Objective, close to the observed objects, is installed in the bottom of the lens cone. As the important component, which decides the imaging quality and the resolution, it is used to make the first amplification of the object. Objective is usually marked with several key parameters like Numerical aperture, Magnification times, Lens barrel length, Focal length, etc. Such as: NA 0.30; 10×; 160/0.17; 16㎜. And “ NA 0.30” for Numerical Aperture (NA), “10×” for Magnification times, “160/0.17” for the length of Lens barrel and the thickness of the cover glass, “16㎜” for Focal length.
OBJECTIVE
Objective, as the important component, is installed in the bottom of the lens cone. For its close to the observed objects, it has the name as Lens or Objective.
1. The Classification of Objective
For the difference of using conditions, objective can be classified as Dry Lens and Immersion Lens. And Immersion Lens can be further classified as Water Immersion Objective and Oil Immersion Objective (commonly used magnification factor is 90-100 times).
For the different magnification factor, objective can be also classified as Low Power Objective (under 10 times), Medium Power Objective (about 20 times), High Power Objective (40-65 times).
For the different aberration correction situation, objective can be classified as Achromatic Objective and Apochromatic Objective. Achromatic Objective is common used, and it can correct the aberration between two coloured light in spectrum, while Apochromatic Objective can correct three coloured light’s aberration, with the higher price and the less usage.
2. The Key Parameters of Objective
The key parameters of objective include Magnification times, Numerical aperture, Working distance, etc.
A、Magnification times means the ratio of the image’s size we can see and the corresponding specimen’s size. It refers to the radio about length instead of area. For instance, if the magnification factor is 100×, it means that it can magnify a 1μm specimen’s length to be a 100μm’s length image, so the image’s area is magnified to be 10,000 times.
The total microscope magnification is the product of eyepiece magnification and objective magnification.
B、Numerical aperture (NA or A), also called mirror mouth rate, is a key parameter of objective and condenser, is proportional to the resolution of microscope. Dry lens’s numerical aperture is 0.05-0.95, and oil (cedar oil) immersion lens’s is 1.25.
C、Working distance refers to the distance between the bottom of the front lens (at the clearest moment of the specimen) and the top of the cover glass. The working distance of objective is related to the focal length of objective. The longer focal length makes the lower magnification and the longer working distance. For instance, objective 10× with the mark: 10/0.25 and 160/0.17, means the objective magnification is 10, the numerical aperture is 0.25, lens barrel length is 160mm, the standard thickness of cover glass is 0.17mm. Objective with 10× has the effective working distance for 6.5mm, while the one with 40× for 0.48mm.
3. Objective is used for providing the first amplification of specimen. It is the most important component deciding the performance of microscope-the resolution.
Resolution is also called Distinguishability or Resolving Power. The performance of the resolution is shown as the resolving distance, which means the minimum distance between two distinguished object points. In the distance of distinct vision (25cm), normal human eye can see two object points with 0.073mm distance. And this is the distinguish distance of normal. The shorter distinguish distance means the higher resolution, and the better performance.
The performance of the microscope’s resolution is determined by the resolution of objective, which is determined by the numerical aperture and the wavelength of lighting.
When using the common central lighting method, the resolving distance of microscope is
d=0.61λ/NA
d - resolving distance of objective, unit: nm.
λ – wavelength of lighting, unit: nm.
NA – numerical aperture of object
For instance, if the numerical aperture of oil immersion objective is 1.25, its visible wavelength range is 400-700nm, the average wavelength is 550nm, then d=270nm, approximately equals a half of the wavelength of lighting. Generally, microscope with visible light has a resolution limit, which will be 0.2μm.