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11-10-2008, 08:00 PM   #1
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Converter Issue

Hi All
I have just been loaned a TAMRON Pz-AF MC7 2X Tele-converter.
Here's the problem..............
When I put it on my DA 18-55, it works great at 18mm. It focuses slowly (jumps around) but I DO get focus lock. When I zoom out to 55mm, it will NOT focus, even on manual.
When I put it on my DA 50-200, same thing. Great at the low end, but won't work at the high end.
It focuses at 50mm on the 50-200, but NOT at 50mm on the 18-55????
Throw it on the Sigma 175-500............same thing! Focuses at 175, but not 500.
Again.............Focuses at 175 on the 175-500, but not at 175 on the 50-200??

I'm very confused.
The only thing I can come up with is maybe it's some kind of physical/mechanical issue? When I zoom out, maybe one of the elements gets in the way? Shifts the focal point?
I don't know.

Suggestions???

11-10-2008, 08:40 PM   #2
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QuoteOriginally posted by stgmgr Quote
Hi All
I have just been loaned a TAMRON Pz-AF MC7 2X Tele-converter.
Here's the problem..............
When I put it on my DA 18-55, it works great at 18mm. It focuses slowly (jumps around) but I DO get focus lock. When I zoom out to 55mm, it will NOT focus, even on manual.
When I put it on my DA 50-200, same thing. Great at the low end, but won't work at the high end.
It focuses at 50mm on the 50-200, but NOT at 50mm on the 18-55????
Throw it on the Sigma 175-500............same thing! Focuses at 175, but not 500.
Again.............Focuses at 175 on the 175-500, but not at 175 on the 50-200??

I'm very confused.
The only thing I can come up with is maybe it's some kind of physical/mechanical issue? When I zoom out, maybe one of the elements gets in the way? Shifts the focal point?
I don't know.

Suggestions???
Is it possible that as you zoom out, the lens is getting dimmer to the point that the AF is out of range?
Most consumer lenses are not constant aperture, and can lose a stop or more of speed during zooming from short focal length to long.
11-11-2008, 04:39 AM   #3
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QuoteOriginally posted by Wheatfield Quote
Is it possible that as you zoom out, the lens is getting dimmer to the point that the AF is out of range?
Most consumer lenses are not constant aperture, and can lose a stop or more of speed during zooming from short focal length to long.
Yup that would be a problem. At the short end of those lenses even with the TC you still have enough light to focus but at the long end you don't. Don't forget you loose half your light when you use a 2xTC (a f/2.8 lens becomes a f/5.6 lens etc).


John
11-11-2008, 05:16 AM   #4
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Something to remember when looking at either a TC or the pentax 1.7x AF TC..

the 1.7 x AF TC is only gauranteed for F2.8 or faster lenses (f4.5 when TC attached) Many including myself have posted repeatidly that it works fine with F4 lenses, (F6.7 when TC attached) BUT that is the real limit.

Pentax AF regardless of camera etc, simply won;t focus reliable below F6.7. I believe most other makers are the same.

Sure you cn get lucky in bright sunlight, but that is about it.

11-11-2008, 05:23 AM   #5
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Yep. Generally it's recommended to have at least an f/2.8 lens to use a teleconverter. But in practice, it works pretty well down to f/4. At f/5.6 your getting pretty iffy performance.

A 1.5X teleconverter will loose 1 f-stop, a 1.7X teleconverter will loose 1.5 f-stops, and a 2X teleconverter will loose 2 f stops.

F-number - Wikipedia, the free encyclopedia

At 55mm on the kit lens with a 2X teleconverter, you're asking the auto focus system to operate at f/11.

It's just not going to do that.
11-18-2008, 07:03 AM   #6
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Original Poster
Thanks all for the info.
Guess I'll have to stick to bright sunny days, or better yet, just give it back to my friend.
Thanks again.
11-18-2008, 01:52 PM   #7
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QuoteOriginally posted by stgmgr Quote
Thanks all for the info.
Guess I'll have to stick to bright sunny days, or better yet, just give it back to my friend.
Don't forget manual focus!
11-18-2008, 07:17 PM   #8
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I have always seen there are 2 variants of 1.4x Tamron TC. One is i think MC4 and the other MC7. What are their differences? The MC7 one is more expensive and bigger though.

11-18-2008, 09:23 PM   #9
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QuoteOriginally posted by raider Quote
I have always seen there are 2 variants of 1.4x Tamron TC. One is i think MC4 and the other MC7. What are their differences? The MC7 one is more expensive and bigger though.
The MC7 is a seven element converter, the MC4 is a four element converter. The quality is very much better with the 7 element design. That's where the higher price comes in. There are more elements to grind and match up.
11-20-2008, 01:38 AM   #10
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Thanks Canada Rockies for the explanation.
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