Olympus MC-20 teleconverter with macro lenses

This page describes tests of the Olympus MC-20 teleconverter coupled to legacy macro lenses. You may also be interested in my separate extended review and tests of the MC-20 used with the Olympus 300 mm f/4 Pro.

Most macro lenses produced between the late 20th century and the present continuously focus from infinity to a maximum magnification of 1x. The traditional method to increase this magnification is by inserting extension tubes between camera and lens. Most modern macro lenses, however, use one or more "floating" optical groups to correct aberrations at different focus distances.

Although the term "floating optical group" evokes the image of a floating object free to go wherever wind and water currents take it, the position of floating optical groups is strictly controlled by precision mechanical cams, and highly reproducible. There is nothing random about the position of a floating optical group. The "floating" term only means that this optical group moves with respect to other optical groups when focus changes (or a manual adjustment ring is turned in some lenses).

The main problem with adding extension tubes to a lens with floating optical groups is that the extension tubes change the distance between lens and sensor as well as the magnification (with respect to the magnification displayed on the focusing scale), and therefore the lens is forced to work outside its design parameters. This is likely to increase aberrations and negatively affect image quality.

One traditional method of avoiding this problem is to reverse the lens when used above 1x, so that its rear points toward the subject, and adding extension rings as necessary between lens and camera. When reversed, the focusing ring of lenses with floating groups should also be adjusted to indicate the inverse of the actual magnification, so that the optical system is still used within its design parameters. For example, with the reversed lens giving a magnification of 2x, the focus ring should be adjusted to show a magnification of 0.5x (i.e., 1/2). Light follows the same path thorough the lens optics regardless of the light direction, so this strategy still uses the lens at its design optimum. There are still a number of possible problems that cause a degradation of image quality, including increased diffraction and the fact that lens resolution is computed with a certain sensor pixel-count in mind, and sooner or later reaches its limits when magnification is increased beyond its design maximum. An even bigger problem with modern lenses is that, once reversed, their electronics are no longer connected to the camera. In some cases, this prevents their aperture to close to the desired setting when the camera shutter is triggered. Electronic functions that need access to the various motors and actuators in the lens no longer work, including automatically stopping down the aperture to the preset value during exposure, lens-based image stabilization, autofocus, in-camera focus bracketing and in-camera focus stacking.

Some photographers have reported that using a teleconverter instead of extension rings solves the above problems. Modern teleconverters trasmit electronic information between lens and camera, and with a teleconverter a lens with floating groups still works within its design parameters throughout its focusing range. Unfortunately, current teleconverters for mirrorless cameras are meant to be used only on long telephoto lenses, and have front optical elements that project from their front mounts and prevent use with other lenses. Some measure of success has been reported by adding extension tubes with electronic signal transmission between lens and teleconverter, but this brings back the problem of forcing the lens to work outside its optical design parameters.

There are additional methods for using legacy lenses (not only macro lenses but other types as well) at 1x magnification and above. One of these is mounting a lens focused at infinity on the camera, and attach a second lens, this one usually reversed, at the front of the first one. This effectively builds an infinity-corrected system comparable, in principle, to the infinity-corrected optical paths used in modern compound microscopes. This infinity system has interesting optical properties. At a given magnification, its effective aperture is faster than the front lens alone, although working distance is lower (the effects of a teleconverter are the opposite in both respects, see below). Two identical camera lenses connected front-to-front yield a 1x magnification, and tend to cancel out some of each other's aberrations. If the focal lengths of the two lenses differ, magnification is equal to the ratio of rear over front focal lengths. If the reversed front lens is designed for a 35 mm SLR/DSLR, the working distance of the system is around 40-50 mm regardless of the system's magnification, or twice as much if the lens is designed for a large-format SLR.

Stacked lenses is another term indicating a comparable system of two lenses, one mounted on the camera (sometimes via an extension) and the second reversed atop the first. This term does not necessarily denote an infinity system, as the first lens may not be focused at infinity. The magnification of such a system may differ from the ratio of the two focal lengths, and the image quality is largely unpredictable and must be tested for a specific combination of lenses, distances and magnification.

A characteristic of systems of two lenses connected front-to-front is that the diaphragm of the lens closest to the subject may not be optimally placed to control the aperture of the system. A diaphragm mounted in the space between the two lenses may work better. The diaphragm of the rear lens is typically left fully open, as it typically reduces the size of the image circle rather than reducing the system aperture. At high magnifications, both lenses are usually left fully open to limit diffraction to a minimum.

Adding a convergent achromat (or a purpose-designed multi-element convergent optical system) at the front of a macro lens can also be a way to magnify the subject above 1x, albeit at the expense of working distance. Also in this case, only tests can tell whether a given setup is useful.

The complexities and uncertainties of these alternative optical systems deserve a longer discussion than feasible on this page. All above methods are discussed, for example, on photomacrography.net and closeuphotography.com. These links are also recommended for information on using tube lenses coupled with infinity-corrected microscope objectives at magnifications substantially higher than 1x.

Like the teleconverter method discussed on this page, two-lens systems in most cases are not that interesting at magnifications of 1x and below, where a modern macro lens gives excellent image quality, straightforward use, and access to camera functions like focus stacking and focus bracketing.

On this page, I discuss a simple alternative approach to use these teleconverters with macro lenses while avoiding extension tubes altogether. It is common among photographers to adapt legacy lenses, originally designed for film SLRs and DSLRs, to mirrorless cameras via mechanical adapters devoid of optics, and often devoid of electronic connections between camera and lens. Legacy macro lenses are prime candidates for using on mirrorless cameras when manual focus, manual aperture and lack of image stabilization are not major obstacles. The present approach is not applicable if these electronic functions are desired (in this case, there is no substitute for native lenses). The above figure shows the legacy macro lenses used for this test.

Strictly speaking, the CoastalOpt (now Jenoptik) 60 mm f/4 Apo is not a legacy lens because it is still being manufactured (on special order), but it is included in this study because it is one of the best macro lenses ever made. Incidentally, the Sigma 180 mm macro is one of many legacy Sigma lenses marketed as Apo. However, none of these lenses, unlike the CoastalOpt 60 mm, is a true apochromat.

All the tested lenses have manual focus and manual aperture when used with ordinary lens adapters on Micro 4/3 cameras. These lenses reach 1x, except for the CoastalOpt 60 mm (0.67x) and Micro-Nikkor-P 55 mm (0.5x). In the present test, all lenses were focused at their maximum magnification.

In the field, traditional macro techniques can make it easier to use these lenses, e.g., setting the chosen magnification on the lens, then gradually approaching the subject and tripping the shutter when the subject is at the desired focus in the viewfinder. This technique can be aided by sequence-shooting while approaching the subject, and choosing the best images back at home.

Tip - Most mirrorless cameras briefly display the recorded images in the viewfinder and/or LCD screen, before resuming live view. When shooting sequentially, this function may interrupt live view, which essentially forces you to shoot blindly. In most cameras, it is possible to switch off this function in the viewfinder and/or the rear LCD screen, but you need to manually activate this choice in the menu. Half-pressing the shutter button usually also interrupts the inspection of recorded images and restores live view.

Teleconverters and macro lenses

Teleconverters magnify the center of the image circle by a given factor, usually 1.4x or 2x, and decrease the effective lens speed by one or two stops, respectively. Modern teleconverters work together with camera and lens electronics, and the camera displays the effective aperture when a teleconverter is used. When shooting with teleconverters on telephoto lenses, this is all you need to know. When using a teleconverter in macrophotography, there are a few more things to be aware of.

One of them is that certain cameras do not take magnification into account to adjust the displayed aperture. They just display the effective aperture at infinity focus, regardless of the actual lens focus. This is the case, for example, with Olympus cameras.

A teleconverter increases the physical distance between camera and lens, effectively moving the lens forward by a couple of centimeters. This changes the focus distance (i.e., the distance between camera body and subject) and may change the balance of the lens. The working distance of the lens (i.e., the distance between the front of the lens and the subject) remains unchanged by a teleconverter. This, in turn, has a practical consequence that initially left me stumped, but was cleared up by a quick discussion on photomacrography.net. This consequence is that the effective aperture of a lens mounted on extension only (focusing helicoid, bellows, extension tubes etc.) increases by one stop when focusing from 1x to 2x, while the working distance W decreases by

W = f (1 + 1/m)

where f is focal length and m magnification. In this specific example, W decreases from 2f at m = 1 to 1.5f at m = 2. With a teleconverter increasing m from 1 to 2, instead, working distance remains 2f also at m = 2, while effective aperture increases by two stops (see also below). In other words, by using a teleconverter instead of adding extension, you gain focal length and therefore working distance, but lose effective aperture. This may force you to further open up the lens aperture ring to avoid diffraction.

The extra working distance can come in handy with shy or dangerous subjects, and may make the difference between going home empty-handed or with still good-enough images. A higher working distance may also make it easier to position the subject illumination in the studio/lab.

The rest of this section refers to the aperture of legacy lenses, as indicated on their aperture ring. These lenses do not report their aperture to the camera. Legacy macro lenses may behave in two different ways with respect to how they indicate their aperture on the aperture ring. Most lenses indicate the nominal aperture (not adjusted for magnification), including the lenses discussed on this page. Some lenses instead display the effective aperture (adjusted for magnification), and consequently open up their diaphragm when focused from infinity to 1x.

Some versions of the Micro Nikkor 55 mm f/3.5, for example, compensate for effective aperture by using a curved (as opposed to straight) rail to couple the aperture ring to the diaphragm. As the optical assembly slides forward within the barrel when the lens is focused close, the curvature of the rail causes the cam connected to the diaphragm to change the amount by which the diaphragm is closed. You need to find out which of the two alternatives applies to the specific macro lenses you are using. In the following discussion, I assume that the aperture ring always shows its nominal aperture.

The increase of effective aperture with a teleconverter requires you to be aware that the lens + teleconverter has a different optimal aperture range (as displayed on the aperture ring of a legacy lens), compared with the lens alone. In terms of diffraction, at the same magnification you need to open the aperture by one stop with a 2x teleconverter to restore the same amount of diffraction blur produced by the lens without teleconverter.

In practice, many lenses need to be stopped down by one or two stops to give the best image quality, so you may need to compromise between the need to open up the aperture to reduce diffraction, and the need to stop down to increase optical performance. Test your combination of camera, lens and teleconverter to find out which aperture range works best for you.

If your lens is sensitive to flare and central hotspot (e.g., the CoastalOpt 60 mm f/4 Apo), a lens shade can often solve this problem. As mentioned here, using the lens on a smaller sensor allows the use of a narrower and/or longer lens shade, which further helps and may make illumination of the subject easier. When shooting with a teleconverter, you can use an even narrower and/or longer lens shade than without teleconverter, which may help even more.

Tests

All following test images were shot with a metal-on-glass test target with thin lines spaced 10 μm apart. This is a highly reflective target, and therefore produces low-contrast images when shot with the diffuse flash illumination coming from both sides of the target that I used for this test. Each lens was shot with the MC-20 teleconverter and at the maximum magnification provided by the lens focus helicoid. The captions of the following test images give the nominal aperture (as displayed on the focus ring of the lens). All figures are 1:1 pixel crops of the central region of the image.

The following discussion of nominal vs effective aperture, for simplicity, ignores the effects of the pupil ratio of the lens by assuming it is equal to 1. If you need a more precise calculation, you need first to measure the pupil ratio of the lens, then compute the effective aperture A' as

A' = A ((m / P) + 1)

where A is the nominal aperture, m the magnification, and P the pupil ratio. Disregarding the pupil ratio, the above formula becomes

A' = A (m + 1)

The main thing to remember is that, at 1x magnification, the effective aperture is twice the nominal aperture displayed on the aperture scale of the lens (i.e., two stops more). At 0.5x, the difference is approximately one stop. The MC-20 teleconverter adds another two stops to the effective aperture, so in total the effective aperture at 2x magnification becomes four stops more than the nominal aperture of the lens. For example, nominal f/4 in these conditions is effective f/16.

Sigma 180 mm f/3.5

Over the years, this lens has continued to impress me for its remarkably high resolution, although its contrast is not exceptional. Its resistance to flare is quite good, however. The present test confirms my impressions. With 13 elements (of which 2 SLD) in 10 groups, internal focusing and a 9-blade diaphragm, this is a modern macro lens. See for example the technical review on DPreview.

The fully internal focusing mechanism leaves the front and rear groups immobile. The diaphragm moves together with the internal focusing group. It also visibly closes a little when focusing from infinity to 1x. This is not a "compensating aperture" that adjusts for the increase in effective aperture while focusing, because this type of aperture opens up when focusing from infinity to 1x. There must be other optical reasons for the behavior of the aperture in this lens, probably related to the shortening of effective focal length that usually accompanies internal focusing at close distances.

   

   

At nominal f/3.5, i.e. fully open, the effective aperture is f/14, which is already visibly affected by diffraction on a 20 Mpixel Micro 4/3 sensor. Notwithstanding this unavoidable limitation, the lines on the test target are quite clearly resolved. In fact, f/3.5 gives the best resolution for this lens with MC-20. The (darker) lines on the test target are much thinner than the (lighter) spaces between lines, which allows one to qualitatively judge resolution at a scale smaller than the line-pair spacing. In principle, the thinner the dark lines are rendered, the higher the resolution.

There is a substantial drop of resolution between f/8 (effective f/32) and f/11 (effective f/44). At f/8, contrast is lower than in the preceding images, but the lines are still well resolved. At f/11, the lines are just barely visible, and completely gone at f/16.

There is some color moiré in the images, caused by the firmware anti-aliasing being not completely effective in removing aliasing caused by the slightly oblique lines. This is a limitation of the camera, rather than the optics used for this test.

No axial chromatic aberration is visible in the in-focus areas. There is some (of the green/magenta type) in out-of-focus areas, but not excessive.

Micro Nikkor 105 mm f/2.8

   

   

At f/2.8, resolution is high but contrast is low. The best nominal aperture is f/4, followed by f/5.6. I discussed this lens here.

CoastalOpt 60 mm f/4 Apo

This is a manual-focus and manual-aperture lens designed primarily for extended-spectrum imaging (near-UV to near-IR). It is, however, also one of the best macro lenses ever made, in spite of not reaching 1x. CoastalOpt recommends using an extension ring to achieve a higher magnification than the 0.66x allowed by its focusing helicoid, in spite of the lens using a fixed rear correcting group (in practice, equivalent to a floating group).

   

This lens is best fully open (f/4) and at f/5.6. At f/8 difraction already hides almost completely the fine line pattern (although thin lines spaced farther apart are still individually resolved along the top and bottom of the test image). The maximum magnification of this lens (0.66x) is lower than in the preceding lenses, and the results do not imply that this lens is worse than the preceding ones, or is affected by diffraction to a higher extent.

The test images display the type of fine-scale artifacts I observe with other very high resolution lenses, like the Printing Nikkor 105 mm f/2.8. These artifacts are probably caused by the lens outresolving the sensor's individual pixels and thereby forcing the camera's anti-aliasing algorithm beyond its design specifications. The f/8 image, in spite of showing only faint traces of the lines of the test target, still displays the type of color moiré associated with the line spacing of the target being resolved at the scale of individual pixels.

A curious side effect noticed during this test is that the center hotspot for which this lens is well known, with the MC-20 added, does not arise until the aperture is closed well beyond the limits at which diffration substantially blurs the image. In spite of using a wide and relatively short lens shade suitable to avoid vignetting on full-frame, the lens only begins to display a faint center hotspot at nominal f/22, and a definitely visible hotspot at nominal f/32. This suggests that the hotspot is caused by strongly oblique light rays that hit the sensor from peripheral regions of the lens, which are mostly cut off by the narrow and projecting front element of the MC-20 acting as a sort of internal baffle.

Nikon Micro-Nikkor-P Auto 55 mm f/3.5

   

The oldest lens in this test also performs well, with the best nominal aperture being f/4, and f/3.5 very slightly worse in contrast. All fine detail is already gone at f/5.6, which is reasonable given the low maximum magnification of this lens (0.5x). My initial fears that this lens could prove to have too low a resolution for the small Micro 4/3 pixels proved unfounded. The antiquated single-layer coatings of this lens may contribute to the relatively low contrast, although the small number of optical elements (5 elements in 4 groups) minimizes the impact of this problem.

Also this lens shows a little color moiré suggesting that the lens slightly outresolves the sensor, albeit to a lesser extent than the CoastalOpt 60 mm.

Test conclusions

The Olympus MC-20 teleconverter doubles the maximum magnification attained by macro lenses without decreasing their working distance. With all tested lenses, the MC-20 produces fine image detail, sometimes at or near the limit of the 20 Mpixel Micro 4/3 sensor resolution. Image resolution is especially good with lenses that reach a maximum native magnification of 0.5x-0.7x. In almost all cases, image resolution is best with the lens fully open (as long as the lens itself performs best fully open).

With teleconverter, the CoastalOpt 60 mm Apo appears to produce better image resolution at nominal f/5.6 than the Micro-Nikkor-P 55 mm at the same nominal aperture. The 60 mm focused at 0.66x + teleconverter gives an effective magnification of 1.32x at effective f/13. The 55 mm focused at 0.5x + teleconverter gives an effective magnification of 1x at effective f/11. Although the 55 mm has a slight advantage in effective aperture, the slightly higher magnification of the 60 mm gives it a clear resolution advantage at nominal f/5.6. At f/4, the 60 mm still has an edge over the 55 mm, but not as evident as at f/5.6. Both lenses with teleconverter produce images that approach the maximum sensor resolution, but the 55 mm is closer to this limit because of its lower magnification, at least with the test target used for this study. In this case, the slightly higher magnification beats the slightly higher lens speed in terms of recorded fine subject detail.

Using a 2x teleconverter with a macro lens focused at 1x magnification on Micro 4/3 requires a nominal lens aperture of f/2.8 to avoid diffraction. f/3.5 is slightly worse but still quite good. Nominal f/4 or f/4.5 is probably at the limit of usability. A macro lens that needs to be stopped down to nominal f/5.6 or f/8 to give an excellent image quality at 1x magnification, on the other hand, would not be a likely candidate for use at 2x magnification with a 2x teleconverter, and might instead be worth testing at 2x on extension. As a whole, the transition from a relatively good resolution of the test target lines to no resolution at all of the target lines takes just one aperture stop. It is therefore important to know where this limit goes, and not to accidentally exceed it (at least, if maximum resolution is desired).

Summary

The MC-20 teleconverter can be used on Micro 4/3 cameras to double the maximum magnification of legacy macro lenses without shortening the working distance. In the four tested macro lenses with Nikon F mounts, image resolution with the MC-20 is high to very high. However, an f/3.5 lens focused at 1x magnification, coupled with a 2x teleconverter and used fully open, is already moderately affected by diffraction. A macro lens that requires stopping down to nominal f/5.6-f/8 to give excellent image quality at a magnification of 1x would not be a likely candidate for use with the MC-20.