Objectives

Another important objective parameter is a numerical aperture that affects the objective resolution. Common numeric apertures are from 0.02 to 1.40. Definition: Numerical Aperture is a dimensionless number that characterizes the range of angles over which the objective can accept light. It is a measure of the ability of an optical system to gather and resolve fine details of an object. Formula: NA is calculated using the formula NA = n * sin(α), where n is the refractive index of the medium between the lens and the specimen, and α is the half-angle of the maximum cone of light that can enter the objective. Importance: Higher NA values generally correspond to better resolution and improved image quality. As you noticed, NA depends on the refractive index of medium. For example, this index is 1.0 for air, 1.33 for water, 1.515 for cedar oil. That is why immersion objectives were invented. Adding a drop of a liquid between the objective and specimen may significantly improve the system resolution and image quality.

Total microscope magnification is the objective power multuplied by the eyepiece power. For example, 40x objective and 15x eyepiece will give you 40x15 = 600x magnification.

The practical limit of light microscope magnification is generally around 1000x to 2000x. This limit is determined by several factors related to the physics of light and the capabilities of optical systems. Beyond this range, the limitations of optical microscopy become more pronounced, and alternative techniques such as electron microscopy are often employed for higher magnifications. That is why you will not find a LOMO objective with larger than 125x power, or LOMO microscope eyepiece with larger than 17x power (2125x total magnification that is slightly beyond the practical limit). Other manufacturers may have even 500x objectives, but I don't know how well they perform with their relatively low (0.75) numerical aperture.

Objective power

Based on the magnification and numeric aperture, microscope objectives are grouped to low power, medium power and high power.

Low power objectives have 0.02 - 0.25 numeric aperture and 1x - 10x magnification. Such objectives have long work distance and large field of view.

Medium power objectives have 0.30 - 0.65 numeric aperture and 10x - 40x magnification. Their work distances are 0.5 - 8mm, and their field of view is 0.2 - 2mm.

High power objectives have 0.7 - 1.60 numeric aperture and 40x - 125x magnification. They have very short work distances (down to hundredths of a mm).

Aberrations correction

Different objectives have different degrees of aberration correction.

Achromats are most simple optical systems. Their chromatic aberration is corrected for two wavelengths. The image they produce have some color tint.

Achromatic objectives give sharp enogh image in the center , but image is soft at the edge of the field of view due to curvature.

Image quality of achromats is suitable for usual everyday applications like observing or picking an object.

These objectives are divided into a few groups:

1) objectives for transmitted light, 160mm tube length, 0.17mm cover glass;

2) objectives for reflected light, 190mm or infinity tube length, without cover glass;

3) epiobjectives for reflected light, bright or dark field, 190mm or infinity tube length, without cover glass.

Achromats of low and medium power are used with Huygens eyepieces. Objectives of high power are used with compensating eyepieces.

Achromatic objectives are widely used in biology, mineralogy, metallography.

Apochromats have better optical system. Their chromatic aberration is corrected for three wavelengths. The image they produce is colorless. Their chromatic aberrations are about 10 times lower than with achromats.

To achieve this degree of correction, these objectives use more complex optical construction and special glass, including fluorite and quartz.

All apochromat objectives have chromatic magnification aberration, and you should use compensating eyepieces with them therefore.

Also, they have some image field curvature. This can be compensated with homal eyepieces.

Apochromats can be divided into two groups:

1) objectives for transmitted light, 160mm tube, 0.17mm cover glass;

2) objectives for reflected light, 190mm or infinity tube, without cover glass.

You should always use compensating eyepieces with achromatic objectives.

Planachromats and Planapochromats contain 9-12 glass elements. Theu have flat field of view evenly sharp in the center and at the edge. They usually have wider field of view, and can be used with wide-angle eyepieces that is useful for taking photos. Also, they have increased to 45mm parfocal height.

Planobjectives are divided into two groups:

1) planachromats for transmitted light, 160mm tube length, 0.17mm cover glass;

2) planobjectives for reflected light, infinity tube, wuthout cover glass.

These objectives are usually used with compensation eyepieces for microphotohraphy and microprojection.

Special objectives are made for particular microscopes.

1) objectives for gelatine emulsion researches are purposed for MBI-8M, MBI-9 and MIRE-2 microscopes;

2) objectives for ultraviolet light are purposed for MUF-5 and MUF-6 microscopes. They are made of few types - quartz-fluorite, mirror, mirror-lens.

3) objectives for infrared light are purposed for infrared microscopes;

4) microplanaras are purposed for macrophotorgaphy with FMN-2 device.

Other features

Objectives can use different media between their front element and object. This can be air (dry objectives) or liquid (immersion objectives). Immersion objectives have a band of corresponding color - black for oil and glycerine immersion, white for water immersion.   Other important parameters are tube length, work distance and parfocal height. Tube length is a microscope tube distance from the objective flange to the eyepiece flange. Common distances for LOMO objectives are 160mm and 190mm. Infinity tube means that there is an additional achromatic lens is installed in the tube behind the objective. Work distance is a distance from the top surface of a cover glass to the first objective glass holder. Parfocal height is a distance from the object plane to the objective flange.

Some general info about objective body marks.

• А - objectives for research using the phase-dark-field contrast method
• Л - objectives for research in luminescence light
• П - objectives for research in polarized light
• Ф - objectives for phase contrast research
• АЛ - objectives for dark-field contrast method research in luminescence light
• ЛК - contact objectives for fluorescent microscopes
• ФЛ - objectives for phase contrast research in luminescence light
• ВИ - water immersion
• ГИ - glycerine immersion
• МИ - oil immersion
• АПО - apochromatic objective
• ПЛАН - planachromatic objective
• ПЛАН-АПО - planapochromatic objective

LOMO objective parameters

I'll gradually add links to objectives in our catalog for convenience. Please check back later.