=0.706 mm
Commonly Thread Standards
RMS M20.32 x 0.706 (WJ4/5"x1/36"), Olympus, Zeiss, and most Biological microscopes use M25 M25 x 0.750, Leica, Nikon Biological microscope use M26 M26 x 0.706, Olympus, Zeiss, Mitutoyo Industry microscope use M32 M32 x 0.750, Leica, Nikon Industry microscope use M48 M48X0.7, for stereo microscope lens
Olympus Biosystems and most brands in market primarily use the RMS M20.32 standard. Leica and Nikon Biosystems employ other metric threads such as M25×0.750,
Olympus Industrial Systems use M26×0.706. Leica and Nikon Industrial systems, on the other hand, utilize M32×0.750.
The tooth profile angle of threads of different standards varies. The profile angle of British thread is 55 °, and the profile angle of standard metric thread is 60 °. This parameter has no impact on installation, but the thread size needs to be consistent.
Thread Convert Ring
When the thread size of the objective lens does not match the thread of the microscope objective converter, the installation of the objective lens can also be completed by using an objective lens thread variable diameter ring. For example, the thread outer diameter of the M26-RMS variable diameter ring is RMS, and the thread inner diameter is M26, so that the M26 threaded objective lens can be installed on the RMS threaded aperture objective lens condenser.
It is usually recommended not to use an adapter ring to switch the same objective lens between the two manufacturers, since the quality of the images may not be satisfactory. Please consult a sales engineer when selecting.
Practical Advice For Users
-
Do not forcefully tighten: The RMS thread is relatively fine (36 TPI) and very precise. When installing the objective lens, feel a slight resistance when turning it to the bottom. Do not use pliers or excessive force, otherwise it may cause thread slippage and even damage the calibration of the internal lens group of the objective lens.
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Keep the threads clean: Dust or oil stains sticking to the threads are not only difficult to clean, but may also change the installation height of the objective lens, affecting the alignment. Regularly blow clean the converter holes with air.
-
Confirm parameters when purchasing replacement parts: If you want to equip an old microscope with a new objective lens, in addition to confirming the optical system and defocus distance, be sure to confirm the thread size of the objective lens converter.
Huang Xin