Celestron Advanced Biological Microscope 500 Microscope Basics - Page 11

Objective Lenses

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Objective Lenses Stereo Microscope Objectives for compound microscope Objectives Mounted Objective on Stereo Microscope The objective lenses are the most important components of microscopes and thus will be discussed in greater detail here. Their basic function is to gather the light passing through the specimen and then to project the image up into the body of the microscope. Then, the eyepiece lens system further magnifies the image for your eye to see. Most quality microscopes use glass for the objectives and even for beginner microscopes, stay away from plastic objectives lenses as the quality level is quite inferior. The objectives are the lens system closest to the specimen. There is one objective for each eyepiece in a compound microscope. For stereo microscopes, there are objective pairs (one objective lens for each eyepiece lens) which give the 3-D effect. On compound microscope objectives, there is printed the following information on each one - power, DIN tube length, N.A., cover slip thickness, universal color ring. Tube length of the objectives usually have a DIN (interchangeable) of 185mm or 195mm. Objectives vary in power from 1x to 160x in compound microscopes but the most common power range is from 4x to 100x. Most compound microscopes have three or four (occasionally five) objectives usually of 4x, 10x, 40x, and 100x (oil immersion) which revolve on a nosepiece (turret) to give different magnifying powers. The 4x, 10x, and 40x are called "dry" objectives which means they operate with air between the objective and the specimen. The 100x is called a "wet" objective which means it operates with immersion oil between the lens and the specimen. For stereo microscopes, they usually have one or two objectives lenses which normally are 1x, 2x, 3x, or 4x. In addition, there are zoom models which operate from about 0.5x up to 5x. The extent of corrections for lens errors (aberrations) and flatness of the image field determines the usefulness and cost of the objectives for compound microscopes. The least expensive objectives are achromatic types and these are fine for all microscopes costing under about $ 750.00. The cost of objectives increases if using fluorites or semi-apochromatic types and the most expensive are the apochromats. Flatness of field (field curvature) refers to how well the specimen is focused across the entire field of view. In achromatic objectives the flatness of field is usually about 50% to &70% of the field where they are in sharp focus in the center and the outside of field becomes out of focus but you can refocus in the outer portions to see that area sharply. Flatter field objectives (micro plan or semi-Plan) are the semi-apochromats which are flat for about 70% to 85% of the field and apochromats (Plan types) are flat for about 90% to 100% of the field. Plan objectives are the ultimate for quality and a microscope typically will cost more than $ 750.00 to have these type objectives in them. N.A. (numerical aperture) is a number that expresses the ability of a lens to resolve fine details in an object being observed especially those close together. As the N.A. number increases, the resolution becomes better. The N.A. may vary from 0.04 (low power) to 1.4 (high power Plan wet objective). The N.A. will be marked on the objective and the typical N.A. for the following are; 4x=0.10, 10x=0.25, 40x=0.65, and 100x=1.25. Resolution (actual not theoretical) is the separation distance of two details (points or lines) lying close together still seen as separate. The higher the resolution, the closer the two points may be to one another and still be distinguished as two points. Resolution comes from the objective and not the eyepieces as the eyepieces only magnify the resolution. Sometimes objectives have a color ring (universally used) to aid in identifying the magnification: black (1x), brown (2x), red (4x), yellow (10x), green (20x), turquoise (25x), light blue (40x), dark blue (60x), white (100x). Another number on the objective (like 0.17) refers to the thickness in millimeters of the cover glass that was assumed by the lens designer in computing best performance for the objective lens. 11

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11
Stereo Microscope
Objective Lenses
Objectives for compound microscope
Objectives Mounted
Objective on Stereo Microscope
The objective lenses are the most important components of microscopes and thus will be discussed in greater detail here.
Their basic function is to gather the light passing through the specimen and then to project the image up into the body of the
microscope.
Then, the eyepiece lens system further magnifies the image for your eye to see.
Most quality microscopes use
glass for the objectives and even for beginner microscopes, stay away from plastic objectives lenses as the quality level is
quite inferior.
The objectives are the lens system closest to the specimen.
There is one objective for each eyepiece in a compound microscope.
For stereo microscopes, there are objective pairs (one objective lens
for each eyepiece lens) which give the 3-D effect.
On compound microscope objectives, there is printed the following information on each one – power, DIN tube length, N.A., cover slip
thickness, universal color ring.
Tube length of the objectives usually have a DIN (interchangeable) of 185mm or 195mm.
Objectives vary in power from 1x to 160x in compound microscopes but the most common power range is from 4x to 100x.
Most
compound microscopes have three or four (occasionally five) objectives usually of 4x, 10x, 40x, and 100x (oil immersion) which revolve
on a nosepiece (turret) to give different magnifying powers.
The 4x, 10x, and 40x are called “dry” objectives which means they operate
with air between the objective and the specimen.
The 100x is called a “wet” objective which means it operates with immersion oil
between the lens and the specimen.
For stereo microscopes, they usually have one or two objectives lenses which normally are 1x, 2x, 3x, or 4x.
In addition, there are
zoom models which operate from about 0.5x up to 5x.
The extent of corrections for lens errors (aberrations) and flatness of the image field determines the usefulness and cost of the objectives
for compound microscopes.
The least expensive objectives are achromatic types and these are fine for all microscopes costing under
about $ 750.00.
The cost of objectives increases if using fluorites or semi-apochromatic types and the most expensive are the
apochromats.
Flatness of field
(field curvature) refers to how well the specimen is focused across the entire field of view.
In achromatic objectives
the flatness of field is usually about 50% to &70% of the field where they are in sharp focus in the center and the outside of field
becomes out of focus but you can refocus in the outer portions to see that area sharply.
Flatter field objectives (micro plan or semi-Plan)
are the semi-apochromats which are flat for about 70% to 85% of the field and apochromats (Plan types) are flat for about 90% to 100%
of the field.
Plan objectives are the ultimate for quality and a microscope typically will cost more than $ 750.00 to have these type
objectives in them.
N.A. (numerical aperture
) is a number that expresses the ability of a lens to resolve fine details in an object being observed especially
those close together.
As the N.A. number increases, the resolution becomes better.
The N.A. may vary from 0.04 (low power) to 1.4
(high power Plan wet objective).
The N.A. will be marked on the objective and the typical N.A. for the following are; 4x=0.10,
10x=0.25, 40x=0.65, and 100x=1.25.
Resolution
(actual not theoretical) is the separation distance of two details (points or lines) lying close together still seen as separate.
The higher the resolution, the closer the two points may be to one another and still be distinguished as two points.
Resolution comes
from the objective and not the eyepieces as the eyepieces only magnify the resolution.
Sometimes objectives have a color ring (universally used) to aid in identifying the magnification: black (1x), brown (2x), red (4x),
yellow (10x), green (20x), turquoise (25x), light blue (40x), dark blue (60x), white (100x).
Another number on the objective (like 0.17) refers to the thickness in millimeters of the cover glass that was assumed by the lens
designer in computing best performance for the objective lens.