Everything About Microscope Objectives

Microscope objectives are a critical part of the optical system of any compound microscope. The two main parts of the optical system of a microscope is the eyepiece and objective. In this article we review everything about microscope objectives.

What is an objective lens turret?

compound microscope objective lenses, microscope lens turret, objective lens magnification, microscope turret, numerical aperture

The turret is the assembly that contains multiple objective lenses. The turret is rotatable so you can easily switch between objectives, and change the magnification you are viewing your specimen. You always want to make sure when you rotate the turret, that the objective lens clicks into place, which is the proper position so it can be focused.

What are the numbers on an objective lens?

There are four numbers typically on an objective lens. If it also says “OIL” that means its a oil immersion objective and it can only be used with immersion oil, see our separate video on that topic.

Magnification: The top left number is the power of just the objective lens, not to be confused with total magnification. So for example 40 means a 40x objective lens.

Tube Length: The tube is the optical tube on the microscope that holds the eyepiece and along where the light travels from the objective lens. There are standard diameters and lengths for these tubes. For the lengths, the German standard DIN is 160 mm and the Japanese standard JIS is 170 mm. Each objective states which tube length standard is used to calculate the magnification. If you use an objective lens with the wrong standard of tube, the total magnification calculated by multiplying the eyepiece magnification by the objective magnification will not be correct.

Cover Slip Thickness: The least important specification is the cover slip thickness. This is the thickness of a cover slip that is assumed when the optics where designed. What that means is that the best focal plane isn’t the stage surface or the top of a standard thickness glass slide, it’s designed to focus with a cover slip.

Numerical Aperture: The numerical aperture of an objective describes the ability of a lens to capture light from a sample and focus light onto a detector or eyepiece. The higher the numerical aperture, the greater the resolution and the ability to distinguish between closely spaced objects in the sample. Numerical aperture is an important parameter in microscopy, as it determines the resolution and depth of field of the image. A higher numerical aperture lens can capture more light, which results in a brighter and clearer image with greater detail. However, higher numerical aperture lenses also have a shorter depth of field, which can make it more difficult to image thick samples.

What are the colors on an objective lens?

Each objective lens has a color ring that for a quick visual reference of the magnification range. The chart below lists the relative magnification and color from 1x -100x.

1x – 1.25xBlack
1.6x – 2xGray
2.5x – 3.2xDark Red
4x – 5x Red
6.3 – 8xOrange
10x – 12.5xYellow
16x – 20xLight Green
25x – 32xGreen
40x – 50xLight Blue
63x – 80xBlue

What does parfocal mean?

Parfocal optics refers to a property of optical systems, such as lenses or microscopes, in which the focus remains relatively constant when changing magnification or adjusting the zoom. In other words, if you focus on an object at a certain magnification and then increase or decrease the magnification, the object will remain in focus without the need to adjust the focus knob again.

This property is particularly useful in microscopy or other applications where you need to zoom in or out on a sample while maintaining a sharp focus. Parfocal optics make it easier and faster to switch between magnifications and maintain clarity, as you can quickly change the magnification without having to constantly adjust the focus.

It’s worth noting that not all optical systems are parfocal. For example, some zoom lenses or microscopes may require manual focus adjustments when changing magnification. However, parfocality is a desirable feature that is often found in high-quality optics.

Parfocal distance is the height from the turret to the slide, which remains fixed in a parfocal microscope. The objective lenses are designed so that even though they are different powers, the don’t have to be refocused between switching. Any parfocal system may not be perfect though, but then only a bit of a fine focus may be needed between switching objectives. As you increase any optic in power, that means a smaller working distance (WD). The working distance is measured from bottom of the optical system to what you are viewing.

What does paracentric mean?

Paracentric optics refers to a type of optical design used in microscopy, photography, and other fields that involves aligning the light path so that it passes through the center of the objective lens, rather than through the edge. This helps to eliminate distortion and aberrations in the image, resulting in a clearer and more accurate representation of the object being observed or photographed.

In paracentric optics, the light rays are refracted at a certain angle as they pass through the lens, allowing them to converge at the focal point without being affected by any peripheral aberrations. This makes it possible to obtain high-quality images even at high magnifications, as well as to observe small details and structures that might otherwise be difficult to see.

Paracentric optics are commonly used in biological and medical research, as well as in industrial and engineering applications where precise measurements and observations are required. They are also used in astronomy, where they help to eliminate distortions caused by atmospheric turbulence and other factors that can affect the quality of the image.

What are the types of objectives?

Different objectives have different corrections in terms of optical aberrations or errors. There are three main types of optical aberrations that can be corrected for in microscope objective lens designs: spherical aberration, chromatic aberration, and field curvature. As you increase in the number of corrections, the complexity of the lens design increases and so does the cost.

Objective TypeSpherical AberrationChromatic AberrationField Curvature
Achromat1 Color2 ColorsNo
Plan Achromat1 Color2 ColorsYes
Fluorite2-3 Colors2-3 ColorsNo
Plan Fluorite3-4 Colors2-4 ColorsYes
Plan Apochromat3-4 Colors4-5 ColorsYes

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