Microscope objectives usually have text printed on them, which can sometimes be confusing. The objective shown below is a
Meiji metallurgical microscope objective.
This is what the microscope objective inscriptions stand for:
- BD Plan EPI - this is a Brightfield / Darkfield Plan EPI objective lens. The BD means it can be used for both brightfield and darkfield microscopy. Plan EPI is a higher quality objective that has a flat field of view. You can read more about plan vs. semi-plan objectives here.
- 50x / 0.75 - the objective magnification is 50x and 0.75 is the numerical aperture. This is a number that expresses the ability of a lens to resolve fine detail in an object being observed. It is derived by a complex mathematical formula and is related to the angular aperture of the lens and the index of refraction of the medium found between the lens and the specimen. To get the best possible image, you should have a condenser system that matches or exceeds the N.A. of the highest power objective lens on your microscope.
- ∞/0 - The objective lens is infinity corrected and the 0 is the thickness in mm of the cover slip that was used in computing the corrections for the objective. The image distance is set to infinity, and a tube lens is strategically placed within the body tube between the objective and the eyepieces to produce the intermediate image. Infinity optical systems allow introduction of auxiliary components, such as differential interference contrast (DIC) prisms, polarizers, and epi-fluorescence illuminators, into the parallel optical path between the objective and the tube lens with only a minimal effect on focus and aberration corrections. Older finite, or fixed tube length, microscopes have a specified distance from the nosepiece opening, where the objective barrel is secured, to the ocular seat in the eyepiece tubes. This distance is referred to as the mechanical tube length of the microscope. The design assumes that when the specimen is placed in focus, it is a few microns further away than the front focal plane of the objective. Finite tube lengths were standardized at 160 millimeters during the nineteenth century by the Royal Microscopical Society (RMS) and enjoyed widespread acceptance for over 100 years. Objectives designed to be used with a microscope having a tube length of 160 millimeters are inscribed with this value on the barrel.
- F=200 - the objective focal length of the tube lens is 200mm.
- WD 0.35 - the working distance of this objective lens is 0.35mm. This is the distance that must be between the tip of the objective lens and the specimen in order for the specimen to be in focus when looking through the microscope eyepieces.
If an objective is an oil immersion objective lens, it will typically have "oil" printed on the objective.
