Petteri's Pontifications
My musings about photography, mostly.
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Converter versus Converter

Converter versus Converter

I've acquired two inexpensive 1.4x teleconverters: the "generic" Tamron-F 1.4x MC4 (identical to the Teleplus and cheap-o Kenko), and the decidedly more up-market Sigma EX 1.4x APO. I finally got around to doing a shootout between the two. Given Sigma's solid reputation for optics, I was expecting the Sigma EX to win in a walk... but it turns out it wasn't as simple as that.

Build and design

The Tamron and the Sigma side-by-side. Note the protruding front element on the Sigma: this means it can't be used with many lenses, as it'd get in the way. If you want one, be sure to check for compatibility.

There aren't that many things you can put on a TC, but somehow the Tamron and the Sigma manage to be different anyway. The Tamron is clearly built for economy: it has a metal body and metal mounts both ways, and the element coatings look pretty good to the eye, but paint and finish are pretty minimal. The TC is wired through: it reports the aperture setting on the attached lens, without accounting for the light loss. The Sigma looks and feels much nicer: it's heavier, has the pretty classy EX textured-gray finish, the printing is neat, and it reports the aperture correctly. Both come with a baggie to carry them in, and caps for both ends. The build definitely prejudiced me towards the Sigma... but they do say you shouldn't judge a book by its cover.Note also that the Sigma, like the Canon 1.4x, has a protruding front element that means that it can't be used with many lenses with rear elements close to the back.

Test set-up

I shot a scene with plenty of detail with my Canon 200/2.8L and each teleconverter at f/4, f/5.6, and f/8. Then I shot the same scene without the teleconverter, and, just for kicks, with both teleconverters stacked. Then, I compared the teleconverter shots to each other and to the bare 200 mm shot up-rezzed by 1.4x. Finally, I downsampled the teleconverter shots by 0.71, and compared that to the non-teleconverter shot, sharpened somewhat to normalize contrast. I'm omitting discussion of the f/5.6 shots, as they don't really show anything that's not in the f/8 ones.

Performance at f/4

Edge crops

Tamron

Sigma

No TC, up-rezzed

Center crops

Tamron

Sigma

No TC, up-rezzed

A couple of things strike the eye immediately: while it's clear that both teleconverters do gain resolution, they also introduce some rather unpleasant artifacts. Both do nasty things to the bokeh: they introduce color fringes, and the Tamron also smears it out circularly. The Tamron is visibly softer at the edges, although both are pretty good in the center. On the other hand, the Sigma introduces some rather nasty red-green shift, very similar to the Tokina 17. Neither loses much contrast. Color-wise, the Sigma is slightly warm, the Tamron possibly slightly cool. ("Possibly," because the light was changing as I made the shots, so I can't be 100% certain it's not because of this.) The Tamron is sharper in the center, but the Sigma is more even across the frame.

I was particularly struck at how good the up-rezzed 200 mm shot looks. Very few optical artifacts visible, and still looking pretty sharp. The tell-tale is in the arcing structures: they've started to develop the beginnings of jaggies. But in any case, there's clearly a lot of room to crop and up-rez even without a teleconverter.

I'll hand this round to the Sigma: despite the red-green shift, it maintains bokeh better than the Tamron, and does gain a significant amount of resolution against the up-rezzed shot. The up-rezzed 200 is very close, though: not quite as sharp, but with none of the annoying teleconverter artifacts, and it has one more very good stop of brightness in it. In less than bright daylight, I'd definitely consider shooting without the TC and up-rezzing afterwards. For birding, I'd pick the Tamron, though, as edge softness doesn't matter much for that mission.

Performance at f/8

Edge crops

Tamron

Sigma

No TC, up-rezzed

Center crops

Tamron

Sigma

No TC, up-rezzed

On to round 2. Here, the roles are reversed. Both teleconverters perform pretty well: the smeared bokeh and edge softening are largely gone. (In fact, they were largely gone by f/5.6 -- there's really very little difference between that and f/8.) There is an obvious gain in resolution over the up-rezzed frame. However, Sigma's red-green shift is still there, and actually more obvious because of the overall better sharpness, whereas the Tamron's halation-softness is gone, and the smeared bokeh isn't smeared anymore either. So this round goes squarely to the Tamron.

"Don't be an idiot"

Stacked, at f/11.

Finally, from the "Don't be an idiot, that couldn't possibly work" department: I stacked the two TC's to get an effective 400/5.6. Guess what? It worked! Not spectacularly well, but it does produce usable quality. I'm sure a dedicated 2x teleconverter would do a much better job. I plan to own one, one of these days.

Conclusion: I'm keeping the Tamron

I have to say I expected better from Sigma's EX: while it maintains bokeh better and is more even across the frame, the red-green shift it introduces is a definite minus, and the yellowish color cast isn't very nice either. The Tamron is sharper in the center even wide-open, and its nasties go away almost completely just one stop down. As it's probably the cheapest 1.4x available, it's definitely excellent value for money. The Sigma is better wide open, but the Tamron is better for birding and capable of better quality stopped-down.

In practice, this means that I'll stop down to f/5.6 whenever possible, and if not possible, I'll still get a decently sharp center... or just leave off the TC and crop afterwards.

Neither teleconverter is bad, though. Both gain a good deal of resolution, especially stopped-down, lose very little contrast, and aberrations are largely kept under control. I have a feeling that to get much better than this, I'd have to shell out for the much more expensive Canon... or, of course, get a 300/4. Neither makes much sense for my needs, so the Tamron's staying. Anyone want to buy a good-as-new Sigma 1.4?