What Are The Magnification Of Microscope?

A microscope is an instrument that magnifies an object so that it may be seen by the observer. Because cells are usually too small to see with the naked eye, a microscope is an essential tool in the field of biology. In addition to magnification, microscopes also provide resolution, which is the ability to distinguish two nearby objects as separate. A combination of magnification and resolution is necessary to clearly view specimens under the microscope.

How Total Magnification Is Calculated

Magnification tells us how much bigger an object appears under a microscope compared to its real size. Most compound light microscopes have two types of lenses — the ocular lens and the objective lens. The ocular lens is the lens on the eyepiece. The objective lens is the lens closest to the object or slide being observed. To find the total magnification, multiply the power of the objective lens by the power of the eyepiece (ocular lens).

For example, a 40x objective lens × 10x eyepiece = 400x total magnification. If you are using an ocular lens with 10x magnification and an objective lens with 50x magnification, your total magnification is 500x.

Total visual magnification of the microscope is derived by multiplying the magnification values of the objective and the eyepiece. For instance, using a 5x objective with a 10x eyepiece yields a total visual magnification of 50x and likewise, at the top end of the scale, using a 100x objective with a 30x eyepiece gives a visual magnification of 3000x.

Objective Lens Magnification Levels

Most microscopes have a rotating disc with at least three objective lenses attached, so the observer can choose an appropriate magnification. Objective lenses come in different powers, with the most common magnifications being:

• 4x (Scanning): Great for scanning larger areas.
• 10x (Low power): Used for broader overview of specimens.
• 40x (High power): Good for seeing more detail in cells.
• 100x (Oil immersion): Used for the highest magnification and resolution; requires a special oil to reduce light refraction and improve clarity.

Objectives typically have magnifying powers that range from 1:1 (1x) to 100:1 (100x). An important feature of microscope objectives is their very short focal lengths that allow increased magnification at a given distance when compared to an ordinary hand lens. The primary reason that microscopes are so efficient at magnification is the two-stage enlargement that is achieved over such a short optical path, due to the short focal lengths of the optical components.

Ocular Lens and Eyepiece Magnification

Standard eyepiece magnification

A standard microscope eyepiece magnifies an object 10x. However, eyepieces that magnify 15x, 20x, and even 30x or higher are also available.

Eyepiece magnification range

Eyepieces, like objectives, are classified in terms of their ability to magnify the intermediate image. Their magnification factors vary between 5x and 30x, with the most commonly used eyepieces having a value of 10x–15x.

Virtual image formation

The eye of the observer sees the secondarily magnified image as if it were at a distance of 10 inches (25 centimeters) from the eye; hence this virtual image appears as if it were near the base of the microscope.

Standard Magnification Table for Compound Microscopes

The total magnification is determined by multiplying the magnification of the ocular and objective lenses. The following shows the standard combinations using a 10x ocular lens:

• Scanning (4x objective) × 10x ocular = 40x total magnification
• Low power (10x objective) × 10x ocular = 100x total magnification
• High power (40x objective) × 10x ocular = 400x total magnification
• Oil immersion (100x objective) × 10x ocular = 1000x total magnification

Classroom compound light microscopes theoretically offer a maximum magnification of 1000x. Analog microscopes that use light and mirrors to magnify objects usually max out at about 1,500x magnification. This is because light wavelengths cause the image to appear unclear at that magnitude of magnification. Electron microscopes, however, can produce images that exhibit impressive clarity all the way up to 200,000x magnification since electrons have much shorter wavelengths.

How Tube Length Affects Magnification

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. Many industrial microscopes, designed for use in the semiconductor industry, have a tube length of 210 millimeters. The objectives and eyepieces of these microscopes have optical properties designed for a specific tube length, and using an objective or eyepiece in a microscope of different tube length will lead to changes in the magnification factor (and may also lead to an increase in optical aberration lens errors).

Modern research microscopes are very complex and often have both episcopic and diascopic illuminators built into the microscope housing. These additional lenses will sometimes introduce an additional magnification factor (usually around 1.25–1.5x) that must be taken into account when calculating both the visual and photomicrographic magnification. This additional magnification factor is referred to as a tube factor in the user manuals provided by most microscope manufacturers. Thus, if a 5x objective is being used with a 15x set of eyepieces, then the total visual magnification becomes 93.75x (using a 1.25x tube factor) or 112.5x (using a 1.5x tube factor).

The Range of Useful Total Magnification

The range of useful total magnification for an objective/eyepiece combination is defined by the numerical aperture of the system. There is a minimum magnification necessary for the detail present in an image to be resolved, and this value is usually rather arbitrarily set as 500 times the numerical aperture (500 × NA). At the other end of the spectrum, the maximum useful magnification of an image is usually set at 1000 times the numerical aperture (1000 × NA). Magnifications higher than this value will yield no further useful information or finer resolution of image detail, and will usually lead to image degradation.

Exceeding the limit of useful magnification causes the image to suffer from the phenomenon of empty magnification, where increasing magnification through the eyepiece or intermediate tube lens only causes the image to become more magnified with no corresponding increase in detail resolution. 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, but use of these lenses may lead to empty magnification — a situation where the image is enlarged, but no additional detail is resolved.

Limitations on Maximum Magnification

Higher magnification does not always guarantee a clear image. While the theoretical maximum magnification of a classroom compound light microscope is 1000x, achieving this level of magnification is not always practical due to several factors:

1. Optical quality: As magnification increases, so does the risk of optical aberrations, which can distort the image. The optical quality of the lenses, especially at high magnifications, plays a crucial role in the clarity of the observed specimen.
2. Depth of field: At high magnifications, the depth of field becomes extremely shallow. This means that only a thin section of the specimen will be in focus at any given time, making it challenging to observe three-dimensional structures or moving organisms.
3. Resolution: Resolution, or the ability to distinguish between two closely spaced objects, also becomes a limiting factor. While the microscope can magnify the image, it may not have the resolution to clearly distinguish the details.
4. Specimen preparation: High magnification requires meticulous specimen preparation. Specimens need to be thin enough to allow light to pass through and transparent enough to reveal meaningful details.
5. Field of view: Higher magnification narrows what you can see. The working distance — the space between the lens and specimen — also gets smaller, which can be tricky for certain procedures.

Digital Microscopy and Magnification

In digital microscopy, the magnification steps are achieved in a different manner. The slide is scanned, i.e., photographed in full resolution, and the resulting image can be zoomed in and out to get the best scale view of the sample. The simplicity of the continuously variable magnification of a digital zoom makes moving between an overview and the desired level of detail incredibly easy. Digitally, any part of the image can also be magnified to a size far larger than it would ever appear through an eyepiece.

Digital magnification is the process of enlarging the image captured by a microscope's camera using software. This can be done on a live video feed or a still image, and is similar to zooming in on a photo with your phone or computer. The system doesn't change the physical optics — it simply increases the size of the pixels that make up the image. Optical magnification, by contrast, is achieved through the physical lenses of the microscope and increases the level of detail and resolution you can actually see. Scaling up a digital image beyond its resolution limit leads to pixelation — a grainy or blurry image where fine detail is lost.

In conventional light microscopy, the magnification of the objective lens is very straightforward: a 20x magnification means the image is magnified to a size 20 times larger than the original. Thus, when viewed with 40x magnification, objects seem twice as big as with a 20x lens. Magnification serves a useful purpose only when it is possible to see more details of an object in the image than when observing the object with the unaided eye.