The right combination of objective and eyepiece

To obtain a good microscopic image it is important that the objective and eyepiece are optically matched. In general, microscope manufacturers take care of this and most of the time it is best to use an objective with an eyepiece from the same manufacturer. But not always……. On a regular basis I see that certain combinations of objective and eyepiece from different brands give a better image than a combination from the same manufacturer.

Having the right match of objective and eyepiece is more important for photography than for visual observation. During photography, optical aberrations become noticeable. The tests in this section are mainly aimed at the use of eyepieces for photography.

To determine if a certain objective-eyepiece combination is a good match, the image quality can be judged with a diatom slide or an object micrometer. But also a stained or fresh plant slide can be used. With low power objectives like 2.5x - 10x, I find white sand to be a very useful material to detect chromatic aberrations.

For the tests I used an Olympus PEN E-PL1 camera with a Sigma 30 mm lens or an Olympus 17 mm Pancake lens as relay optics between eyepiece and sensor. With the Pancake lens it is possible to photograph the entire microscope field and to detect optical aberrations up to the edge of the field of view.

 

Compensating eyepieces

The image of most objectives with a finite mechanical tube length (usually 160 or 170 mm) is not fully corrected and still contains residual errors that can be removed by matching eyepieces. Zeiss for example, introduced an opposite aberration in the eyepieces to level out the residual errors of the objective. These eyepieces are called compensating eyepieces and they eliminate the remaining aberrations in the image from the objective. The higher the objective magnification, the more optical aberrations will be there in the uncorrected image from the objective. These errors are greatly reduced with compensating eyepieces. Compensating eyepieces are often indicated with a letter C or K. For example, Zeiss had a K (Carl Zeiss Jena and Zeiss-Winkel), PK (Carl Zeiss Jena) or C, CPL, Kpl, KF (Zeiss Oberkochen) written on the eyepieces. Higher power achromats and all better corrected objectives such as planachromats, (plan) fluorite objectives and (plan) apochromates require compensation and they generally work best with a compensating eyepiece from the same manufacturer. Many low power achromats (2.5x -10x) however require less correction and some are clearly over-corrected by compensating eyepieces, resulting in an image with clear chromatic aberrations.

A good example of an optical mismatch between objective and eyepiece from the same manufacturer is the Carl Zeiss 3.2/0.07 objective. Carl Zeiss Oberkochen only made compensating eyepieces (see above) and those eyepieces work fine with all Zeiss objectives produced in Oberkochen (including the 10/0.22 achromats) except for the 3.2/0.07 achromat. German microscopists sometimes call the 3.2/0.07 objective "Flashenboden", which refers to the bottom of a glass bottle. This indicates quite clearly that the image is not great when used with any of the compensating eyepieces from Carl Zeiss. But when this lens is combined with an eyepiece that has less or no compensation at all, the image is greatly improved, as the following images show.

Sand grains photographed with a Carl Zeiss 3.2/0.07 objective. Left, a Carl Zeiss C10x eyepiece was used and right, an Olympus 10x eyepiece. The Olympus eyepiece barely compensates and the image is much better than the one obtained with the C10x eyepiece. Blue colour fringes can clearly be seen with the C10x eyepiece.

Section of a stained plant stem photographed with a Carl Zeiss 3.2/0.07 objective and with Olympus, Euromex and Carl Zeiss (CZ) eyepieces. With the CZ eyepieces, strong chromatic aberration is visible towards the edge.

Zeiss-Winkel

Another example where eyepieces from a different manufacturer can give better results are the objectives from Zeiss-Winkel. The Zeiss-Winkel 10/0.25 achromat hardly needs any compensation. This lens works best with a less or non-compensating eyepiece, for example a no-name Huygens eyepiece.

Epidermis of a leaf of Yucca filamentosa photographed with a Zeiss-Winkel 10/0.25 objective. A: Euromex 10x eyepiece. B: Zeiss-Winkel 8x eyepiece. In the lower picture, clear spherical aberration can be seen towards the edges; a substantial part of the specimen is not in sharp focus.

Object micrometer photographed with a Zeiss-Winkel 10/0.25: A: no-name 10x eyepiece. B: Zeiss-Winkel 10x eyepiece. With the no-name eyepiece, this old achromat shows the entire micrometer in sharp focus!

I found out that Zeiss-Winkel 40/0.65 achromats give better images with Olympus P or WF eyepieces than with Zeiss-Winkel eyepieces. Zeiss-Winkel had both compensating eyepieces (indicated with a K) and eyepieces that were less compensating. For this test I used an object micrometer.

Object micrometer photographed with a Zeiss-Winkel 40/0.65. The eyepieces used were, from top to bottom: Olympus P10x, Zeiss-Winkel 10x and Zeiss-Winkel K8x. Chromatic aberration is clearly visible at the edges when using Zeiss-Winkel eyepieces and a better image is obtained with compensating Olympus eyepieces.

Also the older Zeiss-Winkel achromats of 39 mm parfocal length work very nicely with eyepieces from other manufacturers. Both the 42/0.65 and 42/0.85 objectives give minimal artefacts in combination with a Olympus P10x eyepiece while the 10/0.25 achromat works perfectly with a Euromex 10x Huygens eyepiece.

Zeiss-Winkel achromat 42/0.85 gives very little chromatic aberration in combination with a Olympus P10x eyepiece.

Objectmicrometer photographed with Zeiss-Winkel 10/0.25, 39 mm parfocal lenght. The eyepiece used was a Euromex 10x Huygens eyepiece. No significant chromatic aberration is visible. In the lower picture, the objectmicrometer is in sharp focus from the middle to the edge of the field of view.

Compensating eyepieces from Carl Zeiss Oberkochen

The following eyepieces from Carl Zeiss Oberkochen were tested: C10x, CPL W 10x, KF 10x and Kpl 10x W. The C eyepieces have the least correction and the Kpl eyepieces compensate the most. It is best to use Kpl eyepieces for all planachromats, (plan) neofluars and (plan) apochromats. Most disappointing in my opinion are the CPL eyepieces. These eyepieces show a clear pincushion distortion. No wonder that CPL eyepieces were sometimes called ‘Cheap Plastic Lenses’.......

Object micrometer photographed with a Carl Zeiss Plan 25/0.45 and CPL W 10x eyepiece. The bars of the micrometer are curved at the edges indicating pincushion distortion.

Object micrometer photographed with a Zeiss F40/0.65 objective. Left Cpl W10x eyepiece, right Wild 6x photo-eyepiece. With this objective, the pincushion distortion with the Cpl eyepiece becomes even more noticeable while an eyepiece from another brand gives a perfect image.

The effect of pincushion distortion on an object lying at the edges of the field of view is demonstrated with the diatom Arachnoidiscus. The next figure shows the entire field of view of a slide with Arachnoidiscus. The specimen lying in the center of the field is then shifted to the edge of the field of view. Due to pincushion distortion of the CPL W 10x eyepiece, the diatom is distorted to an oval shape.

The Arachnoidiscus specimen in the middle (A) is shifted to the edge (B), thereby becoming oval shaped. In the left image, the distortion is also visible in the other specimens lying at the edge. Carl Zeiss Plan 25/0.45 and CPL W 10x.

Crops from the previous image. A: Arachnoidiscus located in the middle. B: the same specimen, but now shifted to the edge. In addition to oval distortion, chromatic aberration is visible.

I have experience with many eyepieces from different brands but I have rarely seen eyepieces that distort the image as much as the Cpl eyepieces.

In the following images, an object micrometer is photographed with the other eyepieces. Even with the simple C10x eyepiece, the image quality is better than with the CPL W 10x.

Object micrometer photographed with Carl Zeiss Plan 25/0.45 and C10x eyepiece. Very little pincushion distortion is seen at the edges.

Object micrometer photographed with Carl Zeiss Plan 25/0.45 and KF 10x eyepiece. No pincushion distortion, but slightly less plan correction than with a Kpl 10x W eyepiece.

Object micrometer photographed with Carl Zeiss Plan 25/0.45 and Kpl 10xW eyepiece. Best correction, no pincushion distortion and optimal plan correction.

Crops from the previous images showing the object micrometer at the far left of the field of view. Only with the Kpl 10x W eyepiece, no aberrations are seen over the entire field of view.

Eyepieces for Olympus 37 mm objectives

Olympus microscopes with 37 mm objectives were equipped with different types of eyepieces. Horseshoe stands were often supplied with 2 or 3 eyepieces such as a 5x, 10x and a P10x eyepiece. In addition to these standard magnifications, there were 7x, P7x, 15x and P15x eyepieces. Binocular microscopes were fitted with wide-angle WF10x eyepieces. The P and WF eyepieces have the highest degree of correction and are suitable for the better corrected objectives, planachromats and achromats with higher magnifications. For lower power achromats there are the regular Huygens eyepieces 5x, 7x and 10x. When there is a ‘P’ written on the eyepiece it means that it is a photo eyepiece which is well corrected.

Object micrometer photographed with an Olympus 10/0.25 objective. A: Olympus 10x eyepiece. B: Olympus P10x eyepiece; chromatic aberration caused by over-correction can be seen at the edges.

Object micrometer photographed with an Olympus plan 20/0.40. A: Olympus 7x eyepiece. B: Olympus P7x eyepiece. Clear chromatic aberration is seen at the edges with the 7x eyepiece. The plan 20/0.40 objective clearly needs compensation.

Object micrometer photographed with an Olympus plan 40/0.65 and three different Olympus eyepieces. Chromatic aberration and slight pincushion distortion is visible at the edges with an Olympus 10x eyepiece (A). Better correction is achieved with an Olympus P10x (B) and Olympus WF10x (C) eyepiece. P10 and WF10x eyepieces have a comparable correction, but the field of view is considerably larger with the WF10x.

And then a mix of different manufacturers: the Olympus plan 40/0.65 with a no-name eyepiece on which is written: ‘8xP’. A perfect match, it turns out.

The Olympus plan 40/0.65 with a no-name 8xP eyepiece shows no significant aberrations.

Conclusion

Combinations of objectives and eyepieces from the same manufacturer do not always give the best results. Especially with plain achromats, it is recommended to experiment a bit. To get the best out of achromats, different eyepieces are required. Higher power achromats need compensation whereas low power achromats do not or need less correction. The proper objective-eyepiece combination is best determined experimentally and by critically assessing the microscopic image with suitable test slides.