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A HIGH EYE RELIEF EYEPIECE
for capturing images through a microscope

Giorgio Carboni, January 2012
Translated by Sarah Pogue

 


INTRODUCTION

In order to take photographs through a microscope using a photographic or video camera, the exit pupil of the eyepiece must coincide with the entry pupil of the objective of the camera. This is not always possible because the eye relief of the eyepiece is too low and the front lens of the camera touches the eyepiece of the microscope before it reaches the desired position. The consequence is the more or less severe vignetting of the image. Normally, this inconvenience occurs with cameras equipped with a zoom lens, in which the entry pupil is too internal. This brief article describes the construction of a high relief eyepiece that will permit you to capture photographs and footage even using cameras equipped with a rather powerful zoom.


CONSTRUCTION
 

 

Figure 2 – Clockwise from top: adapter for a photographic camera,
high relief eyepiece, adapter for a 30.5 mm diameter seat.

 

 

To build this eyepiece, you must procure two objectives from an 8x30 pair of binoculars. These are achromatic lenses which usually have a focal length of 125 mm, a diameter of approximately 32 mm, and a thickness of approximately 9.4 mm. To construct this eyepiece (figure 1), you need to have a lathe, otherwise you will need to go to a friend or a craftsman that has one.

The eyepiece in figure 1 must be mounted in place of an eyepiece of a microscope or on the third tube. The connection (1), in aluminium, is destined to be used in the seat of the eyepiece from which you wish to capture your images. Normally, the intermediate image produced by the microscope is 10 mm below the mechanical rest surface of the eyepiece. Verify this. If you wish to exploit the third tube of the microscope, in certain cases the connection 1 must be made in another way. In any case, make it so that the two lenses (7 and 8) are situated at the focal distance from the intermediate image, as shown in figure 1.

For the main tube (3), take an aluminium tube 40 mm in diameter, 2 mm in width and 65 mm in length.

On the upper part of the tube, the two binocular objectives (7 and 8) will be fixed, both with the most convex surface facing upwards. To prevent them coming into contact with each other, a 2 mm diameter ring (5) should be placed between them. The joint between the tube (3) and the “case” which houses the lenses (4) must be somewhat forced by hand, but in such a way that it is still possible to remove it. To avoid jamming, the case should be made of plastic, for example black PVC. The same applies to the retaining ring (6) of the upper lens which should be pushed to fit, but removable when necessary. These push fit connections save you having to resort to threads and they simplify the construction of the eyepiece.

The join between the connection (1) and the tube (3) must instead be free. Both pieces can be made of aluminium because, since the join is free, there isn’t any risk of jamming. A lateral screw (2) serves to block the two pieces once the correct position has been found. To avoid reflections, place some black cardboard or even better a piece of adhesive velvet (black velvet card) on the interior walls of the aluminium.

REGULATING THE LENS DISTANCE
Both lenses must remain at their focal distance (65 mm) from the intermediate image. Under these conditions, the final image will be produced at the infinity. As it is difficult to verify whether or not the image has been focused at the infinity, we will content ourselves with focusing it at a distance of at least 2 m (for example on the ceiling or on a wall).

Mount a slide with a thin and contrasting sample on the microscope. Select the 10 X objective. Turn the microscope on and focus the sample.

From now on, unless stated otherwise, do not touch the focus of the microscope.

Mount the high eye relief eyepiece on the seat that you have chosen and darken the room. Increase the microscope light to its maximum and project the image of the sample onto the ceiling or onto a wall which is at least a couple of meters away. If the image is not in focus, loosen the screw (2) and move the tube (3) up or down until you obtain a clear image, then tighten the screw again. Dim the microscope light. Looking into the high eye relief eyepiece, you should see the focused image neat. Using a piece of frosted glass and a ruler, or the instrument described below, measure the height of the pupil: this should be greater than 50 mm.

If you can use the third tube, then do so: you will gain the alignment of the optical components. You should always obtain a final image that is focused, if not at infinity, at a distance of at least 2 m. As you have seen, you can achieve this by focusing the final image on the ceiling and regulating the position of the binocular eyepieces, acting on the principal tube (3).

In this way, the light that exits from the high eye relief eyepiece is sufficiently parallel. This fact contrasts with the observation that the light exits from the eyepieces as a converging cone of light and that after passing through a vertex (the exit pupil) is divergent. To understand how this is possible, read this paragraph: http://www.funsci.com/fun3_en/upic/upic.htm#2.1  which explains how the light that departs from each point of the intermediate image exits from the eyepiece as a thin cylindrical beam of parallel light (in yellow in the figure).

POSITION OF THE PHOTOGRAPHIC CAMERA
Take the photographic camera and turn it on. If possible, set it to the infinity and open the aperture to its maximum.
Using a tripod, mount the camera above the high eye relief eyepiece. At this point, you should see a bright disk on the screen. Adjust the zoom just enough to make the vignetting disappear in the 4 corners in a symmetrical manner. The image of the sample should appear. Take as much care as possible with the alignment and the coaxiality of the camera with respect to the eyepiece.

The vertical position of the camera should be sufficiently close to the high eye relief eyepiece. The image should occupy the entire screen with the zoom as low as possible. Take various photos while varying the height of the camera in relation to the eyepiece. Take note of the zoom necessary to remove the vignetting. It is not easy to understand if the camera is in the best position, but it appears that there is a certain tolerance.

If you already have the adapter to fix the camera to the eyepiece, mount it. There are universal adapters available on the market which are suitable in many cases. On this page: http://www.funsci.com/fun3_en/upic/upic.htm#3 a ready-made universal adapter is described. Provisionally, you could also use a tripod to position the camera on the high eye relief eyepiece.

Now, you can once again adjust the focus of the microscope with its knobs.

To achieve a precise focus you can set the zoom to maximum, readjust the focus and return to the zoom that you prefer. This method must be controlled, because it is not suitable for all camera types. However, if it works, it is very convenient.

This eyepiece can also be used for normal observations under the microscope, even if this is somewhat inconvenient. If you use two eyepieces such as these, you can also easily use your eyeglasses.

In certain cases, such as a video camera with a high zoom, the height of the exit pupil of this eyepiece may not be sufficient. In this case, try to distance the two eyepieces (7 and 8). Otherwise, you could construct an eyepiece with an even higher pupil using two binocular objectives with a diameter of 50 mm.

http://www.funsci.com/fun3_it/sini/mo/microfoto_digitale.pdf Here you can find more detailed information on this high eye relief eyepiece (in Italian Language only).

In any case, there are eyepieces available on the market with a pupil height of up to 22 mm, but which are capable of supplying a normal magnification of 10 X.


MEASURING THE PUPIL

 


Figure 3 – A small tool for determining
some features of the eyepieces.
 

Figure 4 – The tool is positioned on an eyepiece
and shows the pupil in focus. Here,
you can measure the diameter of the pupil.

Figure 5 – The scale indicates the height of
the eye relief of the eyepiece with a precision
of approximately 0.5 mm.

 

With a couple of hours work, you can construct a tool for measuring the eye relief of an eyepiece with a certain precision (figures 3, 4 and 5). I remind you that the eye relief of an eyepiece goes from the mount to the vertex of the cone of light that exits from the eyepiece. This measurement must be made with the eyepiece mounted on the switched-on microscope and under normal observation conditions. This instrument will also help you to measure the diameter of the exit pupil.

Let’s return to the construction of this small instrument. Procure an L-shaped piece of aluminium whose sides are 25 mm in length and cut a piece approximately 50 mm long. Level off and square the upper edges. With a thin layer of silicone glue fix a piece of frosted glass at the position of the “0” (figure 3). The frosted part of the glass should face downwards. On the inside glue a piece of paper measuring tape so that the zero line touches the glass. This glass can be obtained from a common microscope slide, using a glass-cutter. For the frosting, use an abrasive with a grain of 600 that you can buy from a marble cutter, while the measuring tape can be gotten for free from a DIY store.

The 50 mm length of this small instrument should be sufficient for eyepieces with an eye relief of up to 30 mm. In the case of particular geometries of the eyepieces and the binocular casing, you may need to make the instrument of a different length. Therefore, prepare at least three pieces of frosted glass that you can use to build other eventual measuring tools, for example one suitable to a high eye relief eyepiece.

http://www.funsci.com/fun3_it/guida/guida6/micro6.htm#2.15  How to frost glass.

 


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