Finite and infinity corrected optics

Finite Optical Systems

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	182.5
Deutsche Industrial Normen (DIN)	160mm	45mm	RMS	195

Most currently manufactured finite optical systems use the DIN system, although "student" versions of some Chinese objectives use a 160mm tube length with a 185 conjugate distance (35mm "short barrel" objectives).

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

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.

Links

Nikon's Description of Infinity Optical Systems

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