A Complete Guide for Everything to Know About Microscope Objectives

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

What is an objective lens turret?

The turret is the assembly that contains multiple objective lenses. It’s rotatable so you can easily switch between objectives and change the magnification for viewing your specimen. You always want to make sure when you rotate the turret, that the objective lens clicks into place, securing the proper position for focusing abilities.

What are the numbers on an objective lens?

There are typically four numbers on an objective lens. If it also says “OIL” that means it’s an oil immersion objective and it can only be used with immersion oil (see our separate video and article on topic of immersion oil).

Magnification: The top left number is the power of ONLY the objective lens, not to be confused with total magnification. For example, 40 means a 40x objective lens.
Tube Length: There are standard diameters and lengths for the optical tube on the microscope that holds the eyepiece and where the light travels from the objective lens. However, the term “tube length” refers to an internal optical path length that is standardized to calculate the total magnification.
Numerical Aperture: The numerical aperture of an objective lens 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.
Cover Slip Thickness: The least important specification is the cover slip thickness. This is the assumed thickness of the cover at the time the optics were designed.

What are the colors on an objective lens?

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

Magnification	Color
1x – 1.25x	Black
1.6x – 2x	Gray
2.5x – 3.2x	Dark Red
4x – 5x	Red
6.3x – 8x	Orange
10x – 12.5x	Yellow
16x – 20x	Light Green
25x – 32x	Green
40x – 50x	Light Blue
63x – 80x	Blue
100x	White

What are the benefits of parfocal optics mean?

First, 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.

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, they don’t have to be refocused between switching.

Microscopes with parfocal are 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.

Although the parfocal optical system needs just a bit of a fine focus between switching objectives, it also means the increase in optic power reduces the working distance (WD). Working distance is the distance from the front lens of the objective to the closest coverslip when your image is in sharp focus.

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 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
field curvature

As the number of corrections increases, the complexity of the lens design increases and so does the cost.

Objective Type	Spherical Aberration	Chromatic Aberration	Field Curvature
Achromat	1 Color	2 Colors	No
Plan Achromat	1 Color	2 Colors	Yes
Fluorite	2-3 Colors	2-3 Colors	No
Plan Fluorite	3-4 Colors	2-4 Colors	Yes
Plan Apochromat	3-4 Colors	4-5 Colors	Yes