Sigma 150-600 C with Sigma TC-1401 Teleconverter

This isn’t something that I’d recommend doing, but I know that there are many people interested in putting Sigma’s TC-1401 1.4X teleconverter onto the Sigma 150-600. Sigma has even sold this combination as a kit. So here goes.

Sigma TC-1401 1.4X teleconverter on Nikon D850

The field of view at 840mm is outrageously narrow. On a DX camera, at a 1260mm equivalent, it’s even outrageous-er. I know that isn’t a word, at least not until you try this combination.

The Sigma 150-600 Contemporary is a good lens, but at 600mm it’s pretty much at the limit of its abilities for quality resolution. Adding a teleconverter into the mix certainly isn’t going to help. The camera focus system isn’t going to be thanking you, either. A 2X teleconverter would be out of the question on this lens, both for resolution quality and the ability to auto-focus.

I would be remiss if I didn’t remind the audience that you have to attach your Sigma teleconverter to the lens prior to mounting it to the camera. If you don’t follow this procedure, then it won’t autofocus.

Resolution tests

I performed a resolution test with the lens/teleconverter combination at 840mm and f/9.0, which is with the aperture wide open. This means that the lens was zoomed to its maximum 600mm marking, getting a combined focal length of 840mm. The aperture of f/9.0 is about as dim as I’m willing to photograph moving subjects. Quality will of course improve some when you stop down the aperture.

I figured that there isn’t much sense in testing this teleconverter at small focal lengths, since it would be much wiser to merely remove the teleconverter. If somebody bothers to put a teleconverter onto a lens that already zooms to 600mm, it means they want as much reach as they can get.

I photographed a resolution test chart from 62 feet (!) or 18.84 meters. Even from this far away, only a piece of the test chart could fit into the field of view. This is as far away as I am able to get from the target where I do my testing. Even at this distance, I begin to wonder if air turbulence starts to become a factor.

I perform resolution tests using “live view” with contrast-detect focus, to eliminate focus calibration from being an issue. I set my camera up to use electronic front-curtain exposure to rid vibrations. I also use a wired remote release. Even contrast-detect focus gives variable results, so I pick my sharpest result (from 10 shots) to report.

840 mm resolution chart detail, Sigma TC-1401 f/9.0 on Nikon D850

The 840 mm peak resolution was measured to be an MTF50 of 32.4 lp/mm. This is an equivalent of 1549 lines per picture height. A common minimum “quality” resolution benchmark is an MTF50 of 30 lp/mm. This result just made it into the “good” category. Note that the exif photo data reported above shows 850mm.

You may notice that the other resolution measurements in the shot are less than the 32.4 lp/mm reading. Lens resolution is a lot more complicated than a single number. Generally, these resolution results aren’t too good and fall a bit below what I consider acceptable.

The shot above is un-sharpened raw format, and any chromatic aberration would show clearly. You can see a trace amount of it here, although post-processing would easily remove it.

There is just about zero vignetting with this combination, as you'd expect.

600 mm f/6.3 resolution chart center, Nikon D850, no teleconverter

600 mm f/6.3 resolution chart edge, Nikon D850, no teleconverter

Next, I removed the teleconverter to repeat the test for comparison purposes. Everything else remained the same as the shots that I had taken with the teleconverter attached. The MTF50 peak was measured at 48.0 lp/mm. The equivalent peak resolution here is 2294 lines per picture height. The shot quality looks significantly improved, as expected.

The resolution change is 1549/2294, or 0.68, which means a drop of 32 percent by adding the teleconverter to the lens. This is a better result than just cropping the shot, but not by a very significant margin.

Focus Speed

The cameras I tried (Nikon D850 and D500) struggle to focus with this setup when not in good lighting, but incredibly they can still focus in moderate shade. No birds in flight with this kind of rig, though, unless they’re the big heavy birds. For lesser cameras, focus performance is going to go downhill in a hurry.

I measured focus speed with and without the teleconverter to compare the differences. My usual testing involves setting the lens on its minimum focus distance to find how long it takes to focus on infinity. I do these tests in sunlight, so that I can compare various lenses and cameras in good lighting conditions.

My tests were at 600mm (or 840mm with the attached teleconverter). I used my D850 on some tests, and the D500 on others. I didn’t notice any focus speed differences between these two cameras. They’re advertised to focus down to f/8 (not all focus points, though). The aperture f/9 didn’t generally pose in a problem in the conditions I use this lens, despite being “out of specification” for the camera focus system capability.

If your camera doesn’t have any f/8 focus points, then don’t even consider using this teleconverter. Also, you can notice focus inconsistencies if you pick any non-f/8 focus sensors (try to stick with the center focus point).

Without a teleconverter attached, it takes 0.633 seconds to focus through the full range.

With a teleconverter, it takes 1.183 seconds to focus through the same range. This is the worst performance I have witnessed from a lens to date.

Next, I tried a more “real-world” big lens focus test, going from 50 feet to infinity. I experience conditions like this all the time out in the field. Keep in mind that 50 feet is actually pretty "close" at this extreme magnification.

Without the teleconverter attached, it takes 0.125 seconds to focus through this range.

With the teleconverter, it takes 0.2917 seconds to focus through this same range. This result is perfectly acceptable in most shooting conditions.

Generally, it takes about twice as long to focus when using the teleconverter. For realistic focus distance changes, this is totally acceptable and better than I would have thought. You’re crazy if you routinely focus from minimum distance to infinity out in the field.

Conclusion

I don’t personally think that adding the teleconverter is worth it for this lens. I’d just as soon crop a shot and get only marginally worse results than using the teleconverter. Stop down the aperture, if your subject isn't moving, for a moderate increase in resolution.

I am a real fan of using the Sigma TC-1401 on my 70-200 f/2.8 Sigma Sport, but I just can’t recommend using it on the Sigma 150-600 Contemporary lens.