The achromatic objectives from Zeiss and Zeiss-Winkel

Achromats are the most simple lenses where the chromatic aberration is corrected for only two colours: blue and red. This means that some residual colour may still be present in the microscopic image as an artifact. I found out that the degree to which this residual colour appears is different between the achromats from (Carl) Zeiss and Zeiss-Winkel. The tests described on this page were done with objectives of 160 mm mechanical tube length and 45 mm parfocal lenght. The first objectives with 45 mm parfocal lenght to be produced in a Zeiss factory were made by Zeiss-Winkel in Göttingen during the late 1940s. At the end of the 1950s Zeiss-Winkel became Carl Zeiss and the optical facilities were relocated to Oberkochen. In later years the name ‘Carl’ was dropped and optics were simply labeled as ‘Zeiss’. At the same time, in former East Germany, there was a separate production facility under the name ‘Carl Zeiss Jena’ (CZJ). In this article, the name ‘Zeiss’ refers to the objectives from Carl Zeiss, Carl Zeiss Jena and Zeiss.

Achromats 40/0.65

The achromats used here concerns the objectives with 45 mm parfocal length and 160 mm mechanical tube length from Zeiss and Zeiss-Winkel. For the comparisons I used different kinds of slides and it is striking that the colour of the residual chromatic aberration is different between Zeiss-Winkel and Zeiss. With Zeiss-Winkel objectives, this colour is pink, whereas with Zeiss objectives, blue fringes are seen. The pink coloured fringes from Zeiss-Winkel objectives are much less present than the bright blue colour that is seen with Zeiss optics. The result of this is that images taken with Zeiss-Winkel objectives appear more neutral, so the artifacts are less obvious. Apparently, Zeiss-Winkel used a different kind of compensation to correct for the residual aberration in the objectives. I have the impression that Zeiss-Winkel achromats generally produce less colour artifacts than all other later Zeiss achromats. It is surprising how good these 70-year-old lenses already were. Perhaps Zeiss-Winkel used a different type of glass. Zeiss-Winkel also produced the first Neofluar objectives. These lenses are well known for their better correction and brilliant images. The older Zeiss-Winkel objectives do have more field curvature, but I have never found that really disturbing; the object of interest is usually located in the center of the field of view anyway. When photography with achromats is concerned, I certainly prefer Zeiss-Winkel.

Zeiss-Winkel and Zeiss 40/0.65 achromats arranged chronologically from the late 1940s (1) to the late 1990s (8).

1: Zeiss-Winkel, first 45 mm version, late 1940s
2: Zeiss-Winkel, 1950s
3: aus Jena
4: Carl Zeiss
5: Carl Zeiss
6: Zeiss
7: Zeiss F40
8: Zeiss Plan40

Pinnularia, photographed with the different 40/0.65 achromats.

Crops from the previous images, the differences in colour artifacts become more visible.

The difference between Zeiss and Zeiss-Winkel can also be clearly seen with other objects.

A and B: The diatom Stauroneis, photographed with Carl-Zeiss 40/0.65 (number 5) and Zeiss-Winkel 40/0.65 (number 2). C and D: Stomata of Hosta photographed with Zeiss 40/0.65 (number 6) and Zeiss-Winkel 40/0.65 (number 1)..

At first it seems that the photos taken with Zeiss-Winkel lenses have less contrast than the other images. However, I think that the darker blue colour fringes seen with Zeiss objectives partly enhance the contrast because this colour is more present than the pink colour from Zeiss-Winkel.

Darkfield images of Pleurosigma angulatum. Zeiss 40/0.65 (number 6) with C10x eyepiece (A) and Kpl10x eyepiece (B). With the Kpl10x eyepiece, the blue colour artifacts seem to be even more visible, although very subtle. C: Zeiss-Winkel 40/0.65 (number 1), which gives slightly les artifacts.

Comparison between the oldest and most recent objective in the series. Above: Zeiss Plan 40 (number 8). Below: Zeiss-Winkel 40 (number 1). In the cropped images on the right, the difference with respect to the colour artifacts is clearly visible.

The green algae Pleurococcus. Upper image: Carl Zeiss 40 (number 4). Lower image: Zeiss-Winkel 40 (number 2). In the upper image, clear blue coloured edges can be seen.

Section of a stained slide from a stem of Tilia. Left: Zeiss 40 (number 6). Right: Zeiss-Winkel 40 (number 2) gives a more neutral image. 

The next test is a comparison between the oldest lens in the series, the black Zeiss-Winkel 40 and the much later Zeiss F40 with semi-plan correction. When using Zeiss KF10x/18 eyepieces, I noticed that even at the edge of the field of view, the image quality is better with the Zeiss-Winkel objective. When focusing at objects on the edge, less chromatic aberration and distortion is visible with the Zeiss-Winkel objective. In the middle, it is obvious of course that the F40 has a better plan correction. But in my experience, I find that the overall image quality with Zeiss-Winkel is better, and that while the KF eyepieces are supposed to be a good match with the F40 objective. I am quite surprised by this. An object micrometer shows once again that the blue colour is clearly present when using the F40. With Zeiss-Winkel the bars are really black.

Pollen grains from different plants. Crop from the center of the field. Left: Zeiss F40/0.65 (number 7). Right: Zeiss-Winkel 40/0.65 (number 1) gives a clearer image with fewer artifacts.

Object micrometer from the center of the field photographed with a Zeiss F40 objective (left) and the old black Zeiss-Winkel objective (right). With Zeiss-Winkel, the bars are more black. When zooming in on this picture, the blue colour in the left object micrometer becomes more noticeably.

I am surprised by how good those old black Zeiss-Winkel 40/0.65 achromats already were. I have 4 copies of the black version and they all give clear images with high contrast and minimal artifacts. Once you are used to these Zeiss-Winkel achromats, later Zeiss achromats are likely to disappoint you. That is, if you don’t care so much for the plan-correction. It seems that the improved plan-correction was the only real advancement in the optics of later Carl Zeiss and Zeiss achromats................…

Achromat 10/0.25 versus 10/0.22

As soon as Zeiss-Winkel became Carl Zeiss in the late 1950s, the aperture of te achromats 10/0.25 was adjusted; it changed from 0.25 to 0.22. Zeiss-Winkel had always produced 10x achromats with an aperture of 0.25. The difference in resolving power does not seem like anything, but it is noticeable with critical observations. It wasn't until much later, in the late 1980s, that Zeiss produced 10/0.25 lenses again such as the F10/0.25.

Arachnoidiscus photographed in darkfield with Zeiss-Winkel 10/0.25 (left) and Zeiss 10/0.22. The details are better resolved in the left image.

Achromats 25/0.45

A remarkable case is the Zeiss-Winkel 25/0.45 objective. When compared to a Carl Zeiss or Zeiss 25/0.45, it appears that the Zeiss-Winkel lens that I have in my possession has a slightly higher aperture. The difference in aperture becomes clear with a phase contrast condenser. When using the phase ring 3 (Ph3), with a 25/0.45 objective, annular illumination is obtained. Here, only the outer edge of the aperture becomes visible while central light is blocked. With this illumination technique, the maximum resolving power of a lens is used while the contrast increases. When looking at the back lens of the Zeiss-Winkel 25/0.45 with a phase telescope, it appeared that the illuminated ring at the edge of the aperture was slightly wider compared to a Zeiss 25/0.45 objective. This is only possible when the aperture of the Zeiss-Winkel 25/0.45 is higher. Since I have only 1 copy of the  Zeiss-Winkel 25/0.45, I would like to backup this observation with another objective. Just to rule out differences between objectives.

Comparison between a Zeiss 25/0.45 (A) and Zeiss-Winkel 25/0.45 (B) objective. With annular illumination using the Ph3 phase ring, the illuminated ring turns out to be slightly wider with the Zeiss-Winkel lens.

In the following image, the transition to darkfield illumination is visible with the Zeiss 25/0.45 and annular illumination using the Ph3 phase ring. The edges of the image are already underexposed (A) which is due to the smaller aperture.

Diatoms photographed with annular illumination using the Ph3 phase ring. A: Zeiss 25/0.45. B: Zeiss-Winkel 25/0.45. With the Zeiss 25/0.45 the transition to darkfield is already visible.

Diatom photographed with annular illumination using the Ph3 phase ring. Compared to the Zeiss 25/0.45 (A), the structure is better resolved with the Zeiss-Winkel 25/0.45 (B).

Regarding the chromatic aberration of the 25/0.45 lenses, here too a difference can be seen between Zeiss and Zeiss-Winkel. Again, Zeiss-Winkel provides pictures with fewer artifacts, although the difference is more subtle than with the 40/0.65 achromats.

Comparison between Zeiss 25/0.45 (left) and Zeiss-Winkel 25/0.45 (right). The difference in image quality is subtle but visible when the picture is enlarged. Less colour artifacts are present in the image made with the Zeiss-Winkel lens.

Detailed section of Arachnoidiscus photographed with Zeiss 25/0.45 (upper picture) and Zeiss-Winkel 25/0.45 (lower picture). Colour-artefacts are more pronounced with the Zeiss objective.

Conclusion

The Zeiss-Winkel achromats from the 40s and 50s were surprisingly good. No real improvement, other than the plan correction, is seen in Zeiss objectives that were manufactured at a later date. Older Zeiss-Winkel achromats produce less noticeably colour artifacts than later Zeiss optics.