Finite and infinity corrected optics

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Finite Optical Systems[edit]

The Royal Microscope Society ("RMS") intially defined the physical parameters for a standard microscope system, including and objective diameter and thread pitch and a tube length of 170mm. Other finite standards tend to use RMS threads.

Finite Train Optical Standards
Standard Tube Length Parfocal Distance Threads Conjugate Distance
Royal Microscope Society (RMS) 170mm 31mm RMS
Japanese Industrial Standards (JIS) 170mm 36mm RMS 182.5mm
Deutsche Industrial Normen (DIN) 160mm 45mm RMS 195mm
Current Chinese "student" standard 160mm 35mm RMS 185mm

Most currently manufactured finite optical systems use the DIN system, although "student" versions of some Chinese objectives use a 160mm tube length with a 185mm conjugate distance (35mm "short barrel" objectives). Since both 185mm and 195mm conjugate distance objectives are both marked as with a 160mm tube length, a common problem when buying objectives from China is purchasing a 185mm objective when your microscope is set up for 195mm conjugate distances. While adapters can be purchased to make 185mm objectives work with a 195mm system, it is best to find out beforehand whether the objective is short barreled or not, if it is not clearly advertised.

Microscopes conforming to the same optical standard are compatible with each other in the sense that they fit together and magnify properly, but systems from one manufacturer may apply different corrections for optical aberrations in the eyepiece than what another manufacturer provides. In a DIN system, it is, therefore, important that eyepieces be properly matched to the objectives. This reduces the opportunity to interchange components from different manufacturers without loss of quality.

Infinity Corrected Systems[edit]

Infinity corrected optical systems have objectives that produce a flux of parallel light that must be focused by a secondary (telan) lens, normally located in the microscope head, before going to the eyepiece. The distance between the objective and telan lens is not critical, which allows the insertion of other devices such as polarization films, DIC prisms, fluorescence cubes, and other items in the space between the objective and telan lens (infinity space) without disrupting the optical relation between the objective and eyepiece.

Infinity corrected objectives are not compatible with finite optical systems because the latter lack a telan lens. The big four manufacturers no longer produce finite compound biological microscope systems. Each of the big four microscope manufacturers has chosen a different set of parameters for their infinity system, so an objective intended for one system may not work well in another. In addition to the physical parameters of the system, manufacturers vary in where they correct for lateral chromatic aberration.

Infinity Corrected Optical Train Standards
Manufacturer Reference Focal Length Parfocal Distance Threads Optical Correction Location
Leica 200mm 45mm M25 Objectives and telan lens
Nikon 200mm 60mm M25 Objectives
Olympus 180mm 45mm RMS Objectives
Zeiss 164.5mm 45mm RMS Objectives and telan lens

Other manufacturers may use different standards, or variations of these. Many current Chinese manufacturers conform to the Olympus standard in their basic characteristics. In fact, the Zeiss PrimoStar line (manufactured by the Chinese company Motic) uses a 180mm reference focal length.


Nikon's Description of Infinity Optical Systems

Nikon's explanation for why they chose the standard they did