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Old Aug 6, 2010, 12:10 PM   #21
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To add to one of Scott's comments - I have the DA 55-300, A*300 and DA*300. There's not a huge amount of difference between the A*300 and DA*300 (the DA is very slightly better, but that tends to reflect which one got the best focus and best technique because the DA is a lot heavier so the A is easier for me to shoot). Even though I have a good copy of the DA 55-300 (my second one, first one returned under warranty and replaced), it doesn't compare at all at 300 to the two primes. It's quite nice and seems to be the best of the 300 max zoom options around, but there's a very noticeable difference. As Scott said, some people wouldn't think the differences worth the extra cost of the primes, but it is for me when I want the best quality or are planning on cropping a lot. On the other hand, it's nice to have a light-weight 300 lens for casual stuff.
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Old Aug 7, 2010, 6:40 PM   #22
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Has anyone used the Sigma APO Teleconverter 1.4x EX DG for Pentax with the Tamron 18-250? I understand the loss of the stops, but how is the image quality and does it lose AF? I have read several opinions outside this forum that differ.
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Old Aug 7, 2010, 6:47 PM   #23
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Actually I do not think the tamron 18-250mm will not Auto Focus on the long end with the TC. It may be to dark for the AF system.
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Old Aug 7, 2010, 7:24 PM   #24
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From what I understand the APO TC only works with specific Sigma lenses. Not sure that the Tamron would work because of the part of the TC that extends beyond the lens mount. Not to mention the AF problems because the lens is slow at its longest.
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Old Aug 8, 2010, 9:11 AM   #25
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Thanks for the replies. I did some more research and found this chart www.sigma-photo.co.jp/english/acc/condaitr.htm at the Sigma webiste. When Sigma advertises the TC for the Pentax mount, or Canon, Nikon, etc., they mean select lenses made by those companies, not third party lenses for the mounts. In addition there a lot of exclusions for functions, mode restrictions and special instructions for use.
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Old Aug 8, 2010, 2:28 PM   #26
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Hi pboerger,

I've used my pre-DG Sigma EX AF 1.4x APO TC with a lot of lenses other than those listed on their compatibility chart (which only lists Sigma branded lenses, of course -- probably more of a marketing/possible liability issue rather than a "dedicated to their lenses only" one). The only lenses that I use it with have recessed rear lens elements, and that is really the only limitation that I've seen -- the protruding front elements in both of their TCs. The FA* 300 f4.5 and 2.8 both work well with it alone. I also use it stacked on the P F 1.7x AFA, (AFA on the body, Sigma 1.4x on the FA*300/2.8). This is my preferred 1.4x with this lens even though I've found the Tamron 1.4x PZ AF MC4 to give marginally sharper images -- the Sigma seems so much more robust in build that I trust the big lens on it more than the Tamron.

It also should be mentioned that no Sigma AF TCs have the PZ-type electrical contacts, so they have no chance of working as an AF TC with any SDM or HSM lens. They would be MF only.

The weird thing is that the Tamron is better when using an AF 1.4x stacked with the AFA with the Sigma EX 300 f2.8 APO lens. In MF mode, with the Sigma TC attached, my Sigma 300 doesn't disengage the screw drive for some reason when used like this, so there's extra drag on the focusing ring.

One of the main factors to consider with TC use is the speed of the lens and the limitations of the AF system. Pentax's SAFOX VIII system, especially before the K-7 and Kx, have a practical limit of about f8 in effective max aperture for reasonably reliable AF functionality. The current models are slightly more sensitive, but f8 is still a good Rule of Thumb to use. With rear mount TCs, you need to multiply the max aperture of your lens by the magnification factor of the TC to get the effective max aperture of the combo. Doing this, you get a 1 stop difference for 1.4x, 1.5 stop difference for 1.7x, and 2 stop difference for 2x TCs. The lens' sharpness and contrast wide open (where the AF system works) can also be factors in AF performance and accuracy when you're pushing the limits of the AF system with TCs.

The DA 18-250, at f6.3 at the long end, with a 1.4x TC would give you an effective max aperture of f8.82, which might have a chance to AF in the brightest sunlight (like at noon on June 21 on a clear day at most any location other than the Equator) with the newest models, but would be iffy at best in anything less. The added lens speed headroom is one reason why I suggested faster prime teles for their ability to use TCs in previous posts on this thread.

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Old Aug 8, 2010, 4:10 PM   #27
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Thanks for a very complete answer. Amazingly, after several read throughs it made sense. Sounds to me like I should save me money for a bigger lens rather than the compromises needed to go cheap.
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