Praise for the humble Achromat

Depending on the degree of correction, there are roughly three categories of microscope objectives, namely achromates, fluorites and apochromates. Achromats correct the chromatic aberration for 2 colours, fluorites for 3 and apochromates for 4 colours. Apochromats have the highest level of correction and therefore the highest resolution can be achieved with these objectives.

Achromats are often seen as second-rate objectives and less suitable for photography, a common misunderstanding. In my opinion, there is a place for all three categories of objectives and the better corrected objectives cannot always replace the achromats. In many cases, contrast and depth of field will be better with achromats compared to higher corrected lenses. As the numerical aperture (NA) of an objective increases, the higher the resolving power and the shallower the depth of field. A thick slide often does not benefit from the use of a high NA apochromate. I often see images made with apochromats that are disappointing because of the limited depth of field. The use of an achromat in such cases would have resulted in a better impression of the object. It is good to think about which type of objective the object needs. What is important, depth of field or resolving power?

In the following examples I will show a number of comparisons between the different objective classes. It is clear that good photos can be taken with normal achromats. The numbers that are mentioned correspond to the objectives shown in the image at the bottom of this page.

For an overview of a leaf of Ceratophyllum, a Zeiss-Winkel achromat 10/0.25 (A, No. 1) is just as good as a Carl Zeiss Neofluar 10/0.30 (B, No. 2). The achromat gives a little bit more depth of field, which improves the overall impression of this object.

Left: Cosmarium (top) and Arcella (bottom) photographed with a Zeiss-Winkel 40/0.65 achromat (No. 3). Right: the same objects with a Carl Zeiss 40/1.0 apochromat (No. 4).

The following comparison is between two completely different objectives: a no-name water immersion achromat 63/0.85 and a Carl Zeiss Neofluar 63/0.9. The 37 mm water immersion objective was attached to an old gray Euromex horseshoe stand. Above all expectations, this objective turns out to be worth it’s weight in gold because water immersion can be applied at a numerical aperture of 0.85. The objective is therefore much less sensitive to deviations in cover glass thickness. The result is an image with high contrast. The Neofluar 63/0.9 objective has a correction collar to adjust for the cover glass thickness. These kinds of dry lenses with high NA are not my favorite, you keep adjusting the cover glass correction all the time and if that is not done properly, the image will be ruined.

Section of Pleurosigma angulatum photographed with a no-name WI 63/0.85 achromat (A, No. 5) and a Carl Zeiss Neofluar 63/0.90 (B, No. 6). The contrast in image A is slightly better.

Darkfield image of Stauroneis photographed with a Leitz 25/0.50 achromat (A, No. 7) and a Leitz planapochromat 25/0.65 (B, No. 8). Right: Pinnularia with Leitz achromat 25/0.50 (C) and Leitz planapochromat 25/0.65 (D). More chromatic aberration is visible with the achromat, but the depth of field is better. There is also more glare visible in the darkfield image from the planapochromat; a higher aperture often gives lesser results in dark field.

Mix of different types of pollen grains photographed with a Leitz 25/0.50 achromat (left, No. 7) and a Leitz planapochromat 25/0.65 (right, No. 8). The image on the right is of better quality, but it is debatable which image gives the best subjective impression. Due to the better depth of field with the achromat, the boundaries of the pollen grains are clearer.

Spirogyra photographed with a Zeiss-Winkel achromat 25/0.45 (left, No. 9) and a Carl Zeiss Neofluar 25/0.60 (right, No. 10).

Oedogonium photographed with Zeiss-Winkel achromat 40/0.65 (left, No. 13) and Carl Zeiss Neofluar 40/0.75 (right, No. 12).

Stauroneis, photographed with a Carl Zeiss achromat 40/0.65 (left, No. 11) and Carl Zeiss Neofluar 40/0.75 (right, No. 12).

Mix of different algae photographed with a Zeiss-Winkel 40/0.65 achromat (left, No. 13) and a Carl Zeiss Neofluar 40/0.75 (right, No. 12). The colours are better with the Neofluar while the achromat gives a better depth of field.

The following examples show that decent images can be made with simple achromats.

Cymbella, photographed with Zeiss-Winkel achromat 40/0.65 (No. 3).

Zygotes of Spirogyra. Zeiss-Winkel achromat 40/0.65 (No. 3).

Closterium, photographed with oblique illumination. Carl Zeiss achromat 40/0.65 (No. 11).

The objectives

The objectives used for these tests are shown below. The numbers are indicated with the previous images.

1: Zeiss-Winkel 10/0.25

2: Carl Zeiss Neofluar 10/0.30

3: Zeiss-Winkel 40/0.65

4: Carl Zeiss Apo 40/1.0

5: WI 63 / 0.85, no-name

6: Carl Zeiss Neofluar 63/0.9

7: Leitz 25/0.50

8: Leitz Pl Apo 25/0.65

9: Zeiss-Winkel 25/0.45

10: Carl Zeiss Neofluar 25/0.60

11: Carl Zeiss 40/0.65

12: Carl Zeiss Neofluar 40/0.75

13: Zeiss-Winkel 40/0.65

Conclusion

Achromats should deserve a little more appreciation. These objectives are good all-rounders and they are also fully usable for microphotography. I think that better corrected objectives are chosen way too often for visual observation and photography. This is not necessary. In fact, in some cases the image from an apochromat will even be disappointing because the object to be examined needs more depth of field. Resolution comes at the expense of depth of field. And many objects lack the fine details that justify the use of a high NA apochromat. Achromats are simply build and contain fewer lenses than, for example, (plan) apochromats, which translates into a better contrast.

There will be microscopists who only use the highest corrected objectives and who consider the achromats to be inferior. However, the expensive high quality objectives they use are no guarantee for good images.

It can be an eye-opener to show micro photos to people who have no idea about microscopy. These people look at these images with completely different eyes than an experienced microscopist. My experience is that, in comparison tests, the photos taken with achromats are usually picked out as being the best because of better depth of field which gives objects clearer boundaries and thus better overall impression.