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Looking Down and Looking Through: The Optics of a Microscope 1: The Eyepieces

Although the modern microscope is an array of optics, mirrors and lenses, in general, there are only two set of optics which are adjusted and changed by the user when viewing samples- the eyepieces and the objectives.

For optimal viewing and imaging of specimens down the microscope, the eyepieces and objectives must work harmoniously with each other. Microscope manufacturers design these optical components to work together, so this is something to bear in mind if you are changing eyepieces or objectives between instruments. If you are buying an ‘off-the-shelf’ instrument, then the optics fitted will be designed and matched in such a way as to complement each other. If, on the other hand, you are building your own research grade instrument, then the choice of objectives will ultimately determine the matching eyepieces and vice versa.


The eyepieces (also known as ‘ocular lenses’ or ‘oculars’) are where the final image of your specimen ends up and is viewed. The eyepieces add a final additional magnification factor on top of that from the chosen objective (this is typically a 10X magnification). Eyepieces may look like relatively simple components of the microscope- indeed, some are simply a metal tube with lenses top and bottom, but in reality (and in many research grade microscopes) these tubes contain groups of lenses which work together to give a corrected view of your specimen as well as complimenting the optical set-up and properties of the objectives.

Whether the eyepieces are made up of two or more lenses, the only visible lenses are at either end of the eyepiece. The lenses closest to your eyes are known as the ‘Eye Lens’, whilst those which face into the body of the microscope are known as the ‘Field Lens’.

The eye lenses are usually surrounded by rubber eyecups. These serve two purposes- if you wear glasses, then these allow you to view your specimens down the microscope without bumping or damaging your glasses on the metal body of the eyepiece. Secondly, they block out some of the ambient light giving you a clearer view of what’s on the stage. They can be rolled back or removed if you would prefer not to use them.

If you use microscopes within a shared facility, then cleanliness and hygiene should be important factors. If you have any sort of eye infection, then you should refrain from using shared microscopes until this has cleared up. Many eye infections are highly contagious, so don’t go spreading them around your lab! After using a shared microscope you should also wipe clean the eyepieces and cup (as well as the other cleaning of the microscope before you finish). I’ve nothing against mascara and eyeliner, but some people really don’t suit it!

Diopter Adjustment

Adjusting eyepieces to suit your vision is a relatively simple step. This is known as ‘diopter adjustment’ and is used to adjust the focus and vision differences between eyes which many people have (unless you have perfect normal visual acuity or ‘20/20 vision’). The first simple step is to adjust the distance between the eyepieces to suit the anatomy of your own head. The eyepieces on a binocular set-up are sometimes mounted on a horizontal ‘slider’ and can be moved to suit the distance between the eyes. Alternatively, these may be mounted in separate housing which can be revolved.

The next step is to make the diopter adjustment. If you have a look at the two eyepieces in a binocular set-up, then you’ll see that one of them has a ring adjuster around the eyepiece tube. Firstly, look at your specimen with the fixed eyepiece only. Adjust the focus of the specimen using the focus on the microscope body. Without adjusting the microscope focus, look down the adjustable eyepiece and slowly turn the diopter ring until your specimen comes into sharp focus. Once this is set, then this adjustment stays the same no matter which objective you use (unlike Kohler Illumination which I’ll cover in a future article).

Author: Martin Wilson

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