Why do some microscope objectives need a tube lens while others do not? What is a tube lens anyway?
To understand tube lenses, we need to look at how a microscope works, because only a certain category of microscope objective needs them. The 19th century was a very dynamic time in microscope development, and attempts were made to standardize the many different objective sizes so that several of them could be used on the microscope in the revolving nosepiece without moving the stage. These objectives produced an intermediate image at a standardized distance of 160 mm (also 170 mm at Leitz), which passed upwards through the tube (the tube connection between the objective and eyepiece), where it was then magnified again through the eyepiece and projected into the eye of the microscope operator. An intermediate image is the image provided by the objective with its nominal magnification, which is also printed on the outside of the housing. This intermediate image is recorded by the eyepiece and magnified. The intermediate image could therefore theoretically be seen or imaged with a camera if it were captured by a focusing screen.
But there is no advantage without a disadvantage: there was now too little space inside such lenses to insert additional lenses with specific optical tasks. No optical element could be inserted behind the lens, as the light rays left the lens already bundled and fixed at the point where they were to be picked up by the eyepiece.
The tube lens
The solution to this problem was to remove a certain lens from the objective and mount it higher up in the tube. This turned it into a tube lens. The light rays no longer left the lens focused on a specific point in the tube (on the left in the diagram), but were directed completely straight ahead, virtually into infinity (on the right in the diagram). All rays of light now did not run towards each other in order to focus, but parallel, i.e. virtually straight ahead to the tube lens.
Additional lenses or filters could now be placed between the objective and the tube lens as required. The light rays was then bundled in the tube lens and focused on a specific point, where it was then picked up by the eyepiece and magnified. Because the light emitted from the lens was directed towards infinity, these newer designs were referred to as infinity lenses, recognizable by the horizontal 8 as the infinity symbol. Strictly speaking, the tube lens is therefore a functional part of the infinity lens. Those of the earlier generation with the indication "160" or "170" were called finite lenses.
Microscope lens on the camera
If we now want to attach a microscope lens directly to our camera, we need to take two important things into account. On the one hand, we have to replace the microscope tube – the tube connection between the objective and the eyepiece – e.g. with a tube, several extension rings screwed together or a bellows device. The objective therefore needs a certain distance to the camera sensor, which is as large as it would be in the microscope. And on the other hand, we need to clarify whether our objective requires a tube lens or not. If we are using an older finite lens, which you can recognize by the label "160" or "170", no tube lens is needed, unlike for the more recent infinity optics.
Color corrections through eyepieces
You might assume that the older finite lenses can all be screwed onto the camera without any additional accessories, but unfortunately it's not quite that simple. This is true for many, but not for all. This has to do with the fact that some other lenses were left out due to the lack of space in order to place them higher up in the eyepieces, because there was plenty of room. These are often lenses that were used for color corrections. The Leitz company is an example of this, because its eyepieces with this color correction, the Periplan eyepieces, are indispensable for the objectives from a certain period, regardless of whether the image is to reach the eye of a person using a microscope or the sensor of a camera. If this color correction is missing, we can see purple and green color fringes on the contours of the image, known as "chromatic aberrations".
There are comparable products from other companies, also with the correction of other imaging errors. You should therefore clarify individually for each microscope objective whether it requires an eyepiece with a certain optical performance. If not, it already has an internal correction of color errors ("chromatic aberrations") and other imaging errors such as distortions in the peripheral area. Such lenses can be used on the camera without an eyepiece, and they are highly sought after in focus stacking and therefore usually quite expensive, especially if they have an apochromatic correction. In a separate article, I will focus specifically on microscope objectives and their suitability for focus stacking.
Enlarger objective as a tube lens
Theoretically, for infinite objectives we would always need exactly the same lens that is installed in the corresponding microscope. Fortunately, these lenses can also be replaced by optics that were actually created for completely different purposes. An example of this are various enlarger lenses from the analog era with the appropriate focal length from 135 mm, because they also focus the light rays on a specific point and we have to adapt them to our camera so that the image sensor is located at this point. Focus stacking experts like to use high-end optics from companies such as Schneider Kreuznach (e.g. Componon S 135mm f5.6) or Rodenstock (Rodagon 150mm f5.6), which used to be very expensive. Such enlarger optics from the analog era do not even cost a lot of money because there is hardly any other use for them today. All you have to do is adapt them to the camera and microscope objective.
Telephoto lens as a tube lens
Instead, you can also use some conventional telephoto lenses with the appropriate focal length, which you may already have in your lens collection. This also focuses the light onto our camera sensor. However, it should have a sufficiently long focal length (at least 135 mm, preferably more). The aperture must be fully open and the focus set to infinity.
Such a telephoto lens even makes it easier to mount the microscope lens on the camera, as it acts as a kind of adapter: it sits directly on the camera bayonet and a small microscope lens can be easily attached to its front thread. This is possible, for example, with step-up and step-down adapter rings and a round plate that can be screwed in and has an RMS thread in the middle (Royal Microscopy Society, 20.32 x 0.71 mm).
Instead, you can also use a snap-on holder from Raynox close-up lenses if you can find a round plate with a 43 mm external thread that has an RMS thread in the center. You can screw a microscope objective into it, and the adapter can be clamped onto a front thread between 52 and 67 mm. These round plates are occasionally found with a 39 mm external thread, and such a plate would have to be adapted with a 43 mm/39 mm step-down ring. An alternative is the "M43 (Raynox Snap) to RMS" adapter offered by the Traumflieger webshop (www.traumflieger.de) in Germany.
Autofocus
Depending on the lens combination, you can expect the corners to be darkened or even a circular image. On the other hand, the autofocus of your lens may well work with this constellation, which would make things quite convenient. Unfortunately, this combination of telephoto lens/microscope lens only offers you an extremely small depth of field. But this problem can also be solved: If your camera is capable of internal focus stacking ("focus bracketing"), which is generated by the autofocus of your lens, you may also be able to do this with this lens combination, which would compensate for the extremely shallow depth of field of this constellation. Alternatively, you can work in the conventional way with manual or automated stacking and software post-processing.
Close-up lens as a tube lens
Close-up lenses, which are usually screwed in front of a camera lens in order to be able to get closer to the object, are simpler because the minimum distance becomes smaller. Many of these are also suitable as tube lenses, and the focal length is also a decisive factor for them. One of the best-known examples are two models from Raynox: DCR150 and DCR250. They offer excellent image quality for relatively little money. Others would be the "Live Size Adapter" (LSA) from Sigma or the "Achromatic +4 Diopter" lens from Century Precision Optics.
The focal length of these lenses often does not exactly match the objective requirements, but microscope objectives can tolerate certain deviations, albeit with a corresponding change in image scale. Many objectives used in metallurgy, for example, have a number on the housing that stands for the required focal length of the tube lens, e.g. Mitutoyo or HLB. However, this does not mean that the lens in question can only be used with exactly this tube lens focal length, or that it only produces optimum sharpness in this pairing, as is generally assumed. In fact, this number only indicates that exactly this tube lens focal length must be used to achieve the nominal magnification – i.e. the value specified on the housing as the magnification. For certain applications in metallurgy or material testing, an exact magnification is indispensable, but for our photographic purposes it is of secondary importance.
Therefore, we can certainly deviate with the tube lens focal length. How far, however, depends on the optical flexibility of the lens in question, and this tolerance can be enormous. This can go so far that the smaller tube lens focal length (and the correspondingly reduced distance between sensor and tube lens) almost halves the image scale. The image sharpness is then somewhat worse than with the ideal focal length, but the imaging performance can still be better than with a lens with half the magnification.
An example: HLB Planapo 10x with an NA value (numerical aperture; objective aperture angle) of 0.28, which is intended for a tube lens with 200 mm focal length and an extension of 200 mm, is used with a tube lens of only 125 mm focal length (Raynox DCR250) with a low extension of only 125 mm. The magnification thus achieved should be slightly more than half the nominal value of 10x.
For comparison, a Mitutoyo M Plan Apo 5x is used, NA value 0.14, equipped with a tube lens of 208 mm (Raynox DCR150), with an extension of 210 mm. Tube lens focal length and focuser are almost an ideal match. Here the 10x objective performs close to its best, but is still clearly inferior to the 10x objective in terms of detail, despite its massive deviation at tube focal length. The reason for this is that a lens with a higher magnification power and higher NA value also has greater fine detail. However, this example shows how flexible lenses can be in this respect and that it is definitely worth experimenting here.
In principle, you can adapt microscope objectives that require a tube lens to a camera in three ways: by using an enlarger lens, a telephoto lens or a close-up lens as a tube lens replacement. But don't expect any miracles, because you will pay elsewhere for the greater fine detail that you achieve with these lenses compared to a conventional photo lens at the same image scale. You will have an extremely shallow depth of field and will need a tripod. The best way to use such a combination is under laboratory working conditions, with a stationary subject, stable base and high-quality linear stage, manually or motor-controlled.
If you plan to use this lens on your camera in the garden for freehand subject hunting, it will be very challenging, even if you can use the autofocus and internal focus stacking. So don't be disappointed if you experience a steep learning curve – this is quite normal, and if it gets too difficult, start by working with a tripod and linear stage. Which microscope objectives are generally suitable for this purpose will be discussed in a separate article.
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