How To Change The Magnification On A Microscope?

Microscopes achieve magnification through a combination of two optical systems: the objective lens and the eyepiece lens. Understanding how these lenses work together is essential to knowing how to change the total magnification on a microscope.

How Total Magnification Is Calculated

Magnification tells you how many times larger the observed image is compared to the object's actual size. Total magnification is calculated by multiplying the power of the objective lens by the power of the eyepiece.

• Objective lens: the main lens closest to the specimen
• Eyepiece (ocular lens): the lens you look through, usually 10x
• Example: a 40x objective lens × 10x eyepiece = 400x total magnification

Eyepieces have magnification factors that vary between 5x and 30x, with the most commonly used eyepieces having a value of 10x–15x. Objectives typically have magnifying powers that range from 1x to 100x, with the most common powers being 4x (or 5x), 10x, 20x, 40x (or 50x), and 100x.

Changing Magnification by Switching Objective Lenses

Objective lenses come in different powers, each suited for different levels of detail:

4x or 10x (low-power)

Great for scanning larger areas of a slide.

40x (high-power)

Good for seeing more detail in cells or bacteria.

100x (oil immersion)

Used for the highest magnification and resolution. Requires a special oil to reduce light refraction and improve clarity.

Switching between objective lenses is the most direct method for changing magnification on a compound microscope.

Changing Magnification by Swapping Eyepieces

If you change your standard 10x eyepieces, you can increase your overall magnification without changing your working distance — the space between the lens and the specimen. The downside of using higher magnification eyepieces is a smaller field of view (FOV) or field diameter (FN).

Changing Magnification with Auxiliary Lenses

By adding an auxiliary lens to the bottom of the scope, you can either increase or decrease magnification; however, the working distance will change.

• Reduction lenses (1.0x or less) increase working distance.
• 1.5x and 2.0x auxiliary lenses decrease the working distance significantly.
• Since the optics are physically closer to the specimen when using an auxiliary lens, you will get better image resolution.

Stereo microscopes are used for all kinds of production and inspection work in all kinds of industries. Some applications require the working distance to be long enough to get a particular specimen to fit or be able to work with your hands under the microscope in the field of view. If you want to increase magnification and a shorter working distance does not cause a problem, then adding an auxiliary lens may be the best route.

Changing Magnification by Adjusting Tube Length

Total magnification is also dependent upon the tube length of the microscope. Most standard fixed tube length microscopes have a tube length of 160, 170, 200, or 210 millimeters, with 160 millimeters being the most common for transmitted light biomedical microscopes.

Using an objective or eyepiece in a microscope of a different tube length will lead to changes in the magnification factor and may also lead to an increase in optical aberration lens errors. If you don't want to change objectives or get a new eyepiece, making the tube longer by having the eyepiece sit further back increases the primary image distance and decreases the object distance at the same time, which helps increase the image-object distance ratio and get a higher magnification. The only drawback is that you must make sure the objective can accommodate a smaller working distance; otherwise, the image can't come into focus if the objective is hitting the glass slide.

In modern research microscopes, a set of parallelizing lenses is sometimes added to shorten the apparent mechanical tube length. These additional lenses can introduce an additional magnification factor — usually around 1.25–1.5x — referred to as a tube factor in user manuals. For example, if a 5x objective is used with 15x eyepieces, the total visual magnification becomes 93.75x (using a 1.25x tube factor) or 112.5x (using a 1.5x tube factor).

Magnification Changers in the Optical Pathway

Manufacturers may also provide additional lenses — sometimes called magnification changers — that can be rotated into the optical pathway to increase the magnification factor. These lenses usually have very small magnification factors ranging from 1.25x up to 2.5x. However, use of these lenses may lead to empty magnification, a situation where the image is enlarged but no additional detail is resolved.

How Changing Magnification Affects Field of View

It is a common misconception that at 1000x magnification, items will be visible under the microscope that are not visible at 400x. This is not typically true — you can view the same samples at 400x that you will view at 1000x; they will just take up a greater portion of the microscope's field of view at 1000x.

• Higher magnification narrows the field of view.
• The depth of field — the range in focus — becomes shallower at higher magnification.
• Working distance, the space between the lens and specimen, gets smaller at higher magnification.

The Limits of Useful Magnification

The range of useful total magnification for an objective/eyepiece combination is defined by the numerical aperture (NA) of the system.

1. The minimum magnification necessary for detail to be resolved is usually set at 500 times the numerical aperture (500 × NA).
2. The maximum useful magnification is usually set at 1000 times the numerical aperture (1000 × NA).
3. Magnifications higher than this value will yield no further useful information or finer resolution of image detail, and will usually lead to image degradation.
4. Exceeding the limit of useful magnification causes the image to suffer from empty magnification, where increasing magnification only causes the image to become more magnified with no corresponding increase in detail resolution.

High magnification alone doesn't guarantee a clear image. Resolution — the ability of a microscope to show two nearby points as separate — determines how sharp and detailed the final image appears. A higher numerical aperture means better resolution, which is why oil immersion lenses are used when the clearest image is needed.