Microphotography with ‘hybrid’ eyepieces

Introduction

When using microscopes with finite optics (mostly 160 or 170 mm mechanical tube length) there are roughly two ways to perform microphotography:

1. Afocal method. With this method, there is a lens or camera objective between eyepiece and camera sensor. The camera lens acts as a relay optic and captures the image that is corrected by the eyepiece.

2. Projection by means of a special projective or photo-eyepiece. Here there is no (camera) lens between the eyepiece and sensor and the image is projected onto the sensor by the respective eyepiece or projective.

There is a third way, however, which I would like to leave out of consideration here and that is the direct projection of the image from the objective onto the sensor, without the use of an eyepiece.

For a long time I have used the afocal method, but after a lot of experimenting with eyepieces that can serve as projection eyepieces, I have become convinced that the latter method produces better images. With the afocal method, there is extra glass between the eyepiece and the sensor. It is certain that this will never improve the image in the center of the microscope field. Any additional optical element in the optical path can potentially degrade the image. And on any additional glass surface, disturbing dust speckles can settle.

Projectives are special eyepieces that correct the image formed by the objective and project it onto the sensor. For photography with DSLR cameras, projectives with low magnifications are particularly suitable. A highly wanted projective is for example the NFK 1.67x, to be used with the Olympus BH2 microscope system. However, the price for this projective on the second-hand market is often absurd.

Not all microscope manufacturers have produced useful projectives in the past. Carl Zeiss, for example, went the afocal path at one point and because of this, photography with Zeiss microscopes with 160 mm mechanical tubelenght was and is mainly done with a relay lens between eyepiece and sensor. If you want to photograph with for example a Zeiss Standard microscope without any relay optics, an ordinary observation eyepiece (e.g. a Kpl10x) can be used as a projective when the position of this eyepiece in the photo tube is raised a few mm. However, the results with this are variable and there is often a clear chromatic aberration at the edges of the image when photography is done this way.

Hybrid eyepieces

I own a fairly large collection of eyepieces and at one point I wondered what would happen if I would combine two different Zeiss eyepieces and used them as a projection eyepiece for photography. So, I exchanged the eye lens of a Zeiss Kpl8x eyepiece with the eye lens of a Kpl10x eyepiece. I had only expected a bad image, but the result was surprising. Not only did the image on the camera appear parfocal with the visual image, there was also very little chromatic aberration at the edges. The result was significantly better than when using a raised Kpl10x eyepiece. The fact that it works better this way than raising a normal eyepiece is probably caused by the increased distance between field lens and eye lens. Apparantly, the eyepiece is functioning like a projection-eyepiece this way. With some photo-eyepieces, like for example the Carl Zeiss FK10x, it's also possible to vary the distance between the two lenses. In any case, the chromatic aberration is largely canceled. I also experimented with Leitz and Olympus eyepieces. I have called these mixed eyepieces 'hybrid eyepieces'. Some results are shown below.

If you want to experiment with this method, it is important that an adapter is used with which the distance between camera sensor and eyepiece can be varied.

Zeiss

Hybrid eyepiece Kpl10-8-10: field lens Kpl10x - barrel Kpl8x - eye lens Kpl10x

I tested the combination of Kpl8x and Kpl10x eyepieces with a stage micrometer on a number of different Zeiss objectives. I started with just screwing the Kpl10x eye lens onto a Kpl8x eyepiece. This combination works well and parfocality is achieved. The combination of both lenses of a Kpl10x eyepiece screwed onto a Kpl8x barrel gives the most satisfactory results and a slightly larger portion of the field of view is captured. Some results of this combination are shown below. This hybrid eyepiece produces significantly less chromatic aberration compared to a normal Kpl10x or Kpl W10x/18 that is used in a raised position. There is a slight pincushion distortion, but this is within an acceptable range. Reasonable to good results were achieved with all Zeiss objectives that I have tested, including various (plan) achromats, Neofluars and (plan) apochromats. The tests were done with a Zeiss Standard 16 with monocular and trinocular tube. In addition, several horseshoe stands with 160 mm mechanical tube length were used. The hybrid eyepiece was not raised in the tube and the distance between camera sensor and eyepiece was adjusted until the image on the camera was 100% parfocal with the visual image. The tests were done with both an Olympus PEN E-Pl1 camera and a Canon 600D camera.

Hybrid Kpl10-8-10 eyepiece in use as a projection eyepiece with the Zeiss Standard and Canon 600D camera. The field lens and the eye lens of a Kpl10x eyepiece were screwed onto the barrel of a Kpl8x eyepiece. The distance between the eye lens (L) of the eyepiece and the bayonet connection of the Canon 600D is approximately 21 mm. The precise distance should be optimized until the image on the camera display is 100% parfocal with the visual image.

Stage micrometer photographed with a Zeiss Plan 10/0.25 and hybrid eyepiece Kpl10-8-10. Camera: Canon 600D.

Stage micrometer photographed with Carl Zeiss Plan 25/0.45 (upper image) and Zeiss Planapo 25/0.65 (lower image). Hybrid Kpl10-8-10 eyepiece and Canon 600D camera.

Stage micrometer photographed with Zeiss Plan 40/0.65 (top) and Zeiss Planapo 40/1.0 (bottom). Hybrid Kpl10-8-10 eyepiece and Canon 600D camera.

Comparison between eyepiece projection with a hybrid eyepiece Kpl10-8-10 (top) and a raised Kpl W10x/18 eyepiece (bottom). Chromatic aberration is clearly visible with the normal eyepiece. Objective: CZ Plan 25/0.45. Camera: Canon 600D.

Arachnoidiscus, photographed with Carl Zeiss Plan 25/0.45 objective and hybrid Kpl10-8-10 eyepiece. This is the complete section as it was captured by the sensor of the Canon 600D.

Left: a moss photographed with a Zeiss Planapo 25/0.65 and hybrid Kpl10-8-10 eyepiece. Right: The orange rectangle indicates the portion of the microscopic field of view that is photographed using this eyepiece and a Canon 600D camera. The circle refers to an eyepiece with a field number of 18, such as a Zeiss Kpl W10x/18 eyepiece.

Hybrid eyepiece Oly5-Kpl10: field lens Olympus-5x - barrel Olympus-5x - eye lens Kpl10x

A combination that also works well is an Olympus 5x Huygens eyepiece on which a Zeiss Kpl10x eye lens is screwed. The original Olympus field lens remains in place. Zeiss purists may be shocked by this combination, but all that matters is the end result. And that's not bad at all, as can be seen in the images below that were taken with the Canon 600D camera. With this hybrid eyepiece, a somewhat larger part of the field of view is photographed and, moreover, the pincushion distortion is virtually absent. The distance from the eyepiece to the sensor of the Canon 600D was adjusted until no more vignetting occurred. The image on the camera was parfocal with the visual image.

Installation of hybrid eyepiece Oly5-Kpl10 on the Zeiss Standard. A: Olympus 5x Huygens eyepiece, the lenght of the metal part is 49 mm. B: Zeiss Kpl10x eyepiece. C: the black ring of the Kpl10x eyepiece is applied to the Olympus 5x eyepiece. D: Finally, the Kpl10x eye lens is screwed on. E: hybrid eyepiece in the photo-tube of the Zeiss Standard. F: Canon 600D on the Zeiss Standard. The distance between the eye lens of the eyepiece and the bayonet connection of the Canon 600D is approximately 19 mm. The precise distance should be optimized until the image on the camera display is 100% parfocal with the visual image.

Stage micrometer photographed with Zeiss Planapo 25/0.65 and hybrid eyepiece Oly5-Kpl10. Camera: Canon 600D.

Left: Stage micrometer photographed diagonally with Carl Zeiss Plan 25/0.45 and hybrid eyepiece Oly5-Kpl10. Right: The part of the field of view with fieldnumber 18 that is captured with the Canon 600D when using this hybrid eyepiece.

Stage micrometer photographed with Carl Zeiss Planapo 40/1.0 and hybrid eyepiece Oly5-Kpl10. Camera: Canon 600D.

Histological section of trachea, photographed with Carl Zeiss Planapo 40/1.0 and hybrid eyepiece Oly5-Kpl10. Camera: Canon 600D.

Hybrid eyepiece C5-PeriGF10: field lens Zeiss C5x - barrel Zeiss C5x - eye lens Leitz Periplan GF10x

The most recent combination I tested consists of a Carl Zeiss C5x eyepiece with the eye lens of a Leitz Periplan GF10x eyepiece. The black sleeve of the C5x eyepiece is removed. If this combination is placed in the photo-tube of a Zeiss trinocular tube (the old 'sliding' version), the eyepiece is raised about 11.5 mm because there is a diaphragm in this photo-tube. The hybrid eyepiece is resting on this diaphragm. This situation turned out to be perfect. A very large part of the field of view was captured with the Canon 600D's sensor, fully parfocal and with minimal chromatic aberration. In addition, there is no vignetting. When a monocular tube (without a diaphragm) is used, the position of this eyepiece in the tube must be raised with 11.5 mm. This hybrid eyepiece has proven to be the best combination so far for Zeiss 160mm objectives.

Construction and placement of hybrid eyepiece C5-PeriGF10 for Zeiss Standard. A: Carl Zeiss C5x eyepiece. B: eye lens of Leitz Periplan GF10x eyepiece. C: C5x eyepiece with the eye lens and black sleeve removed, the lenght of the metal part is approximately 49mm. D: Leitz Periplan GF10x eyepiece is screwed onto the C5x barrel. E: Hybrid eyepiece in the photo-tube of the Zeiss standard where the eyepiece is raised approximately 11.5 mm.

Left: stage micrometer photographed with Carl Zeiss Plan 25/0.45 and hybrid eyepiece C5-PeriGF10. Right: part of the field of view with field number 20 that is photographed with the Canon 600D.

Stage micrometer and histological slide photographed with Carl Zeiss Planapo 40/1.0 and hybrid eyepiece C5-PeriGF10. Camera: Canon 600D.

Stoma in the epidermis of Tradescantia zebrina photographed with Carl Zeiss Neofluar 63/1.25 and hybrid eyepiece C5-PeriGF10. Camera: Canon 600D.

Comparison between photography with hybrid eyepiece and afocal method using a diatom slide. An Olympus PEN E-PL1 camera was used. Left image: C5-PeriGF10 hybrid eyepiece. Right image: Sigma 30mm camera lens and Kpl10xW eyepiece. Objective: Zeiss Plan 40/0.65. The results are similar.

Stage micrometer photographed with Olympus PEN E-PL1 camera and Zeiss objective Plan 40/0.65. Upper image: hybrid eyepiece C5-PeriGF10. Lower image: afocal method using Sigma 30 mm lens and  Kpl10xW eyepiece.

Hybrid eyepiece C5-Kpl10: field lens and barrel of Zeiss C5x eyepiece - eye lens of Zeiss Kpl10x eyepiece

When this combination is set parfocally, not the entire sensor of the Olympus PEN E-PL1 is exposed and there is some vignetting. This hybrid eyepiece could be ideal for smaller sensors such as a 1 inch sensor. The image quality is very good.

Stage micrometer (left) and Arachnoidiscus (right) photographed with hybrid eyepiece C5-Kpl10 and Olympus PEN E-PL1 camera. Objective: Carl Zeiss Plan 25/0.45.

Leitz

Hybrid eyepiece L6-Oly5-PeriGF10: field lens Leitz 6x eyepiece - barrel Olympus 5x eyepiece - eye lens Leitz Periplan GF10x

I tested different eyepiece combinations with the Leitz Dialux-II and Laborlux-12. For example, with Leitz NPL Fluotar objectives on the Laborlux-12 I get good results with an eyepiece that is composed from the parts of 3 different eyepieces: a Leitz 6x field lens, the metal barrel of an Olympus 5x eyepiece and the eye lens of a Periplan 10x eyepiece. Quite exotic, but it works well with NPL-Fluotar objectives. With the the Leitz EF achromats this combination produces slightly more chromatic aberration. The combination of an Olympus 5x barrel with both lenses of a Leitz Periplan GF10x also works well for both EF achromats and NPL Fluotars but the part of the field of view that is captured will be smaller.

Hybrid eyepiece L6-Oly5-PeriGF10 composed of 3 different eyepieces. The lenght of the Olympus barrel is 49 mm. Right: Stage micrometer photographed with this eyepiece and a Leitz NPL Fluotar 25/0.55 objective on the Leitz Laborlux-12. Camera: Canon 600D.

Dracaena epidermis photographed with Leitz Fluotar 25/0.55 and hybrid eyepiece L6-Oly5-PeriGF10. Microscope: Leitz Laborlux-12. Camera: Canon 600D, uncropped image.

Hybrid eyepiece Peri8-Oly5-Peri8: field lens Leitz Periplan 8x - barrel Olympus 5x - eye lens Leitz Periplan 8x

One of the most recent and best combinations that I have tested sofar consisted of the field lens and eye lens of an older version Leitz Periplan 8x eyepiece, screwed onto the barrel of an Olympus 5x eyepiece. This hybrid eyepiece worked well with all Leitz 160 mm and 170 mm planachromatsNPL Fluotars and planapochromats that I tested, both on the Leitz Laborlux-12 and the Leitz Dialux-II. Several achromats, planachromats, NPL Fluotars and planapochromats were tested. With this hybrid eyepiece, the image of the camera was 100% parfocal with the visual image. There was no significant pincushion distortion detected.

A: Hybrid eyepiece Peri8-Oly5-Peri8 that was constructed by screwing the field lens and eye lens of a Leitz Periplan 8xB eypiece onto the barrel of an Olympus 5x eyepiece. B: Original Leitz Periplan 8x eyepiece. C: Hybrid eyepiece with a total lenght of 54 mm (including lenses), the lenght of the metal part is 49 mm. D: Ihagee adapter attached to the photo-tube of the Leitz Dialux-II. E: Connection of the Canon 600D to the photo-tube. The distance from the eye lens to the bayonet mount is 37 mm and the camera image is exactly parfocal with the visual image.

Stage micrometer photographed with hybrid Peri8-Oly5-Peri8 eyepiece. Upper image: Leitz Plan 25/0.50. Lower image: Leitz Pl Apo 25/0.65. Camera: Canon 600D.

Hybrid eyepiece L6-Oly5-Peri10: field lens Leitz 6x - barrel Olympus 5x - eye lens Leitz Periplan 10x

This combination worked well with Leitz 160mm and 170mm achromats / semi-planachromats. The eyepiece is parfocal with the Olympus PEN E-PL1 camera on the Leitz Laborlux-12 and is shown in the following image.

Hybrid eyepiece L6-Oly5-Peri10 consisting of the field lens of a Leitz Huygens 6x eyepiece, the barrel of an Olympus 5x Huygens eyepiece (lenght: 49 mm) and the eye lens of an older version Leitz Periplan 10x eyepiece. The total lenght of the eyepiece including field lens and eye lens is 55 mm. The last two pictures show the camera coupling to the Leitz Laborlux-12. In the second photo from the right the Olympus PEN E-PL1 camera, where the distance from the eye lens to the bayonet mount is 30 mm and the setup is almost parfocal. On the far right, the Canon 600D, where the distance from the eye lens to the bayonet mount is 19 mm and the setup is 100% parfocal.

Stage micrometer photographed with Leitz EF 25/0.50 and hybrid eyepiece L6-Oly5-Peri10. Camera: Olympus PEN E-PL1 connected to the Leitz Laborlux-12.

Stage micrometer photographed with Leitz EF 25/0.50 and hybrid eyepiece L6-Oly5-Peri10. Camera: Canon 600D connected to the Leitz Laborlux-12.

Slide of a plant stem photographed with Leitz EF 25/0.50 and hybrid eyepiece L6-Oly5-Peri10. Camera: Olympus Canon 600D connected to the Leitz Laborlux-12.

Olympus

Olympus 37 mm objectives. Hybrid eyepiece E5-P7: field lens and barrel of Euromex 5x eyepiece - eye lens of Olympus P7x

I exchanged the eye lens of an unbranded 5x eyepiece (belonging to a Euromex horseshoe stand) with an Olympus P7x eye lens. It turned out to be a great find. This combination worked very well with most Olympus objectives of 37 mm parfocal lenght. There is no pincushion distortion. Good results (both with Canon 600D and Olympus PEN E-PL1 camera) were achieved with planachromats 10/0.25, 20/0.40, 40/0.65 and achromats 40/0.65 en 60/0.80. Only the 100/1.25 achromat gave lesser results but fortunately this objective is not so populair among many microscopists. The normal achromats with low magnifications (4x and 10x) are being overcompensated, which leads to chromatic aberration. This is because the hybrid eyepiece contains the eye lens of a compensating P7x eyepiece. This over-correction is no different with the afocal method: here too, for Olympus 37 mm achromats 10x or less, non-compensating eyepieces must be used. The following hybrid eyepiece worked well for the low power achromats: of an unbranded 6x eyepiece (Euromex), the eye lens was replaced by an unbranded 10x (also Euromex) eye lens.

Hybrid eyepiece E5-P7 consisting of the lower part of a no-name (Euromex) Huygens 5x eyepiece and the eye lens of an Olympus P7x eyepiece. Total lenght of this hybrid eyepiece including the eye lens is 59 mm, the lenght of the metal part is 52 mm. The distance from the eye lens to the bayonet mount of the Canon 600D is approximately 34 mm. The construction is very compact, so that a stable connection is possible, even on a horseshoe stand, in this case an Olympus HSC. At the right, in orange, the part of the field of view is seen that is captured by the sensor of the Canon 600D. The area that is captured without vignetting is as large as it can be when using eyepieces with field number 20.

Stage micrometer photographed with a 37 mm Olympus Plan 20/0.40 objective and hybrid eyepiece E5-P7. Camera: Canon 600D.

Stage micrometer photographed with 37 mm Olympus Plan 20/0.40 objective. Here, an Olympus FK2.5x projective was used in combination with the Olympus OM-Mount Photomicro Adapter L. It is clear that a smaller part of the field of view is captured compared to the hybrid eyepiece. Camera: Canon 600D.

Leaf of a moss taken with a 37 mm Olympus Plan 20/0.40 objective and hybrid eyepiece E5-P7. Camera: Canon 600D, attached to a Olympus Horseshoe stand.

Olympus 37 mm objectives. Hybrid eyepiece L6-P10: field lens and barrel of Leitz 6x - eye lens of Olympus P10x

This eyepiece proved to work well with Olympus 37 mm objectives and can be made exactly parfocal when it is raised approximately 10-11 mm in the tube. A very slight barrel distortion occurs, which is however hardly disturbing.

Stage micrometer (left) and pollen of a lily (right) photographed with hybrid eyepiece L6-P10 and Olympus objective F40/0.65, a semi-planachromat. The photos were taken with the Canon 600D mounted on the Olympus HSA microscope, a simple horseshoe stand that was used in schools.

Olympus 45 mm objectives. Hybrid eyepiece E5-P7: field lens and barrel of Euromex 5x eyepiece - eye lens of Olympus P7x

As for the 45mm Olympus objectives, in theory they should require a different correction than the 37mm lenses because they were used with different eyepieces and projectives. Nevertheless, reasonable to good results were achieved with most of the Olympus EA, A, D Plan en S Plan objectives. I performed the tests using both a Zeiss microscope and an Olympus BH2 microscope. The reason for using a Zeiss stand is the normal 23.2 mm tube it contains, which makes it possible to use an Ihagee adapter and to adjust the distance between eyepiece and camera sensor. The distance between the eyepiece and the camera sensor was set in such a way that no more vignetting occurred and the sensor of the Canon 600D was just fully exposed. With the Olympus BH2 I could not set this distance freely and I used a temporary set-up with which the results were still quite good. The only objectives that gave lesser results were the achromats 100/1.25.

Stage micrometer (left) and pollen (right), fotographed with Olympus S Plan 40/0.70 and hybrid eyepiece E5-P7.

Zeiss-Winkel

Hybrid eyepiece E5-P7: field lens and barrel of Euromex 5x eyepiece - eye lens of Olympus P7x

Zeiss-Winkel objectives, both of 39 mm and 45 mm parfocal lenght, harmonize well with the Olympus eyepieces that were intended for the 37 mm Olympus objectives. For afocal photography I usually use Olympus P10x or WF10x eyepieces. The 5x-P7x hybrid eyepiece described above for Olympus objectives also worked well with Zeiss-Winkel objectives 25/0.45 and higher. Zeiss-Winkel objectives give a strong field curvature, so that with flat and thin slides the image becomes blurry towards the edges. However, this is not a problem for less flat specimens or for imaging pond life. Some may wonder: why use such old lenses with field curvature for photography? The answer is simple: in my opinion, these old Zeiss-Winkel achromats give a cleaner image than many much more modern (plan) achromats.

Stage micrometer and slide with pollen photographed with a Zeiss-Winkel 40/0.65 objective and hybrid eyepiece E5-P7. On the left, the image was focussed on the center, the field curvature is clearly visible at the edges. On the right, the image was focussed on the edge and here there is minimal chromatic aberration. Camera: Canon 600D.

Hybrid eyepiece L6-P10: field lens and barrel of Leitz 6x - eye lens of Olympus P10x

This combination also works well with Zeiss-Winkel objectives 25/0.45 and higher. The parts for this hybrid eyepiece are easier to find than for the E5-P7 eyepiece. Parfocality is achieved by raising the eyepiece approximately 10-11 mm. Leitz 6x and Olympus P10x eyepieces are often found on the second-hand market.

Stage micrometer (left) and mitosis slide of an onion (right) photographed with Zeiss-Winkel 40/0.65 and hybrid eyepiece L6-P10. Much of the field of view is photographed with reasonable planarity. If you consider that this is an old achromat from the late 1940s, this result is far from bad. I have seen modern (semi) planachromats that gave worse results. Camera: Canon 600D.

Hybrid eyepiece ZW8-ZW12.5: field lens and barrel of Zeiss-Winkel 8x - eye lens of Zeiss-Winkel 12.5x

The low power objectives from Zeiss-Winkel need much less compensation than the high power objectives. The above combination of 8x and 12.5x eyepieces worked well with Zeiss-Winkel objectives 2.5/0.06, 4/0.10, 10/0.25 and 16/0.32.

Carl Zeiss Jena

Hybrid eyepiece C5-PeriNF10: field lens and barrel of Carl Zeiss C5x eyepiece - eye lens of Leitz Periplan NF10x eyepiece

I tested some Carl Zeiss Jena (CZJ) (plan)achromats and apochromats with 45 mm parfocal length with a hybrid eyepiece made up of the lower part of a Carl Zeiss C5x eyepiece and the eyepiece of a Leitz Periplan-NF10x eyepiece. I compared the results obtained with this eypiece with the afocal method in which I used a CZJ PK12.5x eyepiece in combination with an Olympus 17 mm pancake camera lens. The image quality obtained with the hybrid eyepiece was comparable to the afocal method and perhaps even slightly better. The sensor of the Olympus PEN E-Pl1 camera was not fully exposed with this hybrid eyepiece which causes some vignetting. This eyepiece is very suitable for cameras with a 1-inch sensor. I tested the hybrid eyepiece with the Nikon 1 J1 camera and the 1" sensor was fully exposed.

During testing, it became clear to me that the residual errors in different CZJ objectives are not present to the same extent, as is the case with objectives from Carl Zeiss Oberkochen. Even between different CZJ apochromats there is a difference in the required compensation. With CZJ PK10x/12.5x eyepieces it is visually noticeable that, for example, the CZJ 6.3/0.10 apochromat is overcompensated compared to the CZJ 16/0.40 and 40/0.95 apochromats. There is also a small difference between higher power CZJ (plan) achromats (≥ 40) and apochromats. Finally, the low power achromats (16) require less compensation and therefore another hybrid eyepiece is needed: C8-A10, in which the eye lens of a CZJ A10x(14) eyepiece is screwed onto the barrel of a Carl Zeiss C8x eyepiece. The differences in compensation described here are also seen with afocal photography and the use of CZJ PK eyepieces. CZJ had clearly not brought the compensation for the different objective classes to the same level.

Slide of Arachnoidiscus photographed with CZJ Apochromat 16/0.40. Top two images: C5-PeriNF10 hybrid eyepiece focussed at the center (left) and at the edge (right). Bottom two images: afocal method with CZJ PK12.5x eyepiece and Olympus 17 mm pancake lens focussed at the center (left) and at the edge (right). Camera: Olympus PEN E-Pl1.

Diatom slide photographed with CZJ Apochromat 40/0.95. Left: C5-PeriNF10 hybrid eyepiece. Right: afocal method with CZJ PK12.5x eyepiece and Olympus 17 mm pancake lens. Camera: Olympus PEN E-Pl1.

The filamentous algae Spirogyra (green) and Melosira (brown) photographed with objective HI 100/1.25 (aus Jena). The image was projected onto the 1" sensor of the Nikon 1 J1 camera using hybrid eyepiece C5-PeriNF10.

Hybrid eyepieces and microscope cameras

Eyepiece cameras can only be used to a limited extent with microscopes with a finite mechanical tube length. Two problems arise here:

1. Most microscope cameras have a small sensor. If there is no optical element in the camera that reduces the image size, only a very small part of the microscopic field of view will be captured.

2. For microscopes with a finite mechanical tube length, the image coming from the objective is not corrected for residual aberrations and appropriate optical compensation must be applied. With eyepiece cameras without internal optics, there is no correction of residual errors because the camera takes the place of a compensating eyepiece. But also with eyepiece cameras that contain a reduction lens, there will be no correction because the optical elements in such cameras do not compensate for the residual errors in the image formed by the objective.

Both problems can be solved quite easily with hybrid eyepieces. Here, a microscope camera is used without internal optics and the image is both being reduced and corrected by the hybrid eyepiece and projected onto the sensor.

I have put together a hybrid eyepiece for Zeiss 160 mm objectives in which a large part of the field of view can be projected onto a very small sensor. For this test I used the BRESSER MikrOkular Full HD eyepiece camera. This is a very basic 2 Mp eyepiece camera from the lower price range with a sensor of 5.86 x 3.28 mm. The camera has no internal optics and is inserted into the tube of the microscope instead of the eyepiece. In this way, only a very small part of the field of view is photographed and, moreover, chromatic aberration occurs. With the hybrid eyepiece, a very large part of the field of view with field number 20 could be projected onto this small sensor, parfocally and without vignetting. To create the correct parfocal distance, a custom-made metal or plastic sleeve can be placed between the eyepiece and camera. The camera is then slid into this sleeve. How the hybrid eyepiece was constructed is shown in the following image.

Hybrid eyepieces also work well with C-mount cameras. Some cameras are testet here: 'Combining hybrid eyepieces with microscope cameras'.

Hybrid eyepiece C5-ZW5-Kpl10. The eyepiece is composed of the parts of 3 different eyepieces. A: Zeiss C5x eyepiece without eye lens B: The upper barrel from a Zeiss-Winkel 5x eyepiece, this contains a screw thread. C: Eye lens of Zeiss Kpl10x eyepiece. D: Hybrid eyepiece in which the distance between eye lens and field lens is 75 mm.

Comparison between images made with the BRESSER MikrOkular Full HD eyepiece camera using a slide with the diatom Cymbella (top) and a slide with mitosis stages from the root tip of an onion (bottom). In the two photos on the left, the camera was used as intended, i.e. slid into the tube without the intervention of an eyepiece or projective. In the right images, the hybrid eyepiece C5-ZW5-Kpl10 was placed in the tube and the distance from the camera to the eyepiece was adjusted so that the image from the camera was parfocal with the visual image. The photos on the right show a much larger part of the field of view and in addition, the chromatic aberration is corrected. Objective: Carl Zeiss Neofluar 63/1.25.

The original eyepieces

Anyone who owns a collection of eyepieces from different brands can make many combinations. There is a good chance that a combination can be made that works well as a projection eyepiece. At the moment I am testing other combinations than the ones discussed here. The following image shows the original eyepieces that served as the basis for the hybrid eyepieces.

The original eyepieces that were used and combined with each other. From left to right: Zeiss Kpl10x, Zeiss Kpl8x, Leitz 6x B, Olympus P7x Bi, no-name 5x (Euromex, metal part: 52 mm), Olympus 5x (metal part: 49 mm) and Leitz Periplan GF10x.

Conclusion and comments

Combinations of different eyepieces, here called hybrid eyepieces, can give good results as a projection eyepiece. Considerably less chromatic aberration occurs compared to an ordinary eyepiece that is raised. Apparently, by increasing the distance between field lens and eye lens, some of the chromatic aberration is eliminated and the hybrid eyepiece is functioning as a projective. Slight pincushion distortion may occur with some combinations like the Kpl10-8-10 hybrid eyepiece. The level of pincushion distortion is in my opinion still acceptable and will not be noticeable in most slides. The degree to which chromatic aberration occurs at the edges may be slightly more or even less compared to the afocal method. With the combinations I tried, the chromatic aberration was minimal. It is recommended to try out different combinations of eyepieces.

The results were satisfactory with Olympus PEN E-Pl1 (micro 4/3), Canon 600D (APS-C) and Nikon 1 J1 (1" sensor) cameras. It should also work well with other cameras.

With certain combinations, like the Kpl10-8-10 hybrid eyepiece, the part of the field of view that is photographed may be slightly smaller compared to some afocal setups. On the other hand, with the E5-P7, Oly5-Kpl10, L6-Oly5-Peri10 and C5-PeriGF10 hybrid eyepieces, a considerable part of the field of view was captured. 

I personally find it not that disturbing when a smaller part of the viewfield is recorded. However, with histological slides, it may be useful to photograph as large a portion of the field of view as possible. With many other slides, objects that you want to photograph will normally be located in the center and not at the edge of the field of view. When a larger part of the sensor is used for an object to be photographed, this will be beneficial for the photographic resolution.

The advantage of this method compared to the afocal method is the simple and compact camera setup and the lack of extra glass between eyepiece and sensor. A camera lens can give rise to light reflections and is a place where dust particles can settle and these become clearly visible in the photo. With the hybrid eyepiece, 'hotspots' will also occur less often: a small illuminated spot in the center of the field of view that can be very disturbing for photography. Moreover, there is no need to invest in a camera lens or an expensive and hard to find projective or photo-eyepiece. And finally, there is no extra weight of a camera lens, not unimportant when a DSLR has to be placed on, for example, a horseshoe stand.

Sometimes it is good to do some crazy experiments instead of sticking to the usual 'rules'. I can recommend it to everyone.

 

It would be great receiving feedback from people who are going to try this method. Please refer to this website when the method is going to be mentioned elsewhere.