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10-26-2013, 10:33 AM   #1
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Tamron teleconverters

Hi,
Can someone please let me know which Tamron teleconverter is better 2X AF 4MC or the 7MC? Will they work on the K5 and how is the image quality?
How much can I pay for either of them?

thanks.
TJ

10-26-2013, 11:05 AM   #2
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7MC in theory, but either will degrade image somewhat. On SDM lenses (non-screwdrive), the autofocus will reportedly not work with the Tamron 2x TCs even if it has proper contacts, unlike the Tamron-F AF-Pz 1.4x, which works fine...
10-26-2013, 12:30 PM   #3
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Hi tjwaung,

Personally, I haven't found a 2x TC that gives what I consider acceptable IQ, but I've never tried what is universally considered the best, the Pentax A Rear Converter 2x L (I do have an A RC 2x S, and it doesn't cut it, IMO). I get better results from 2 stacked 1.4x TCs, so if I want 2x, I'd rather used the stacked 1.4xs than a single 2x. The only exception to this is the Tamron 2x Macro Focusing TC when used to shoot macro. This is the only 2x TC that I will use on a regular basis, and only for macro.

That being said, reputation is that the MC7 is better than the MC4, due to the added elements. I would think that you could get either for under $100 USD. This is probably cheaper than you can buy a 1.4x Tamron AF TC, and this should give you an indication of the utility and IQ potential of the 2x.

Realize that Pentax bodies generally need an effective max aperture of faster than f8 to give the AF sensor enough light to focus reliably, exactly how much faster really depends on the lens' sharpness and contrast wide open when combined with the TC's optics, and a 2x TC adds 2 stops to the max aperture of the lens. This means that the lens you are going to use a 2x with must be faster than f4 to have any chance of reliable AF. My definition of reliable AF is that the camera will be able to lock focus as if the TC was not present, taking into account the loss of light. In other words, the TC should not materially effect the AF performance in bright sunlight with a suitably contrasty subject.

Excuse me for going OT -- I have to disagree with vonBaloney about the PZ 1.4x "working fine" with SDM lenses though. My limited experience, confirmed by reports from a friend who has tried multiple copies (of both lenses and TCs) with just about every SDM tele prime or zoom (brandrx on DPR), says that there are no currently available TCs that AF acceptably with SDM lenses. My experience is that SDM lenses will hunt and not lock focus unless the lens is very close to being in focus when the AF is actuated. With my SDM lens and the PZ 1.4x AF MC4, the lens will hunt and not lock focus up to 3 times, getting closer to focus each time, then will finally lock focus on the 3rd or fourth try, when the prefocus is close enough. This was with f2.8 and f4 max aperture lenses. To me, this is unacceptable, but maybe I'm just being to fussy. . .

I'd suggest that if any users have a PZ AF TC that does actually work well with an SDM lens, that they offer to send their lens/TC combo to Pentax for a substantial fee -- perhaps that would help the lens designers discover the secret to making their own SDM compatible TC -- something that apparently has eluded them thus far. . .

Scott
10-26-2013, 12:34 PM   #4
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QuoteOriginally posted by snostorm Quote
Excuse me for going OT -- I have to disagree with vonBaloney about the PZ 1.4x "working fine" with SDM lenses though. My limited experience, confirmed by reports from a friend who has tried multiple copies (of both lenses and TCs) with just about every SDM tele prime or zoom (brandrx on DPR), says that there are no currently available TCs that AF acceptably with SDM lenses. My experience is that SDM lenses will hunt and not lock focus unless the lens is very close to being in focus when the AF is actuated. With my SDM lens and the PZ 1.4x AF MC4, the lens will hunt and not lock focus up to 3 times, getting closer to focus each time, then will finally lock focus on the 3rd or fourth try, when the prefocus is close enough. This was with f2.8 and f4 max aperture lenses. To me, this is unacceptable, but maybe I'm just being to fussy. . .
I don't think that is an SDM problem, just a light loss problem. The point I was making is that the 1.4 technically "works" -- it will focus if it is bright enough. (Although I can't claim to have tested exhaustively, but I have tested, and I've sold several of them and always offered a return/refund if they didn't work with the buyer's lenses and haven't had a complaint yet.) The 1.5x Kenko with contacts also works -- pretty sure it is the exact same TC as the Tamron with different coatings. (Color is noticeably different, but other than that results and performance seem identical.) Whereas the 2x AF-Pz versions apparently (I've never had one, just going by reports) won't even hunt, they won't do anything, like the contacts aren't even there.

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