Three cheap 4X microscope objectives in test

In the magnification range of four times there are some cheap microscope objectives that perform fairly well in photography (without microscope stand). They are finite objectives. This means that there is a fixed distance between objective and ocular on the microscope, usually 160 mm. These finite objectives will produce an image at the camera sensor without any other lens in-between objective and camera. Nowadays, most microsope objectives are infinite. This means that there has to be a lens between the objective and the camera to get a sharp image.

The microscope objectives tested

Robert O’Toole has just recently tested six cheap 4X microscope objectives. I purchased two objectives based on his suggestions. One objective that he recommended was the AmScope 4X Plan Achromatic Objective Lens with Knurled Ring 4/0.10, 160/0.17. This objective is sold in the USA through Amphoto. It is made in China and is a no-brand. It is sold on ebay for $18 by Amphoto company “Precision*world”. The freight cost to Norway is about the same as the objective price. Another Chinese objective on ebay is a no-brand 4X Plan 0.10, 160/0.17 Achromat with cone tip (making it easy to separate from the Amphoto which has a straight tube). It also lacks the knurled ring. It has the same price as the Amphoto objective, but with free freight to Norway. Robert has not tested this objective. Then, I had access to an Olympus NEA 4X 0.10, 160/-. This objective belonged to a student microscope and was made in the late 1990’s. I tested all three objectives to compare quality.

Fig. 1. From left to right: Amphoto objective with knurled ring, ebay nobrand objective with cone tip, Olympus NEA. All objectives are finite 4X plan achromats.

Test rig

The testing set-up I used can be seen in Fig. 2. The tripod was placed in the basement on a concrete floor. The camera used was Sony a7R (full-frame sensor). The light source was a LED headlight, and the exposure time was 1/10 s. I used remote control to avoid camera shake. All three objectives made an image that covered the full frame. The extension was made up from a K&F Concept camera adapter M42-NEX, then three M42 extension tubes, and a cone adapter for microscope objectives with RMS thread. The total length of extension was approx. 130 mm. This gave a magnification of about 3.8X. A macro rail was used to focus precisely. I used a piece of square fabric from a dropdown as motive. The fabric is woven with thin plastic threads.

Fig. 2. Camera set-up for the close-up test. The depth of focus is very shallow so a macro focusing rail is necessary.

Results

The full-frame photo and three magnifications of center, left side and right side of the frame is given for each objective. They are presented in the order of Fig. 1. Amphoto objective in Figs. 3-6, Chinese objective with cone tip in Figs. 7-10, and Olympus objective in Figs. 11-14.

Fig. 3. Amphoto 4x objective, full frame

Fig. 4. Amphoto 4x objective, center magnification, 15X

Fig. 5. Amphoto 4x objective, left side magnification 15X

Fig. 6. Amphoto 4x objective, right side magnification 15X

Fig. 7. Nobrand, cone tip, 4x objective, full frame

Fig. 8. Nobrand, cone tip, 4x objective, center magnification 15X

Fig. 9. Nobrand, cone tip, 4x objective, left side magnification 15X

Fig. 10. Nobrand, cone tip, 4x objective, right side magnification 15X

Fig. 11. Olympus NEA 4x, full frame

Fig. 12. Olympus NEA 4x, center magnification 15X

Fig. 13. Olympus NEA 4x, left side magnification 15X

Fig. 14. Olympus NEA 4x, right side magnification 15X

Conclusions

There were huge differences between the objectives in terms of sharpness, flare, and color rendition. This could be seen already in live-view where on the Sony a7R screen yellow color marking means sharpness. Best overall performance of the objectives tested was the Amphoto. This objective showed true plane sharpness all over the frame. Second came Chinese clone with cone tip. The sharpness was good in center and on the left side, but the right side was unsharp. However, this objective showed the least of stray light, possibly due to good antireflex treatment. When shaking the objective a sound of loose parts could be heard. The least satisfactory objective was the Olympus NEA. This objective was only sharp in the center. Both left and right side were unsharp and showed chomatic aberration. Stray light reduced contrast. This objective had the longest working distance (30 mm), with “cone tip” second (14 mm) and Amphoto third (12 mm). All objectives have a shallow depth of focus. Therefore it is necessary to use focus stacking to get good images.