T-Con help

[AdamW]

Hey, I've got some questions about teleconverters.

1. If I use one with my 70-300 IS, will it still autofocus? Will the IS still work? How many stops of exposure will it lose?

2. If I use one with my 100 macro, will I get 1:1 macro with a greater working distance? Will it still autofocus?

3. Can I use the same teleconverter with either lens?

4. Which one should I buy?

Thanks....

[RichNY]

Here's a great thread on TCs that will answer all your questions and then some....
http://photography-on-the.net/forum/showthread.php?t=41922

[AdamW]

Thanks, Rich. I read that thread before my post, but ended up more confused. It says things that are directly contradicted in the replies. For example, it says a TC won't work with lenses under 135mm, but other posters say it works fine with a 75-300mm. Also, it seems that some brands of TCs are compatible with some Canon lenses, but I'm not clear on what the deal is. Hence my post. But, thanks anyway.

[coldrain]

Hey, I've got some questions about teleconverters.

1. If I use one with my 70-300 IS, will it still autofocus? Will the IS still work? How many stops of exposure will it lose?

2. If I use one with my 100 macro, will I get 1:1 macro with a greater working distance? Will it still autofocus?

3. Can I use the same teleconverter with either lens?

4. Which one should I buy?

Thanks....
A teleconvertor basically does just one thing. It takes the center of the image the lens produces and enlarges it over the entire frame. For the rest it doesn't do a thing.
If a lens gets a certain amount of light per pixel at f2.8 for instance, and you stop down to f4, only half the light reaches each pixel because the hole that lets the light through got half the size.
When the TC crops the image coming from the lens, and spreads that crop out over the entire sensor again, it too has to divide all the light that was meant for the center pixels over all of the pixels, and this is why you have loss of light with TCs.
An SLR usually needs quite some light to be able to reliably measure contrast for autofocus. The "rule" is to have f5.6 or bigger to be able to reliably AF.
Some TCs, like the extenders from Canon and the 1.5x pro extender from Kenko, tell the camera an f2.8 lens has become f4 (or an f5.6 lens has become an f8 lens) with an 1.4x TC. This will mean that with such an extender your camera will turn off AF with a lens of f5.6. Others, like the cheaper Kenko, Soligor and Tamron lines do not tell the camera that, and the AF sensors will keep on trying.

So, here are the answers:

1a. A cheaper Kenko 1.4x TC will still provide AF in good light. Even a Soligor 1.7x TC may still allow AF, depending on the light.

1b. IS will still work, no matter what TC you would use. The lens' IS is totally independent.

1c. With an 1.4x TC the loss of light is about 1 f-stop. So, f5.6 -> f8. An 1.7x TC will have about 1.5 f-stop loss, so f5.6 -> f9.5. A 2x TC will have about 2 f-stops loss of light, so an f5.6 lens becomes f11.

2a. With your macro you will get with an 1.4x TC about 1.4:1 macro at the same working distance (well, a little bit more working distance because of the thickness of the TC :p). With an 1.7x TC it would then be 1.7:1.
For 1:1 macro you indeed could take a bit more distance.

2b. AF will still work fine on the macro lens.

3. This depends totally on how the TC is built. Sigma and Canon TCs and Extenders will not fit either lens. The Soligor and Kenko TCs will fit both.

4. That is up to you. The best ones to consider are:
Kenko 1.5x MC DG. It is actually an 1.4x TC, and will not block AF.
Soligor 1.7x TC. It will give a bit more reach, so also will steal a bit more light.

[RichNY]

Coldy- Great reply. One of the best written explanation posts I've seen up here in a while.

[AdamW]

Coldy--

I was hoping you'd answer my post, and my faith was rewarded. Thanks for a thorough and clear response.

One more question: anyone know where to get a Soligor in the US? B&H, Adorama, KEH, and Amazon all come up empty.

[TenD]

I admit I have a tendency to cut off all research after I have made a purchase, basically so I don't drive myself mad seeing better bargains or the next greatest thing. Because of this I didn't know that Kenko had ditched their fine 1.4x Teleplus Pro 300 for a 1.5x version. I think my older version will try to AF no matter how slow the lens is, I really don't know as I have only tried it with my 300mm f/4.
Anyway, long story short, I have been very satisfied with my Kenko 1.4x. It's very sharp, in tests I have seen it's sharper than the Canon at the edges, with the Canon having a slight advantage in the center.

[Honest Gaza]

Coldy--

I was hoping you'd answer my post, and my faith was rewarded. Thanks for a thorough and clear response.

One more question: anyone know where to get a Soligor in the US? B&H, Adorama, KEH, and Amazon all come up empty.

Can't answer your question. However, if you e-mail Peggy, she may be able to advise. I ordered mine directly through Peggy and had it sent to Australia.

Peggy.Melters@soligor.de

HG

[RichNY]

Adam- I'm ready to try out one of those TCs. Perhaps a few of us can place one larger order and get them shipped over.

[AdamW]

Rich, that's tempting. I'm gonna have to think on it. I think I'll email Peggy and ask her what it would cost.

Gaz, how's it work with the 70-300? (And why is my kit starting to look like yours? ;) )

[Honest Gaza]

Rich, that's tempting. I'm gonna have to think on it. I think I'll email Peggy and ask her what it would cost.

Gaz, how's it work with the 70-300? (And why is my kit starting to look like yours? ;) )

Well obviously there are the limitations that Coldrain has outlined.

Technically, from Soligor's own website :


Focal length must be at least 50mm
AF only works at f/4.5 or better


As a quick experiment just now, I set the 70-300mm to 300mm and Av to f/8.
I then tried focusing on some tree leaves that had a plain sky as the background.

With centre point focus, the lens did a lot of hunting, but with 9-Point AF, it had a much easier job of focusing. So while the recommended Aperture is f/4.5 or better, it is still possible with smaller aperture openings.

Of course, utilising MF overcomes this issue anyway.

[AdamW]

I really rely on autofocus. Between my eyeglasses and the XTi's viewfinder I can only comfortably use MF when the camera's on a tripod or the subject is not moving.

[coldrain]

The Kenko 1.4 300 thingy pro mentioned does cut off AF, so it is not the best of ideas to look at that one.

HG, setting aperture to f8 should not matter at all, since AF is measured with the lens aperture wide open? So I am not sure what your experiment tells you. Aperture at f8 should yield the same AF results as Aperture at f5.6.

Since HG finds that AF works but not optimally, it is up to you to choose.
The Kenko 1.5x (1.4x actually) version I mentioned above should have an AF advantage because the loss of light will be less.

Soligor has no distributor in the US, you have to order it via their website.

[Honest Gaza]

HG, setting aperture to f8 should not matter at all, since AF is measured with the lens aperture wide open? So I am not sure what your experiment tells you. Aperture at f8 should yield the same AF results as Aperture at f5.6.



Well you learn (have something reaffirmed) every day. Yes, I was making the "beginner's" mistake of thinking that the aperture changed as I changed the setting on the camera. But of course, this would only take effect on shutter release (or DOF preview). :o

My bad (thanks Coldrain)

So in retrospect, any AF at all with this lens would be considered a bonus :)