bigcoz Loc: Philly

I was shooting with a Canon 60D, 18-270 Tamron lens. I added a Promaster Spectrum 7 [1.4x teleconverter]. With that combination the camera will not autofocus. Is this a normal problem with teleconverters or is it peculiar to my particular set up?

OddJobber Loc: Portland, OR

Pretty common when you mix brands. Sometimes even with same brands, i.e., the Sigma 1.4X and 2X teleconverters, used on a Sigma 150-500mm will only autofocus when used on a Sigma body.

amfoto1 Loc: San Jose, Calif. USA

Your 60D technically requires an f5.6 or faster lens to autofocus (altho f6.3 will AF by fooling the camera).

A 1.4X teleconverter "costs" one stop of light... so your 18-270mm f3.5-6.3 lens with the TC added "becomes" an effective 25-378mm f5.0-9.0 lens. You might find it capable of AF at the 18mm setting of the lens... but of course where you need it to AF is at the other end of the zoom range, and there just isn't enough light.

Among the current models, only Canon 5D Mark III, 7D Mark II and 1DX are able to autofocus as small as f8 aperture, limited to one or a few of the AF points at the center only. Earlier 1D series can, too. But all other Canon bodies are limited to f5.6 or faster lenses.

Thus, my 300/4 IS focuses just fine on all Canon cameras with a 1.4X teleconverter, as an effective 420mm f5.6. But if I put a 2X teleconverter on it (which costs two full stops of light), making it an effective 600mm f8 lens, it will only autofocus on those few f8-capable models.

One thing you can try is taping up a couple of the contacts on the teleconverter, so the camera doesn't know the TC is there. In good light, with a nice, contrasty subject, you might get AF to work adequately. But in lower light and lower contrast it's likely to hunt and fail to focus at an effective f9.

Google taping up contacts on Canon teleconverter, if you want to give this a try.

Even if you get AF to work, image quality might not be good enough to make it worthwhile to use the combo. In general, teleconverters work best with prime lenses, rather than zooms. And the optical quality of both the lens and the teleconverter need to be top notch, for the combo to be able to provide decent, usable image quality. In other words, there's really no easy and inexpensive substitute for a more powerful lens.

OddJobber Loc: Portland, OR

Ain't that the truth! In my example above, the Sigma 150-500mm is not the sharpest, but acceptable at $700 (used). A "more powerful lens" would be $7000 to $12,000 to give me what I want.

What was not acceptable was the Sigma 1.4X teleconverter. Losing a stop of light and losing auto focus should have been a deal breaker, but I tried it anyway. One review said it gains 40% in magnification but loses 15% in image quality. Since that's a "gotta see it to evaluate it" scenario, I tried it. It went back. (Results on a D7100 were even worse.)

Shot with D800, which is no slouch for details, and I'm an admitted pixel peeper. These are from 20 yards and the little hexagons are six to the inch.

p.s. - These are part of the "O" on a stop sign.

amfoto1 Loc: San Jose, Calif. USA

So in your case... i.e. with that particular combination... it's better to crop the final image than to use the teleconverter.

OddJobber Loc: Portland, OR

Yes. This is not a put down of Sigma. Just in this case the TC is not as good as the lens. They have a newer version out, though, the 1401 that's $100 higher than the 1.4X, but I haven't checked the specs on it yet.

RRS Loc: Not sure

Question?? Are you first attaching the converter to the lens and then the camera to the converter, if not that could cause a problem.

bigcoz Loc: Philly

Gentlemen: Thank you so much for your very lucid explanations. I guess the next step is to buy a Canon $13,500, 600mm telephoto after I win the lottery. Thanks again guys.

Kubie Loc: SE Kansas

Nah, just go for the Tamron or Sigma 150-600mm for $1000-2000. Still might need the lotto, but could get some extras as well!