Photomacrographic lenses, part 2:
Unitron Zoom 0.7-4.5x
Zeiss Luminar 63 mm f/4.5
Zeiss Luminar 25 mm f/3.5
Nikon Plan 2x microscope objective
Nikon M Plan 40x ELWD microscope objective

In an earlier page, I reviewed three photomacrographic lenses and a microscope objective with a variable aperture. I am continuing the reviews of photomacrographic equipment in this page, with two photomacrographic lenses (one of which, the Zeiss Luminar 63 mm f/4.5, proved to be the best in my earlier test) and three rather unusual microscope objectives.

In the semiconductor, electronics and precision mechanics industry, there is often a need of observing, photographing and video filming small objects at high resolution. Quite a large array of specialized microscopes and lenses have been developed for this purpose. However, this equipment is very expensive to start with, and produced in small series. This equipment also tends to be more modern than the microscopes and photomacrographic equipment that were commonly used in university laboratories in past decades. While the latter are relatively frequently finding their way to amateurs, often at reasonable prices (thanks primarily to online auction sites), specialized industrial photomacrographic equipment still is scarce outside the corporate laboratories.

Unitron is a company producing a broad range of both industrial and consumer equipment. Among the industrial range is a 0.7-4.5x Zoom lens (above, leftmost) designed to be used as an objective on industrial microscopes with a 170 mm tube length. Although this lens is intended primarily for visual observation in combination with a (typically 10x) microscope ocular and/or with a video camera, and does not have a variable diaphragm, the specifications of this lens sounded like it could be a possible competitor against other photomacrographic equipment I reviewed on this site, including Zeiss Luminar lenses, the Zeiss Tessovar, reversed macro lenses and enlarger lenses (including the unusual Schneider Betavaron).

The Zeiss Luminar 63 mm f/4.5 (second from the left) is one of the three best lenses for photomacrography in this range of focal lengths, the other being Leitz Photar and Nikon Macro (not Micro) Nikkors of comparable focal lengths (which one is the best among these is partly a matter of personal inclinations). The above specimen is from the next-to-last series made by Zeiss. It is included in this test as the term of comparison, since I tested it also previously in a variety of contexts (in which it always performed best).

The Zeiss Luminar 25 mm f/3.5 (centre) is supposedly of similar performance as the preceding lens. The above specimen is from the very last series made by Zeiss. This series has coloured dots on the barrel indicating the magnification, while the next-to-last series used the same colours in the lens labels. Although there are small differences in mechanical construction, lens coating and perhaps optical formula between the last two Luminar series, I believe any difference in performance between the two series is essentially invisible when comparing pictures.

The Nikon Plan 2x 0.05 (second from the right) is a modern planachromat microscope objective. It is rather unusual in its low magnification, but otherwise a fairly normal example of a good microscope objective. It is optimized for a microscope tube length of 160 mm. The working distance when used as a microscope objective is about 3 mm, which is expected for a low-magnification objective. In these tests, this objective is used as a photomacrographic lens (i.e., projecting an image directly onto a camera sensor, without an ocular).

The Nikon M Plan 40x 0.5 ELWD (Extra Long Working Distance) (rightmost) is specially designed to provide a much higher working distance (over 15mm when mounted on a microscope), compared with the typical one for an objective of this magnification (under 1mm). As a consequence, its optical elements are much larger than usual (the diameter of the front lens element in this case has no relation to its maximum aperture), and the optical formula highly specialized (roughly comparable to a retrofocus camera lens, but with higher specifications). This lens is optimized for a microscope tube length of 210 mm. In my experience, it performs exceptionally well also with a tube length of 160 mm and is sharper than normal 40x/160 objectives, with the only drawback of providing a slightly lower magnification than the nominal one. In addition, this lens is optically corrected for use without cover glass, making it especially suitable for photomacrography. Also this lens was tested here as a photomacrographic lens. It is equivalent to a 5.25 mm lens.

Compared to typical microscope and photomacrographic lenses, the Unitron Zoom is large and heavy (although still small and lightweight, compared to the massive Zeiss Tessovar). Although the Unitron lens has a standard RMS mounting thread, you cannot mount this lens on the revolving nosepiece of a normal microscope. The base of the Unitron lens is too wide, and prevents other objectives from being mounted in adjacent holes. In addition, the length of the lens barrel and the long working distance of this lens make it impossible to use on most normal microscope stands.

The zoom ring of the Unitron lens occupies most of the barrel length, and turns about 270°. In addition to a knurled ring for hand-turning, it carries a ZOOM 1:6.5 label (indicating the zoom ratio of the lens) and a scale from 0.7 to 4.5 (indicating the magnification when mounted on a 170 mm microscope tube). A standard RMS male thread for screwing into a microscope tube or nosepiece is part of a rather massive chrome-plated ring, attached with four screws to the base of the lens barrel (which suggests that this lens might be available also with other mounts).

Turning the zoom ring moves an internal lens group up and down the barrel. Front and rear lens elements remain immobile. Thus, this lens does not extend when zooming. The front end of the barrel has a female thread for attaching matched accessory lenses that change the magnification range of the lens (they were not included with my specimen). The specifications mention 0.5x, 0.75x, 1.5x and 2x accessory lenses. This threaded mount also acts as a lens shade and protection for the front lens element, which is recessed by about 5 mm. The threaded mount does not rotate while zooming, and therefore it can be used to attach light sources close to the optical axis of the lens.

Interestingly, the moving internal barrel has eight holes drilled around its perimeter and visible through the front and rear lens elements. The holes might indicate that diaphragm blades could be mounted onto the moving barrel to provide a variable lens aperture in some lens models (but I have found no indication of this in the literature). These holes are too small to be a way of economizing on weight, and too many to have the only purpose of letting air pass when the lens is zoomed.

The overall magnification range of the Unitron lens is low for a microscope objective. The specifications quote a maximum total magnification of 180x, in combination with a 2x add-on lens and 20x eyepieces, or 45x with no add-on lens and 10x eyepieces. Thus, diffraction likely is not as all-important as at high magnification (most microscopes arrive satisfactorily to 1,000x), and a moderate loss of resolution by stopping down a little may be acceptable, especially if used with video cameras of relatively low resolution. This makes the idea of a variable diaphragm in some models a possibility.

According to the specifications, this lens is parfocal, i.e., it does not need refocusing when zoomed. This, of course, is true only when the lens is mounted on a 170 mm microscope tube. The microscope tube that came with this lens has a threaded section that allows fine adjustments in tube length, probably to achieve perfect parfocality. In any case, small focus adjustments are often required when zooming, because of the narrow depth of field. The lack of a diaphragm does not exclude the use of this lens in photomacrography. In fact, most microscopes and microscope lenses don't have variable lens diaphragms - these optics are designed to provide diffraction-limited resolution at their fixed aperture, and any reduction in their aperture would degrade the resolution and dim the image much more than it would increase the depth of field. Some microscope lenses do have variable diaphragms, floating optical groups and/or sets of insertable funnel diaphragms, but they are used for other purposes than varying the depth of field.

The working distance without additional lenses, as specified in the technical information sheet, is a very high 92 mm (compare this with the 3 mm of the Nikon 2x microscope lens shown above, which is normal for this magnification). The working distance of the Unitron lens is even higher than with the Zeiss Tessovar (75 mm in the base magnification range 1.6x-6.4x). This allows ample room for the placement of light sources and manipulation of the subject. It may also help with live, skittish subjects.

The following pictures show the performance of these lenses in the order mentioned above. All lenses were mounted at the end of bellows extended to 110 mm (which achieves parfocality with the Unitron lens). The bellows were mounted on a Zeiss microscope stand for accurate focusing, and the pictures were exposes with a Nikon SB 800 flash to eliminate any vibration. Exposure was set to iTTL with a Nikon D200 camera controlling the flash as a remote unit. The flash head was placed roughly 4cm from the subject.

All magnification factors are given relative to the size of the camera sensor.

Most microscope objectives are designed to produce a smaller image circle (i.e., the 23 mm diameter available in a standard DIN microscope tube) than the 18x24 mm sensor of a DSLR. However, all microscope objectives tested in this page do produce uniform illumination and sharpness even with the bellows set at their minimum length of 50 mm. On the other hand, both Zeiss Luminar lenses tested here are designed to produce much larger image circles (some photographers use them even with 9x12 cm studio cameras).

When the full frame is examined (Fig. 1), the Unitron zoom provides a good visual sharpness in the focus plane at all zoom settings, but the lack of a variable aperture produces a shallower depth of field than I am used to. In most situations, I would prefer to stop down more and loose a little fine detail to diffraction. This lens is easy to focus with adequate illumination (a 40W lamp placed 20 cm from the subject is quite sufficient for this). The visible illumination level decreases rapidly in the 3.5x to 4.5x zoom range. Contrast is a little too low at low magnifications, but becomes definitely good in the zoom range above 1.5x.

The examination of small crops from the Unitron lens shows a constant good resolution at all zoom ranges (Fig. 2, 3, 4), probably higher in the 2x to 4.5x range than at lower magnifications. Compared with the 63 mm Luminar (Fig. 5, 6), the latter obviously wins, and can be stopped down to n.4 without apparent losses of resolution, which provides a much higher depth of field (Luminar lenses have a diaphragm scale starting at 1, which is full aperture, and graduated in exposure factors, i.e., n. 4 is two stops).

The shallow depth of field of the Unitron lens becomes a real problem between 2.5x and 4.5x (Fig. 3, 4). The Luminar 25 mm in a similar magnification range provides an obviously higher resolution when fully open (Fig. 7). Stopping down to n. 4 (i.e., 2 stops) produces a marked loss of resolution because of diffraction. At this point, resolution is similar to the Unitron lens (but depth of field is higher with the stopped-down Luminar). Even stopping down to n.2 (i.e., 1 stop) causes a loss of resolution very visible when comparing small picture crops. When full frames are reduced in size and compared (Fig. 1 and 9), the Luminar 25 mm can be stopped down at least to n. 8 (i.e., 3 stops) without visible losses, at which point it provides an ample depth of field (for this magnification range).

The Nikon 2x objective provides a remarkably high resolution in the field of focus (Fig. 10), and is quite comparable with the Luminars. However, the working distance of the Nikon lens is only about 3-4 mm, which makes illumination of the subject a real problem. Illumination is only possible perpendicularly to the optical axis, from the side, and light striking the front lens element causes small dust grains on the glass surface to produce out-of-focus round flare points.

The Nikon 40x ELWD objective has a much higher working distance (about 18 mm), and illumination is not a problem. Depth of field is extremely shallow (Fig. 11). Resolution is good (if the almost non-existent depth of field lets you focus on anything at all). The illumination level of the subject is very high for the magnification, and makes focusing easy (provided you have a very precise focusing mechanism - a microscope stand is practically obligatory). Paradoxically, the very high magnification and luminosity of this lens are its main problem. The magnification lies near the high end of what is usual with a composite stereo microscope (Fig. 11, 12), and at the limit of what is regarded feasible in photomacrography. Nonetheless, with a very flat subject, this lens provides a very good resolution, probably better than obtainable with a stereo microscope.

None of the lenses examined here display any visible chromatic aberration, curvature of field, uneven illumination of the field, vignetting, fringing or other aberrations. This is a remarkable result by itself.

In terms of resolution, the above tests show that the Zeiss Luminars are, once more, the best lenses in the test. The Unitron Zoom is not quite on the same level as the Zeiss Luminars, but still provides a high resolution, good contrast, and no noticeable aberrations. In these respects, in spite of being a zoom, it performs as well as, or better than, the fixed-focal-length enlarger lenses I tried. Its main problem is the lack of a variable aperture. If a compact, portable photomacrographic zoom lens with high working distance is needed, then it is hard to find a competitor for the Unitron Zoom. The Schneider Betavaron and Zeiss Tessovar are bulkier and heavier, and the Zeiss Luminar Zoom has a restricted zoom ratio and shorter working distance. Likewise, the working distance of fixed focal length Luminar lenses is shorter, except for focal lengths above 100 mm (which, however, require a much longer bellows extension). If, as I suspect, some models of Unitron Zoom do have a variable aperture, such a lens would be high on my shopping list.

Not surprisingly, the Zeiss Luminar 25 mm f/3.5 performs on par with the Zeiss Luminar 63 mm f/4.5. However, the latter lens is less sensitive to loss of resolution due to diffraction when stopped down (partly helped in this by the longer focal length and lower magnification). The 25 mm, instead, should be used fully open or stopped down only one stop at high magnification if maximum resolution is desired.

Some microscope objectives are potentially useful as photomacrographic lenses, but only when they provide a high working distance. This is the case, for instance, of the Nikon M Plan 40x 0.5 ELWD. Normal microscope objectives, including those that give a much lower magnification, like the Nikon Plan 2x 0.05, in most cases have working distances too short to allow a suitable illumination of the subject.