An almost free low power microscope… If you have a not so cheap biological microscope

WALTER  DIONI                       Durango (Dgo) Mexico
Head of the eagle, from a 10 ctv. Mexican coin - "Köhler microscope" with 10x obj.
All pictures recorded at 640 x 480 px and trimmed or reduced as needed to be included in the article. Objects for the macros were illuminated only with a little handheld torch. Only one shot. None of the pictures were multi-shot amalgamations.

Preliminaries. - Well, do you have a biological microscope with built-in illumination? Equipped with 4x, 10x, and perhaps 40x objectives plus at least a 10x eyepiece?

 This allows you to see the critters of your interest at 40, 100 and 400 powers. Chances are you have a rack and pinion adjustable condenser, and you can set up the so-called Köhler illumination.

I say “so-called”, because if you do not have very high quality, highly prized equipment, it is most improbable that your illuminator has a focusable frontal lens, and an iris lamp diaphragm (field diaphragm). Both requisites are needed, according to good theory and practice, to install “real” Köhler illumination. (See the articles by F. Sterrenburg.)

But we live in the “real world”. And my microscope can not be set up for “real” Köhler illumination. At most I could implement a sort of modern version of the “critical illumination”.

Well, as all this has embarrassed me, I managed to improve the performance of my poor bug.

First of all I assumed that the lamp is focused at the level of the aperture diaphragm, or at least at infinity. That is the least I expect from the manufacturers, which say they provide a “built-in Köhler illumination”.

I then proceeded to improve my equipment using the suggestions of Ian Walker in Micscape, which implies the use of several diaphragm discs to simulate the iris field diaphragm.

Now I turn on the microscope lamp, put a slide on the stage, and focus on it.

The next step was to put, over the lamp lens, a field diaphragm with a lesser aperture than the NA of the 4x objective (I used 10 mm, well suited for my microscope) and to focus the field diaphragm with the aid of the rack and pinion feature of the condenser until its borders are neat, on the focused object plane.

 A low power microscope

Well, well, I’m very curious. What is the outcome of having the field diaphragm focused with my optical system? If the diaphragm is focused on the slide, anything I change for it must be also in focus. Yes or no?

I made some interesting experiments. The first thing that came to my mind as a test object was a millimetre rule. I turned off the microscope built-in light. I put the rule over the frontal lens, were I had the diaphragm before. I use a little hand torch to illuminate the scale. I didn’t touch anything else.

Through the eyepiece I saw a 20 mm segment of the rule. (Fig. 1)  It was in focus with my 4x objective in the working position (see later remarks). I changed it to 10x, and also to the 40x. I viewed 7.5 mm, and 2 mm respectively in focus. (figs. 2 and 3)

001-low power


I now changed to a transparent 1 mm square grid printed on acetate and took a picture at each power (see figs. 4 to 6).


Now, allow me a little disquisition. Have you tried at some time to select material from a sample with a micropipette, using the low power of your microscope? If you don’t have a stereoscope, certainly you will try. What a mess, don't you agree? First of all, in a normal modern microscope you have one 4x as your lowest power objective, and with your 10x eyepiece, you must work at a power no less than 40x, with a working distance between the objective and the slide of more or less 12-13 mm. Second and worst, you must work in the inverse direction. You know that your microscope inverts the image. This is why you put your slides “as seen facing to the back of the microscope”. So, at high power and with an inverted image it is very difficult to make the correct manual actions.

  For this is why the stereoscope is needed: to work “right”, and at a low range of powers, with a long working distance.

And now…a mostly unexpected and really wonderful phenomenon is that the rule I see was the right way round. Right! (Fig. 1)The condenser acted as an invertor and projected a right image through the microscope!!!

Now, I have a digital camera integrated into my micro', and I can capture the image of the field of view on my computer’s screen. Measuring the images with any one of the methods discussed in my earlier MICSCAPE articles, I calculated 12x, 23x and 85x for the three objectives. (Note: the magnification depends heavily on the vertical position of the subject, if you put the object 1 cm higher you still can focus on it but at a lesser magnification.)

I have received a gift: a low power microscope to complete the scale of useful magnifications of the biological micro' (if you have a system like mine). Mine has now 12x, 23x, 40x, 85x, 100x, 400x and as I have a 100x OI, 1000x. And for the three bold figures a long, long, working distance between the condenser and the lens of the built-in lamp. Any way due to resolution problems (at least with my microscope) I consider that the useful magnifications are only the 12x and the 23x.

Exactly what you (and I) may have needed in order to do an occasional macro study, dissection, or comparison between some small objects.

Yes, yes, my friends, I also see the distortion! Wait two minutes, please!

I have never heard of this use of the microscopes.
But it is an inherent consequence of Köhler illumination set up.


Of course what I do not have is a dedicated stereomicroscope; the system does not have the stereo view capability, nor the depth of field of the stereo. And the 4x, even being a planachromatic that gives a neatly plan field of view with a diameter of 4.0 mm in normal usage, gives an image with a clear vignetting effect that limits the “in focus” section of the image to a square of 10 x 10 mm. accounting only for roughly 50% of the total width of the field of view of 20 mm. This must be my working field of view and a mask must be made to apply over the field lens to shut off the out of focus parts of the image.

For the pictures, the solution is the same: one mask  *.ctp  draw in PhotoPaint with a transparent center square.
See fig
. 7 - 4x obj. the same grid of fig.4 with the mask applied.

I think that perhaps this defect is not inherent to the system and that other microscopes with a different condenser would not show it. Anyway, when I changed my standard condenser for another of the two spare Abe condensers I have, things remained the same. The relationship of the usable field with the camera's field of view is of 10 mm to 19 mm i.e. roughly 50%.

The 10x and 40x that embrace a smaller field of view don’t show this defect. The lines over the entire field of view (both visual and camera) are almost orthogonal (figs. 3 and 4).
The problem with the 4x is important only when you try to make a photomicrograph. When working for sampling or dissecting you can use a slightly bigger field than 10 x 10 mm because our brain and the micro focusing device makes the necessary corrections. As the subjects are normally of a certain depth and with many planes, the aberration is less than a concern.

 However, and in spite of this, I (perhaps we) now have the opportunity to take some macros (little flowers, postal stamps, little coins, (fig 6, 7) sand… I leave you to complete the list) or make the dissection of the mouth parts of an insect, or the legs of a copepod … (if you have the appropriate tools and your hands have the needed ability).

Opaque objects- These are the chosen subjects for this “microscope”. The quality of the recorded images, I think, would depend on the quality of the camera you use and of the lighting system, and if you wish to capture the depth of the object, you must do multiple focusing and shots, to recombine them afterwards with some program such as CombineZ or Image J (both are freeware) or the similar commodity of PaintShop Pro (Jasc’s shareware photo processor) and of other photo processors.

Fig. 8 - part of a Mexican coin, obj.x4.
Fig. 9 - Part of a larger mexican coin - x 10 obj.

Also the definition or resolution of the image is impaired compared with the normal microscope image. Try the iris aperture diaphragm as a very modest depth of focus control. Think of the condenser as now being your objective. Try diffuse light, with the 4x objective. Be prepared to supply lots of light for your 10x, and specially to your 40x if you can use yours. Also, use a soft type of illumination.

Fig- 10 - A small branch of an unknown (to me) small plant. The longer leaf is only 4 mm and the flower buttons are ca. 0.25 mm.

The best setup is possibly a combination of 3 or 4 white, high intensity, LED’s (6800 to 8000 candles) regulated with individual switches and potentiometers to allow a careful balance of the light.

Fig 11. The pistil and anthers of a Portulaca flower.

From the higher to the lower anther there is a span of 10 mm.

Viewing and photographing transparent subjects.-  If you don’t use opaque stops the normal light of your microscope can shine through the transparent objects you put on your stage.


Fig. 12 – fly wing – mounted in PVA-G. x 10 Obj.

You could use the transmitted illumination possibilities to view and photograph transparent subjects, even prepared slides, which are suitable for normal microscopy observations, at lower or intermediate powers. However, the system (in my set up) is not very good (for the moment, at least). The definition is not sharp enough to fulfill my expectations. I could not implement a diaphragm system to control the cone (the aperture) of the transmitted light.


Fig 13 -  a small (4.5 mm long) temnocephalid plathyhelminth at approx. 20x  (10x obj. - 10x eyepiece). Not too bad a picture, all the principal organs visible. Compare it with other low power helminthological images on the web. Mayer's Carmine, mounted in Canada Balsam

Trying the  40x and 100x.- I suspected that if I could give enough light to my 100x, without overheating the subjects and the system, making immersion between the frontal lens of the condenser and the objective through an intermediary slide (remember that you must put an oil drop over the condenser top lens, apply a plain and clean slide and put another oil drop between the slide and the objective) must allow me to recover some image. I tried it. I had a poor 100x image (I get this more easily and with much better resolution with the “normal style” of my microscope using the 10x objective and the 10 x eyepiece) I do not think I was supposed to use the macro 100x feature. Even the 85x (x40 obj.) has not enough contrast and definition to warrant its use, especially because defects are worse in transmitted light. But try, with some subjects it works.

Selecting microinvertebrates from samples

At more or less 20x (10x objective) the instrument becomes a useful selection device. Fig. 6 shows the device at work, with the sample in a little Petri dish, over a black velvet background, illuminated by an incandescent light, and with a nematode about to be picked up with a micropipette.


Fig.14 – intestinal sample from an insect. x 10 Obj.

Older microscopes are also useful

It was clear to me that I needed to make another experiment. I took my venerable 72-year-old Koritska, pulled apart the mirror, and made the necessary setup. Over 40 years of owning it, and I never though it could double for a low power one! 
Even with the very old achromatic optic of the Koritska, the focus was very clear on objects placed on the table, between the sturdy legs, or even on a platform supported over the square section legs. An exterior desk lamp with a 50W, internally mirrored bulb, gives enough light to work easily.

Protecting the lenses from reflections 

As I was working in a well-lit room, near a window, I saw that the frontal lens of the condenser was reflecting the eyepiece, nosepiece and the window. And at night the ceiling light was also reflected. The image interferes a bit with a good object imaging and even more so with the photographic images.

I made a cylinder of black cardboard that fits over the frontal lens of the condenser and embraces the objective. This totally corrected the problem. I will make the same adjustment on my new microscope.

With the oldest microscopes with straight tubes, you have the additional problem of working with the microscope vertical, which is not very comfortable, really. What is lost in the old microscopes is the capacity to have transparent objects being lit from the underside. However, with a little brain work most of you can fix the problem.  


I  know very well that these are not high quality images. And that the field of view and resolution capacity of the “low power Köhler microscope” is very far from that of a binocular microscope, even of student quality.

But I remember the times in which I only could manage to buy my second-hand Koritska, and I could not afford to acquire one binocular microscope no matter the cheap it could be.

Over 40 years I have the Koritska making a fine work for me, but unfortunately I never suspected that I could had use it as a modest but useful low power microscope. I think that this article could aid perhaps at some of the so called “silent majority of microscopists” that has modest equipment and budget, and even has not camera, but occasionally need to do some work at less than 40 x.

Refining the instrument

The optical resolution is better than the pictures shown. So if you are willing to implement some working version of the above, I suggest that you:

Make a working stage, with rigid cardboard, cutting one central hole to admit the rim of the lamp (if your microscope has a built in illuminator), and lateral supports to give it stability and allow a resting surface to your arms. This would be not necessary with many old, “mirrored” microscopes you can work on the table, between the microscope feet.

2)      Arrange an illumination system, best with 3 or 4 white LED’s.

3)      Cut cardboard stops to put over the frontal field lens. If you do, make some of them colored or textured to give alternative backgrounds to your subjects.

4)      Taking out the stops allows transmitted illumination. Have a cardboard diaphragm with a hole of more or less the size of the orthogonal field you have determined, and a glass plate to put over the cardboard stage for a more easy manipulation. Close the aperture diaphragm (the condenser’s one) and graduate the intensity of the microscope lamp.

 That is all!


Please! If you make some experiments about this system, let me know of your results! I would be glad of having notices about the behavior of microscopes of different technical levels, and especially with other kinds of condenser and optics. 


Comments to the author, Walter Dioni , are welcomed.


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