Muddled About Magnification? How To Choose The Right Microscope

Deciding which microscopes to buy for your department can be a daunting task. We have put together a few things to consider to help you choose the right microscope for your school.

What Are The Students Studying?

For larger objects, such as flowers, insects and rocks, use a low powered stereoscope. These are binocular microscopes that produce a 3D image using two separate lens systems and typically provide up to 40x total magnification.

When looking at microscope slides, you will need a compound biological microscope. For most year groups, 400x total magnification is sufficient. However, looking at bacteria will require 1000x total magnification. Total magnification is calculated by multiplying the eyepiece magnification by the maximum objective magnification.

Microscopes giving 1000x total magnification should also have a 1.25 NA condenser to achieve the best resolution. A mechanical stage is also highly recommended.

 

Illumination 

Most modern microscopes use LED lighting, which provides a cool, even white light that does not heat up. This means that invertebrates, such as Daphnia, are not adversely affected by remaining under the microscope for extended periods. LED bulbs also have an extremely long life due to their low wattage compared with halogen or tungsten bulbs.

Halogen lighting gives a bright white light, but the bulbs can become hot.

Tungsten lighting tends towards yellow and can also heat up with prolonged use.

 

It’s All About The Lenses

Lenses are all about resolution as well as the magnification of the image.

A lower magnification microscope with better lenses will usually resolve the image more clearly than one with lower quality lenses, giving a better microscopy experience.

DIN is the industry standard for microscope lenses and stands for Deutsche Industrie Norm. The DIN standard uses a 160mm distance from the objective flange, where it screws into the microscope, to the eyepiece flange. Adding accessories can change this distance and blur the image. Using an objective with an ∞ symbol can help compensate for this.

JIS, or Japanese Industrial Standard, uses a 170mm distance from the objective flange to the eyepiece flange.

Most objective lenses in educational microscopes are at least achromatic.

An achromatic, or achromat, objective is a basic objective that helps limit the irregular effects of light entering and reflecting from the slide. It corrects for colour by bringing two wavelengths, typically red and blue, into focus. Unlike higher level semi-plan and plan objectives, achromatic objective lenses do not have a flat field of view. This means there is a smaller area where the image is at its sharpest focus, usually around 60 to 65%. Aberrations may appear in the outer 35 to 40% of the field of view. Although this does affect microscope performance, it is not generally noticeable for lower level microscopy tasks, particularly in younger school years.

Moving upwards in quality, a semi-plan lens will have around 80% of the field of view in sharp focus, while plan lenses will improve this further. As lenses improve in acuity, the price of the microscope will usually increase due to better clarity, improved colour correction and fewer optical aberrations.

 

EA Achromatic Objectives (Achromat Education/Economy)

EA achromatic objectives have been developed especially for education, offering a more affordable lens while still providing reliable quality. They typically consist of several different types of glass lens with differing refractive indices, helping to focus different wavelengths of light at the focal point.

They are suitable for general school microscopy, but they are not as high quality as standard achromatic, apochromatic, semi-plan or plan objectives.

 

Numerical Aperture, NA: Detail Not Size

Numerical aperture is the measure of a lens’s ability to gather light and resolve fine details in the specimen. It is not about the magnification of the image. A lens with a high NA collects more light, giving improved image resolution. However, as NA increases, depth of field decreases, making focusing more selective. It is about finding the right balance to obtain the best image possible.

 

Condensers & Diaphragms

Condensers and diaphragms are used to improve the quantity and quality of light reaching the slide.

They are found underneath the stage and range from a basic fixed height condenser to more advanced condensers that can move independently from the stage, helping to concentrate the light more effectively.

Ideally, the NA of the objective should be equal to or lower than the NA of the condenser.

The diaphragm can be opened or gradually closed to limit the amount of light entering the microscope. This changes the contrast of the specimen and can enhance the image on the slide. However, changing the contrast can also make artefacts on the glass more visible, even when they are not part of the specimen itself.

Using the condenser and diaphragm together helps direct the best light onto the specimen, giving a better viewing experience.

 

A Note Regarding Cover Slips

A number beside the objective tube length on the objective lens indicates the thickness of cover slip required.

If there is nothing marked on the objective lens, the standard thickness of cover slip to use would be 0.13 to 0.16mm. The maximum thickness should be 0.17mm, with the cover slip, sample and water layer assumed to give a thickness close enough to 0.17mm.

Number 1 cover slips are 0.13 to 0.16mm thick: MSL-100-200, 22 x 22mm, and MSL-100-150, 18 x 18mm.

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