• Ed Dozier

Sigma TC-1401 Teleconverter and 105mm f/2.8 Micro Nikkor

Nikon tells you this combination won’t work. Sigma tells you this combination won’t work. Let’s see how this stacks up against reality.


A big problem in macro photography is “working distance”. Lots of insects are fraidy cats, and simply won’t hang around if you get close. Long macro lenses are quite expensive, and big telephotos don’t focus close enough. What to do?



Micro-Nikkor 105mm f/2.8 with Sigma TC-1401


I recently tried the forbidden combination of the 105mm f/2.8 Micro-Nikkor AF-S VR lens with the Sigma TC-1401 1.4X teleconverter. Would it fit? Would I lose autofocus? Would resolution be terrible? Do you get a 40% longer working distance? Does vibration reduction fail? Is cropping a shot better than using a teleconverter? Is camera focus fine-tune available? Does the camera see this combination as 147mm f/4.0 or does it still see 105mm f/2.8?


Lots of questions. Lots of testing to be done.


Notice in the shot above how small the teleconverter is. You can barely even tell that it’s attached. By the way, attach the teleconverter to the lens before attaching it to the camera.


First things first. I checked the clearance between the back of my 105mm lens and the front of the teleconverter. My micrometer said they shouldn’t crash into each other. I couldn’t see any fundamental incompatibilities, so I tried to connect them together. Success.


Autofocus Impact


I had read about autofocus going crazy when you connect this combination to a camera and turn power on. I attached the pair to my D850 and turned on power. No problem. The internet seems to be wrong about that particular issue.


The Nikon D850 is guaranteed to autofocus with lenses as slow as f/8.0. The 105mm, when focused down to 1:1 magnification, has the marked f/2.8 reduced to f/4.8. This means that minimum focus would now get you f/6.7 with a teleconverter (or f/4.0 when focused at infinity). So far, it seems like autofocus could still possibly work.


Next, I tried pressing the AF-ON button. Lo and behold, it could focus. I walked into a dim room and tried to focus again; again it succeeded, but it did quite a bit of focus-hunting. Pressing my luck, I also tried the teleconverter on my D610: autofocus still works, but it hunts a bit more on that camera. Even in bright light, phase-detect focus is noticeably slower and the lens makes a sort of clicking sound while focusing on any camera. Focus takes about ½ second, versus near-instant without the teleconverter. Trying Live View contrast-detect autofocus, I found that it behaves flawlessly, just like the lens does without the teleconverter attached.


The focus chattering didn’t change with either AF-S or AF-C. It was worst with single-point focus, and chattered the least with group-area focus. I couldn’t find a way to stop the chattering entirely. Contrast-detect definitely works better than phase-detect with this teleconverter. Forget about birds in flight.


I don’t think that you’d want to use this combination with regular-distance shooting, such as portraits, while using phase-detect autofocus. For those non-macro scenarios, I’d recommend you simply remove the teleconverter. Everything will work using the teleconverter, but the extra focus hunting and chattering sounds are annoying.


Manual Focus


Focus peaking with Live View manual focus on Nikon D850



Most people probably switch to manual focus with macro lens close-ups, including me. Autofocus at such short distances can be an exercise in futility with any hand-held macro lens. If you haven’t tried focus-peaking mode in Live View and your camera supports it, you should check it out. Just like with other lenses, you need to switch the lens to “manual” to enable focus-peaking. Outdoors, I often use my LCD viewer/magnifier to get a great screen image while also shielding the screen from bright light. Holding the viewer against both my eye and the camera can be helpful for keeping things more steady, too. Focus-peaking makes critical manual focus quite easy. Mirrorless users are spoiled, getting this feature inside the viewfinder.


Vibration Reduction


What about using vibration reduction? This 105mm VR should get me about 4 stops of shake reduction, or shots down to about 1/8 second. I set my camera to use manual exposure with auto-ISO, and set 1/8 shutter with and without VR active. Using VR, the shots are sharp; without, they’re total mush. VR works with the teleconverter just fine.


Lens Exif Data


How about lens information? I tried to set the aperture to f/2.8, which shouldn’t be possible (f/4.0 should be the maximum). Oops. I can set it to f/2.8. The Nikon D610 doesn’t know the teleconverter is attached, and neither does the D850. But I couldn’t help but notice that exposures are perfect, regardless. The photo exif data incorrectly shows “f/2.8”, “105mm” and also notes (correctly) when VR is enabled. Exif distance data with the teleconverter is also incorrect.


Focus Fine-Tune


What are the implications for focus fine-tune? Since the camera doesn’t know a teleconverter is connected, it means that you’d end up overwriting the fine-tune calibration for the 105mm lens without the teleconverter. Not terrible, but certainly not ideal, either. If you want to use phase-detect autofocus, though, you will almost certainly need to set the focus fine-tune for this combination. Remember to set it back to the old value when you remove the teleconverter…


Working Distance


What kind of working distance (from the front edge of the lens) do you get at the minimum focus distance? You get 6.0 inches. That’s the same as no teleconverter, but the lens is at what’s now 1.4X magnification instead of 1.0X. For life-size magnification, you can back off to about 8 inches, or 40% further away.


D850 Focus Shift Shooting (Focus Stacking)


The D850 can help with a major problem in macro photography: depth of focus. The D850 can automatically take shots for focus-stacking. “Focus shift shooting” does work with the teleconverter attached. The lens needs to be kept in auto-focus mode, and the lens needs to be pre-focused onto the target (or a little in front of it). As a test, I chose to NOT use Live-View for these shots. I use aperture-priority mode, and I definitely put the camera onto a tripod for macro shooting. Lens VR is turned off, single-shot mode, and I closed the eyepiece shutter.


For the test, I chose 20 shots, with a focus step width of 3, an interval of “0” until the next shot, exposure smoothing ON, Silent photography ON, and hit the multi-selector middle button after selecting “Start”. Because of selecting “silent photography”, the only vibrations during the automatic shooting sequence were due to the automatic focusing steps of the lens (full electric shutter). Nice.


Getting further from the subject, thanks to the teleconverter, was quite helpful with lighting in this little test.


You can, of course take the series of shots manually in place of using this particular camera feature.


The D850 doesn’t actually do any photo stacking, it just makes the shots to stack. I used the free CombineZP to stack the shots, after using Nikon CaptureNX-D to batch-convert the raw shots into tiff format (CombineZP can’t use raw-format shots). I chose to try the “Pyramid Weighted Average” macro to see what it could do. "Macro" here means running a little program to do a task. After that, I tried the “Do Stack” macro. I prefer the “Do Stack” result. There are several macros to choose from.



First shot of a stack of 20. Skinny focus. f/8



‘Pyramid Weighted Average’ macro stack.



‘Do Stack’ macro.





Resolution Impact


Now, for the biggest question. What happens to resolution? I have read that a Sigma Teleconverter/Nikkor lens combination is very bad for resolution, and the frame edges are horrific.


I don’t have any facilities to measure lens resolution in the macro region, so I’m just going to show resolution results at conventional distances (around 5 meters). You’ll have to infer that the teleconverter resolution impact at regular distances is similar to the macro distance impact.


If the resolution loss using the teleconverter is less than 40%, then it means that using the teleconverter is better than merely cropping the shot. Since it’s a 1.4X teleconverter, the focal length is increased by 40% from 105mm to 147mm.


The following measurements are done using the Nikon D850. As always, my shots are done using un-sharpened raw format. All resolution measurements are in units of MTF50 lp/mm. The resolution plots separate out meridional (tangent) and sagittal (wheel spoke) direction measurements.



MTF50 lp/mm with teleconverter, 147mm f/4.0


The 105mm lens was set to the marked f/2.8 aperture. The ‘exif’ data should have noted a focal length of 147mm and f/4.0. Peak resolution is about 41 lp/mm. Notice that the edges of the full-frame aren’t degraded, compared to the lens center, even wide-open. Already, the resolution is fine (I measure ‘fine’ as being above 30 lp/mm).



MTF50 lp/mm, no teleconverter, 105mm f/2.8


The wide-open resolution plot for the 105mm without a teleconverter is shown above. Peak resolution is about 53 lp/mm at f/2.8.


The resolution loss when attaching the teleconverter is 41/53 = 0.77, or a 23% drop at f/2.8.



MTF contrast plot, wide open with teleconverter


The plot above is the traditional wide-open MTF contrast plot, using the teleconverter. This is 147mm, f/4.0.



MTF contrast plot, 105mm f/2.8 , no teleconverter


The plot above is the traditional wide-open MTF contrast plot, without the teleconverter. This is 105mm, f/2.8. Overall contrast is nearly 10% higher than with the teleconverter attached.



MTF50 lp/mm with teleconverter, 147mm f/5.6



MTF50 lp/mm, no teleconverter, 105mm f/4.0


Attaching the teleconverter at f/4.0 (now 147mm f/5.6) drops the resolution by 49/57 = 0.86, or a loss of 14%.




MTF50 lp/mm with teleconverter, 147mm f/8.0


Nice and sharp at this aperture, with better depth of focus.



MTF50 lp/mm, no teleconverter, 105mm f/5.6


Attaching the teleconverter at f/5.6 (now 147mm f/8.0) drops the resolution by 51/62 = 0.82, or a loss of 18%.





MTF50 lp/mm with teleconverter, 147mm f/11.0


Diffraction is starting to set in and messing with resolution.



MTF50 lp/mm, no teleconverter, 105mm f/8.0


Attaching the teleconverter at f/8.0 (now 147mm f/11.0) drops the resolution by 49/61 = 0.8, or a loss of 20%.



MTF50 lp/mm with teleconverter, 147mm f/16.0


Please ignore the left-side dimple in the meridional-direction plot above. I didn’t realize a shadow had been cast onto the edge of the target, which affected the left edge measurements. Resolution is still okay, but diffraction is getting much heavier.



MTF50 lp/mm, no teleconverter, 105mm f/11.0


Attaching the teleconverter at f/11.0 (now 147mm f/16.0) drops the resolution by 42/53 = 0.79, or a loss of 21%.



MTF50 lp/mm with teleconverter, 147mm f/22.0


Please ignore the left-side dimple in the meridional-direction plot above. I didn’t realize a shadow had been cast onto the edge of the target, which affected the left edge measurements.



MTF50 lp/mm, no teleconverter, 105mm f/16.0


Attaching the teleconverter at f/16.0 (now 147mm f/22.0) drops the resolution by 29/41 = 0.71, or a loss of 29%. The teleconverter at this aperture (f/22) is getting into heavy diffraction, so the resolution loss gets much worse. I consider this aperture to be unacceptable for teleconverter use.


In summary, the resolution drop by attaching the teleconverter is typically about 20%. Since the focal length was increased by 40%, this is definitely superior to merely cropping the picture to get the same field of view. Frame edges are quite good, thus disproving lots of the internet hearsay about the resolution disaster.


Your shots can be tack sharp with or without the teleconverter. Your camera support, accurate focus, and lighting will impact getting sharp photos at least as much as the raw lens/teleconverter resolution. You also probably want to look into focus stacking, since depth of focus is one of the biggest issues in macro photography.


Conclusion


Here’s an “impossible” combination that does the impossible. Aside from stressing the phase-detect autofocus system a bit, you probably won’t even notice if the teleconverter is attached or not. If you’re willing to stick with either contrast-detect focus or manual focus, you’ll be very happy with the shots from this combination. If you’re hooked on phase-detect focus at all times, then you won’t be happy with this combo. But you won’t be complaining about sharpness.


I’m glad I finally got around to testing this setup. I had fallen into the trap of believing what I read on the internet. I intend to use the Sigma TC-1401 and Micro Nikkor 105mm f/2.8 team quite often in the future; it may help a bit with those shy bugs. Lighting is simpler at these longer distances, too.




Sample Photos



Coin at 1.4X magnification. D850 147mm f/14.0 1/30s


Even at f/14.0, the shot above strains to have enough depth of focus at the minimum focus distance. It’s pleading with me to use focus stacking. Nonetheless, it looks pretty sharp to my eye. The coin is 6 inches from the front of the lens. I used Live View contrast-detect focus here, to prove it works, but normally I’d be using manual focus with focus-peaking.



Coin at 1.0X magnification. D850 147mm f/16.0 1/20s


At 1.0X magnification, you get about 8 inches working distance from the front of the lens. Compared to this, the US $1,400.00 Canon 180mm f/3.5L USM lens gets you about 9.5 inches working distance; not that much different. I used Live View contrast-detect autofocus for this shot, too.



Fairy Duster, D850 147mm f/4.0 (wide open) 1/500s


Aside from having almost no depth of focus at this distance, the 105mm wide open with the teleconverter up close is quite nice. I even used phase-detect focus here, just to see if it would work properly.

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