Microscope Activities, 34: Binocular Head Adjustment

In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.

EXPERIMENT 34: Binocular Microscope Head Adjustment

Goal

To learn how to adjust the interpupillary distance and diopter setting(s) when using a binocular microscope, and to understand the reasons behind the need for proper adjustment, particularly for left-eye dominant users.

Level

Basic-Intermediate

Materials Needed

A microscope with a binocular head

Prerequisite

Read and understand Experiment 33: Eye Dominance

Discussion

The most egregious error in using a binocular microscope is the improper adjustment of the binocular head. This, together with improper adjustment of the aperture diaphragm (Experiments 7 and 26), constitutes one of the most flagrant errors in microscope use. If the binocular head is not properly adjusted for each individual microscope user the result will be eyestrain, headaches and inability to keep graticules and reticles in focus or even prevent them from disappearing from view! To adjust the binocular head properly, it is imperative that the microscope user know which of their eyes is the dominant one (perform Experiment 33).

There are three adjustments that need to be made when using a binocular microscope head:

  1. Adjust the focus of the graticule (crosshairs, measuring scale, etc.) by rotating the eye lens of the eyepiece containing the graticule, using the dominant eye. You may do this by removing the eyepiece from the microscope, and making the adjustment while holding it up to a light and looking through it; or, if making the adjustment with the eyepiece in the binocular head, cover the vision of the non-dominant eye with a 3” x 5” card or something similar, or just close the non-dominant eye.
  2. Adjust the interpupillary distance (interocular distance), i.e., the distance between the two eyetubes, so that the exit pupils of the two eyepieces correspond to the distance between your own eye pupils when you are focused at infinity. This is an important point, because when you look at a close object your pupils are closer together (converge); when you look at an infinitely distant object your pupils are farthest apart (diverge); to use the microscope properly, you should be looking “through” your specimen image, while focused at infinity! The average interpupillary distance is 65 mm, and some binocular heads will have this number in red, or in larger numbers on its interpupillary distance scale. Interpupillary distance in adult humans varies, of course, but the vast majority are somewhere between 55 mm and 75 mm, and all binocular heads will have a graduated scale that includes at least this range. There will, occasionally, be people with very narrowly-spaced or very widely-spaced eyes who will be unable to use the binocular head; young children also may not be able to use the binocular head.
  3. Adjust for differences between your two eyes using the diopter adjustment ring. More often than not, people have slight to extensive differences in the visual acuity between their two eyes; one may be more near-sighted (myopia; elongated eyeball resulting in rays coming to a focus in front of the retina) or far-sighted (hyperopia; foreshortened eyeball resulting in rays coming to a focus behind the retina) in one eye than the other. With advancing age recession of the near point of vision requires the use of convex correction lenses, i.e., reading glasses (presbyopia). In addition, there may also be astigmatism (a lens curvature defect in consequence of which rays from one part of an object are not brought to a single focal point) present. All of these optical defects can be corrected through the use of corrective eyeglasses. Some of these differences between the two eyes, however, can be corrected for by the proper setting of the diopter adjustment ring.

So, if I wear glasses does that mean I should keep them on when using a binocular microscope? Not necessarily. One of the eyetubes in a typical binocular head is adjustable for tubelength independently of the other eyetube. This adjustment has the effect of adding or subtracting diopters (a unit used to express the power of a lens; equal to the reciprocal of the focal length in meters); that is, this “diopter adjustment” acts the same as adding a corrective lens of up to ±6 diopters. If you are either near-sighted or far-sighted you do NOT have to leave your glasses on; the diopter adjustment can correct for these defects. If there is a choice, remove your glasses, because often exit pupils are not very high, and leaving eyeglasses on results in vignetted images. If your eyeglasses have correction lenses that are more than ±6 diopters you will need to keep your glasses on.

If you have astigmatism, and it is mild, you may be able to get by without glasses; if your astigmatism is moderate to severe, you must leave your glasses on. [There was an old trick the classic microscopists used to use before the days of high-eyepoint eyepieces if they had astigmatism: they would ask their optometrist or ophthalmologist to tell them their degree of astigmatism, and then, when they were trying to resolve some fine detail in a diatom, e.g., they rotated the specimen until it was in the one orientation where they were astigmatism-free].

In summary, the three adjustments that have to be made in using a binocular-head microscope are:

  1. Adjust focus of graticule using dominant eye.
  2. Adjust interpupillary distance to match exit pupils of eyepieces with pupils of your own eyes.
  3. Adjust diopter ring to make up for difference in visual acuity between your two eyes.

Mechanical Considerations of Binocular Heads

The three adjustments we need to make to use the binocular microscope seem to be pretty straight forward, IF we are right-eye dominant! Most binocular heads are designed for right-eye dominant users, just as most rifles are designed to be used by right-handed, right-eye dominant shooters. Furthermore, there are many different designs of binocular head with different means for adjusting interpupillary distance, and correcting for diopter differences, but few designed to be used by left-eye dominant users. To complicate matters, where two or more people use the same binocular microscope, rarely, if ever, do the users change the necessary adjustments for their own particular use.

You will recall from Experiment 4 on tube length determination and Experiment 28 on the graduated drawtube that the microscope’s mechanical tube length is of extreme importance in controlling spherical aberration. With many binocular heads, the correct mechanical tube length of 160 mm is obtained only when the interpupillary distance of the binocular head is set to 65 mm (the average interpupillary distance). Then, if you close the tubes together, as you would need to do if your interpupillary distance were less than 65 mm; or, if you separate the tubes, as you would need to do if your interpupillary distance were more than 65 mm, you would unintentionally introduce spherical aberration into the final image! If your interpupillary distance were less than 65 mm, you could withdraw your eyepieces slightly, or add spacer rings under your eyepieces to correct the problem by increasing the tube length, but this is a nuisance, so rarely does anybody do it.

Because of the unintentional introduction of spherical aberration, some manufacturers build in automatic tube length correction through a series of gears, e.g., so that as you push the eyetubes together to accommodate < 65 mm interpupillary distance, the tubes automatically lengthen so as to maintain the correct mechanical tube length; likewise, the eyetubes shorten as they are pulled apart >65 mm. Such mechanical compensations are expensive, which is why this feature is only found on the more expensive microscopes.

Figure 34-1 (after Leitz) illustrates the cross section of a conventional rectangular box-like beam-splitter double-prism set arrangement design of binocular head. The eyepieces are shown without graticule, but if there were a measuring scale in one, the eyelens of that eyepiece would be focusable. Notice that one of the eyetubes has a screw-like adjustment to change mechanical tube length (diopter adjustment ring). Notice also that one right-angle prism (right) following the beam splitter is longer than the other one; it is designed that way to make up for the longer path length through glass of the light on the left side; this maintains uniform light intensity in the two eyetubes.

cross section of a conventional rectangular box-like beam-splitter double-prism set arrangement design of binocular head
Figure 34-1. A cross section of a conventional rectangular box-like beam-splitter double-prism set arrangement design of binocular head.

The interpupillary distance adjustment knob is also shown. The lowest optical element is the front view of a prism to deflect the image for inclined eyetubes.

Early in the last century, a microscope designer by the name of Siedentopf came up with a clever solution to the problem of maintaining correct mechanical tube length. Instead of using the conventional rectangular box-like double-prism arrangement in which the eyetubes are pulled apart or pushed together to adjust interpupillary distance, he designed the two eyetubes to “fold” together or apart, much like field binoculars are folded about a central pivot to adjust for interpupillary distance. Still other manufacturers find it more economical, but equally effective, to use the traditional rectangular box with double-prism sets arrangement, but to make both eyetubes independently adjustable for mechanical tube length. We will start with the simplest, most common binocular head intended for right-eye dominant users, and work our way through the alternative solutions for left-eye dominant users.

Adjusting the Binocular Microscope Designed for Right-Eye Dominant Users

Figure 34-2 and Figure 34-3 illustrate a Siedentopf-type binocular head as used on the latest Motic binocular microscope. Figure 34-2 shows the graduated interpupillary distance scale opposite a white-dot reference point; notice that it is set just shy of the average 65 mm. Once you determine your personal interpupillary distance you may simply pre-set this distance on any microscope you are about to use. In this Siedentopf design, you change the interpupillary distance by grasping each side of the head using both hands simultaneously, and “fold” or “unfold” the two sides about the central pivot point until you see a single microscopical image without moving your head from side to side. The mechanical tubelength of the microscope does not change when altering the eyepiece separation distance with this type of design.

Figure 34-3 on the right shows a fixed, non-adjustable eyepiece without scale or crosshairs. If a scale is present the eyelens would have to be focusable, and the right-eye dominant user would start by rotating the eyelens of this eyepiece until the scale is in focus, without looking in the left-side eyepiece.

Next, the right-eye dominant user would look only through the right eyepiece with its now in-focus scale and focus on the specimen using the microscope’s coarse and fine focus; the left eye is not used. At this point both the specimen and the scale are both in sharp focus for the right eye only.

Finally, now blocking the image of the right side, the user looks in the left eyepiece with the left eye, grasps the diopter adjustment knurled ring and rotates it one way or the other until the image of the specimen comes into sharp focus; this step is done without touching the microscope’s coarse or fine focus. Now each side of the binocular head has been adjusted for use by and for that particular user. It is important now to look at the scale at the base of the diopter ring, and to record or remember the setting for future use. Notice that there is a zero indicated; this, theoretically, is the correct setting for users with perfect vision and having an interpupillary distance of exactly 65 mm; most of us will have some different setting. Next, notice that there are scale lines with a positive sign (+) indicated on one side of the zero, and a negative sign (-) indicated on the other side of the zero; these lines represent positive diopters for far-sighted users, and negative diopters for near-sighted users. In Figure 34-3 the diopter setting opposite the fiducial line is slightly off of zero toward the negative (-) side. Whatever this number is, you must memorize it, along with the interpupillary distance, as your particular settings. Now, anytime you go to use any binocular microscope that is not yours you should pre-set your interpupillary distance and diopter setting prior to even looking in the microscope!

The instructions given above are for right-eye dominant users. How will these steps be different for left-eye dominant users? First of all, if there is a graticule in the eyepiece the left-eye dominant user should place the eyepiece with the graticule on the left side of the binocular head, and start by focusing on the graticule using their left eye until it is in sharp focus. Then, using the right eye only with the right-side eyepiece, the specimen is brought into focus using the microscope’s coarse and fine adjustment. Then, back to looking in the left eyepiece only, where the scale is in sharp focus, and rotating the diopter adjustment (not the microscope’s focus controls) until the specimen comes into sharp focus also. Now the microscope is set up for the left-eye dominant user.

There are two huge problems faced by the left-eye dominant user:

  • If the eyepieces are fixed in place, and the scale, pointer, crosshairs, etc are in the right eyepiece, the left-eye dominant user will have to adapt as best they can; it is not an ideal situation.
  • If a polarizing microscope is in use, one focusing eyepiece will have a crosshair or combination crosshair and scale that will be oriented and keyed so as to slip into a slot in the eyetube so that is cannot rotate—this is because the crosshairs now indicate the vibration directions of the polarizer and analyzer (Experiment 15). The slot in the eyetube that receives this specialized keyed eyepiece is always located on the right side of the binocular head, clearly intended only for right-eye dominant users. Left-eye dominant users obliged to use graticules with their non-dominant right eye will often lose sight of the graticule, or the image of it comes and goes. Right-eye dominant users can get some sense of this by using the graticule with their left eye—it just doesn’t feel right; it feels unnatural.

Binocular Microscope Head with Automatic Tube Length Compensation

Figures 34-4, 34-5, 34-6 and 34-7 illustrate a binocular head with automatic tube length compensation. In Figure 34-4 the conventional design binocular head is shown with the eyetubes pulled apart to maximum separation so as to show the entire extent of the interpupillary scale, which is graduated in 2 mm intervals from 54 mm to about 75 mm. Figure 34-5 shows the eyetubes fully pushed together to accommodate users with 54 mm interpupillary distance. What is not apparent in these views is that as the eyetubes are pulled apart and pushed together, they also change length! A side view illustrates this: Figure 34-6 illustrates the eyetubes fully separated to 75 mm; notice that the zero fiducial line easily contacts the white-dot reference point; the eyetubes are fully retracted down into the head to maintain the 160 mm mechanical tube length with the eyetubes fully pulled apart. Compare this figure with Figure 34-7 in which the eyetubes are fully pushed together to 54 mm; notice the space that now appears between the zero fiducial line and the white-dot reference point; the eyetubes have extended out of the head to maintain the 160 mm mechanical tube length with the eyetubes fully pushed together. This is a binocular microscope head with automatic tube length compensation.

The right-side eyepiece in this particular example is both focusing and keyed, because it is the binocular head of a polarizing microscope. The right-side eyepiece has a graticule which is a crosshair or a combination crosshair and measuring scale (eyepiece micrometer), and so the uppermost eye lens is focusable via internal helical screw thread. A locating key built into the eyepiece fits into a slot at the top of the right eyetube so as to keep the crosshairs locked into a north/south east/west orientation. Right-eye dominant users would, of course, start their binocular head adjustment by focusing the eyelens on this right-side eyepiece until the crosshairs are in sharp focus. Left-eye dominant users will again have to adapt to this awkward situation, because there is no slot for the keyed eyepiece in the left eyetube. IF left-eye dominant users simply switch the keyed eyepiece to the left side, it will stick out a couple of millimeters because there is no slot for the key, and if the key is removed there is no way to ensure that the crosshairs are exactly north/south east/west. Optically, it is not a left-eye dominant user-friendly world!

Binocular Microscope Head with Two Independently Adjustable Eyetubes

Figure 34-8 illustrates a unique binocular microscope head made by Zeiss for a polarizing microscope. First of all, note the dual interpupillary distance scales in the center of the photo, graduated in 2 mm intervals from 55 mm to 75 mm; one of the scales is inverted because this particular binocular head was designed to be used on several different models of microscope requiring different mounting arrangements. The interpupillary distance is adjusted by grasping the two eyetubes simultaneously with both hands, and pulling or pushing the eyetubes apart or together until the exit pupils match the eye pupils of the user’s eyes; here the interpupillary distance has been set to about 68 mm. This binocular head does not have automatic tube length correction. Notice that there are knurled rings and scales at the base of both eyetubes. What the user does is after setting their interpupillary distance, they note the reading—in this case 68 mm—and then set both scales to this same number; in doing so, the eyetubes will extend or retract to maintain a true 160 mm mechanical tube length.

unique binocular microscope head made by Zeiss
Figure 34-8. A unique binocular microscope head made by Zeiss.

What is unique about this particular binocular head is that it has key slots machined into both eyetubes; there are four slots total—two machined for north/south east/west orientation, and two for 45° positions; you can just see the 45° position “U”- shaped slots at the base of each eyepiece. What this means is that this binocular head can be used optimally by both right-eye dominant and left-eye dominant users; right-eye dominant users will place the focusing eyepiece with the graticule into the right eyetube slot, as shown in the figure, while left-eye dominant users will insert the focusing eyepiece with the graticule into the left eyetube slot. The adjustment of this kind of binocular head starts by the user placing the eyepiece with the graticule in the eyetube of whichever is their dominant-eye side, and focusing on the crosshair using their dominant eye. Adjust the interpupillary distance, and set this number at that same number at the base of both eyetubes—that establishes the 160 mm mechanical tube length. Then, using only the dominant eye with the crosshair in sharp focus, focus on a specimen using the microscope’s coarse and fine focus controls. Finally, using the non-dominant eye only, rotate the diopter adjustment ring at the base of the eyetube until the specimen is in sharp focus for that eye. Record or memorize the settings for future use.

There are still other binocular head configurations, including no adjustment on the head itself other that interpupillary adjustment, intended to be used with a pair of fully adjustable eyepieces. The principles explained here, however, should be sufficient to adjust any binocular microscope head, and to understand the limitations for left-eye dominant users.

TASK

After you have determined your dominant eye (Experiment 33), examine your binocular microscope head to determine its design type, and note what adjustments are provided for both right-eye dominant and left-eye dominant users. Adjust the binocular microscope head for your use, and record or memorize the settings.

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