Excellent article on Teleconverters with examples

[Scottes]

Bob Atkins over at photo.net put this one out:
http://www.photo.net/learn/optics/tc2/index.html

Also has a link to a Tamron 1.4 vs Canon 1.4 article:
http://www.photo.net/learn/optics/2tcs/index.html

[PhotoSlacker]

Good article... great info for an amature like me

thanks.

[FJC]

Yes, excellent article, really cleared up a lot of questions I had about using TC's, and how much they'd really affect the image quality.

I'm still a bit fuzzy (lol) about what happens with autofocusing when using these, though.

[billhercus]

... Ordered a Sigma EX 1.4TC (Thank goodness) yesterday for my Sigma 100-300 EX f4.

Many 'lessons' on TC's in this Forum led me to this decision. Nothing in that excellent article challenges anything I have read here - thank goodness!

[Scottes]

Originally Posted by FJC

I'm still a bit fuzzy (lol) about what happens with autofocusing when using these, though.

There's a couple of things that happen to AF when using TCs. (At least that I can think of right now...) And others correct me if I'm wrong...

1a) Canon TC on a DRebel/10D: If the maximum aperture is smaller than f/5.6 the camera will disable AF.

1b) One a EOS3 (?), 1D family this is changed to f/8.

1c) You can tape off a couple/few contacts on the TC to stop the camera from knowing the TC exists, so AF will still be attempted. It's not necessarily reliable, it can hunt a lot, and I've heard reports of lenses going back to Canon for service (which may or may not be related, but is suspicious, and may be due to AF motors burning out due to execessive hunting).

2a) Non-Canon TC on any lens: The TC doesn't report the existence of itself to the camera, so the camera doesn't know that it's not supposed to AF. So AF will still be attempted. It's not necessarily reliable, and it can hunt a lot.

2b) Well, at least that seems to be true about older Tamron TCs, and the exact specifics may be otherwise for newer TCs.


I *think* that's it, and I *think* that's accurate.... If nobody jumps on my case in the next day or two then it's accurate.


Edited to clarify aperture - Even though I know what I'm doing, I'm a math guy, and an aperture f/5.6 is "greater" than f/4 even though in reality f/5.6 is a smaller aperture. You know what I mean? I'm sure you do, but it is confusing, and I'll try to stop this in the future. Thanks PaulB.

[FJC]

SO - given that, let's test my knowledge now.

If I put a Canon 1.4x TC on my 70-200mm f/4, it becomes a 98mm-280mm f/5.6 (not counting the DRebel 1.6 crop factor), and will still autofocus fine.

However, if I put a Canon 2x TC on that lens, it becomes a 140-400mm f/8 and now autofocus won't work (but manual will).

Correct?

[PaulB]

Scottes wrote:

1a) Canon TC on a DRebel/10D: If the maximum aperture is greater than f/5.6 the camera will disable AF.

A greater aperture than f5.6 is f4 whilst f8 is a smaller aperture than f5.6.

I think that you really mean that if the effective maximum aperture of the lens + Extender (Canon) is larger or equal to f5.6 then the camera will autofocus; if the effective maximum aperture is smaller than f5.6 then a Rebel/10D will not autofocus.

Hate to be pendantic about it but if we start to mix up accepted nomenclature then no-one will really understand what we are talking about.

FJC has got it spot on though so all was not lost.

[billhercus]

Just tested above Sigma 1.4 TC which arrived 5 mins ago. It is a dreich (miserable, misty) day here with poor light.

No problem autofocusing at most normal apertures tried this minute on 300D with 100-300 f4 EX HSM (Now 140-420 of course)


Paperwork that comes with this converter doesn't even mention this lens but Sigma site fortunately does.

[Cadwell]

Originally Posted by Scottes

2a) Non-Canon TC on any lens: The TC doesn't report the existence of itself to the camera, so the camera doesn't know that it's not supposed to AF. So AF will still be attempted. It's not necessarily reliable, and it can hunt a lot.

2b) Well, at least that seems to be true about older Tamron TCs, and the exact specifics may be otherwise for newer TCs.

Just for clarification... the Sigma 2x and 1.4x TCs DO affect AF in the same way that the Canon units do.

So, for example, a Canon EF 70-200mm F/4L + Sigma 2x TC will yield a maximum aperture of F/8 and so this combo will NOT autofocus on a 300D/10D.

[Pekka]

Has anyone tried Canon 1.4X MkII + Canon 1.4X MkII + 70-200L?

[Nigel Wallace]

Yes, I have a 70-200 f4 L and a canon 1.4 TC mk2 and it is in my opinion great. Autofocus no problem. i am very pleased with this combo

[Cadwell]

Originally Posted by Pekka

Has anyone tried Canon 1.4X MkII + Canon 1.4X MkII + 70-200L?

Why would you want to?

200x1.4x1.4 = 392mm (lose two stops)
200x2.0 = 400mm (lose two stops)

[Pekka]

Originally Posted by Cadwell

Why would you want to?

200x1.4x1.4 = 392mm (lose two stops)
200x2.0 = 400mm (lose two stops)

Because theoretically that would give better quality.

[PacAce]

Originally Posted by Scottes

Edited to clarify aperture - Even though I know what I'm doing, I'm a math guy, and an aperture f/5.6 is "greater" than f/4 even though in reality f/5.6 is a smaller aperture. You know what I mean? I'm sure you do, but it is confusing, and I'll try to stop this in the future. Thanks PaulB.

Scott, I think you just confused yourself. f/5.6 is NOT greater than f/4 just like 1/3 is NOT greater 1/2.

[PacAce]

Originally Posted by Pekka

Because theoretically that would give better quality.

Pekka, 1.4x over 2.0x might give you better quality but how do you figure that 2 1.4x combined would give you better quality (theoretically)? IMHO, I would think that the more external elements you combine together the less quality you would get. So I would think that two 1.4x combined would be of less, or at best of equal quality to that of the 2.0x. The reason being that image quality loss of lens components are cummulative. But this is just an educated guess on my part since I haven't tried either combination.