A 40x lens is also known as what

The objective (lens) is the first set of optical systems that image the object being observed, and is also the most important imaging component in the microscope.
Depending on the application, objective is usually classified into the following categories:

Biological Objective
Metallurgical Objective
Phase Contrast Objective
Polarizing Objective
Dark/Bright Field Objective
Stereo Objective
Monocular Video Microscope Objective
Infinity-Corrected Long Working Distance Objective
NIR Objective
NUV Objective
UV Objective
Telecentric Objective Lens

Some objectives are mounted directly on the microscope body, some separate from the body and are installed when needed.
Different types of microscope objectives are generally not interchangeable. However, when ofthe same type and parameter design the same or similar, the objectives of different models and manufacturers are interchangeable, provided that attention shouldbe paid to the change in magnification, working distance, field of view and image quality.

Usually, on the objective outer casing, there are signs of the following parameters:
Objective Magnification: for example 10X, 40X
Numerical Aperture (N.A.): for example, /1.30
Objective Immersion Media: Oil represents oil, W represents water, and Glyc represents glycerin
Mechanical Tube Length and Objective Cover Glass Thickness: the two parameters are usually written together and separated by /. The finite mechanical is usually 160, 195, etc., and infinite is represented by "∞"; objective cover glass thickness (thickness / mm) is expressed after/, for example /0.17; for specimen that does not use objective cover glass, it is represented by 0, for example, "/0"; for those that do not use objective cover glass or the objective cover glass thickness is smaller than 0.17, it is represented by "/-".
Phase Contrast Objective: represented by PH, for example, PH2, the digit after PH represents the associated ring diaphragm.
Polarizing Objective: represented by POL.
Plan Objective: represented by PLAN or PL
Achromatic: generally, achromatic objective does not require identification
Apochromatic: represented by APO
Long Working Distance: represented by L
There are also objectives that are unique in magnification and medium, and their difference is indicated by color circle.
For objectives that do not have mark, it is necessary to refer to the microscope body for judgment, or refer to the product manual.

Usually, the objective has very fine mounting threads. When there is a need to install the objective /objective frame, be careful to install it. Align the nosepiece installation position, keep it completely “flat”. When it is blocked, remove it and reinstall it. Do not force it in.

Note: although between different manufacturers, some objectives can be used universally, they may still bring magnification error and image quality degradation.

Microscope Magnification

A microscope's total magnification is a combination of the eyepieces and the objective lens. For example, a biological microscope with 10x eyepieces and a 40x objective has 400x magnification. There are however, a few limits to the amount of total magnification that can be reached before empty magnification comes into play. Empty magnification occurs when the image continues to be enlarged, but no additional detail is resolved. This is often the case when higher magnification eyepieces are used. In order to avoid empty magnification, there are a few simple steps that are helpful to follow.

Eyepiece and Objective Combinations for Optimal Magnification

When selecting a combination of eyepieces and objective lenses for the optimal magnification, without ending up with "empty magnification" it is important to consider the numerical aperature (NA) of the objective. The numerical aperture of a microscope objective defines the objective's resolution. Each microscope objective has a minimum and maximum magnification necessary for the details in an image to be resolved. A simple formula for the minimum value is (500 x NA). And for the maximum magnification (1000 x NA). Magnifications higher than this value will result in empty magnification, or an image that has a poor resolution. The table below shows some typical NA values with their corresponding objective and provides a range of useful magnification combinations. The blank boxes in the table would provide empty magnification and should be avoided. For example, pairing 20x eyepieces with a 100x objective would not provide good resolution and would result in empty magnification. To determine this, we took 1.25NA x 1000 = 1250 magnification maximum. However, the combination of the 100x objective x 20x eyepieces = 2000, which is above the maximum magnification.

Range of Useful Magnification based on NA of Objectives

If you have any questions about your microscope's magnification, contact Microscope World.

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