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06-03-2009, 07:47 AM   #1
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Tamron 17-50 and Sigma 18-50

On my side of the world the Pentax has only 1 year warranty and prohibitively expensive compared to these 2.

The main difference is the Sigma is 72mm diameter so does it help better in low light?

any users of both can share?

06-03-2009, 08:07 AM   #2
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The filter size (72mm sigma vs 67mm tamron) does not affect how the lens performs in low light.
06-03-2009, 08:14 AM   #3
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Why not?
I tend to believe that bigger filter size (actually, bigger front element) can "catch" more light in a given time...
06-03-2009, 08:51 AM   #4
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As far as quantity of light transmitted, 50mm @ f/2.8 is 50mm @ f/2.8 regardless of the size of the front element.

06-03-2009, 08:59 AM   #5
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And in other aspects? any differences between different front elements' size?
06-03-2009, 09:13 AM   #6
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The cost of filters.

Seriously, I think it would be spurious to make any qualitative generalizations about lenses based solely upon the size of the front element.
06-03-2009, 12:31 PM   #7
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Well, light absorption by the glass elements actually can make a difference. In theory, a lens with the same f-stop will let in the same amount of light, but you'll find that different lenses will yield slightly different exposures at the same setting. This is due to inherent light absorption by the glass, so in cinematography, they use T-stops instead of f-stops as a measure of light that is transmitted through the lens.
06-03-2009, 03:30 PM   #8
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True, but isn't some of that at least partly to be attributed to the properties of the glass used itself? And the number of groups/elements? and the coatings used?

That's why I stated what I did above. You can't just say a lens with a 68mm front element is going to have better/worse transmittance characteristics than one with a 59mm element. There are many other variables involved than mere diameter.

06-03-2009, 04:50 PM   #9
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Right, in general the difference between the exposures for either lens aren't great enough to take into consideration. I would rather consider size/weight, price, 17mm vs. 18mm, close focusing distance/max magnification, etc.
06-03-2009, 07:36 PM   #10
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Just a couple other things to consider; one objective the other subjective.

- The Tamron comes with a six year warranty and I know from experience no questions and fast turn around.
- The Tamron is occasionally compared favorably against the DA*16-50; I have never seen the same with the Sigma.
06-03-2009, 08:09 PM   #11
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Take a look at this

My thought on the Sigma 18-50mm F2.8 EX DC MACRO - ClubSNAP Photography Forums
06-04-2009, 09:22 AM   #12
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I have only the Tamron, not the Sigma, and I've only had the Tamron for a month or so, but I will share a few initial impressions:

It's definitely sharp as hell. Even at f/2.8, it's quite good, and just gets better from there as you stop down. I find that it produces nice colors, and good contrast, too. AF is pretty quick and accurate, though it could be quieter. I'm very pleased with the lens so far, though I have a few small gripes. First, the lens tends to underexpose in my opinion. Setting EV Comp. to +0.5 corrects it for most shots, but occasionally a full +1.0 is required. I can accept this in exchange for the lens's other good qualities, though. Also, the lens hood is kind of cheesy. It hasn't happened yet, but I always feel like I'm going to crush it with my vice-like grip whenever I attach or detach it. Pentax definitely makes the best hoods for their lenses. Lastly, my copy of the lens does have a slight front-focus issue. I have corrected for it in the K20D's Custom menu and all is well, but it is worth mentioning. Despite my few niggles, I would still heartily recommend this lens. I considered the Sigma, too, and would have exchanged the Tamron for it, if the Tamron didn't work out. I only chose to try the Tamron first because I already had some 67 mm filters, so it was going to be more convenient if the Tamron worked for me, and it has.
06-04-2009, 09:43 PM   #13
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QuoteOriginally posted by Venturi Quote
Just a couple other things to consider; one objective the other subjective.

- The Tamron comes with a six year warranty and I know from experience no questions and fast turn around.
- The Tamron is occasionally compared favorably against the DA*16-50; I have never seen the same with the Sigma.
In your opinion. Tamron or Pentax?
06-04-2009, 10:08 PM   #14
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I have been shooting with the Tamron 17-50 & k20 for a little over a year now. I like the lens a lot & much of what is said above I agree with. I have not had to deal with Tamron's service, but do like knowing I have a 6-year warranty with the lens.

I have not shot with the Da 16-50 or the Sigma 18-50, but did a lot of research beofre I bought the Tammy. I have to believe you would be happy with any of lenses discussed here.
06-04-2009, 10:13 PM   #15
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QuoteOriginally posted by res3567 Quote
In your opinion. Tamron or Pentax?
For me there's no question; the Tamron. My choice though is colored by the fact I don't much care for how Sigma lenses "feel" in my hands. Hate the powdercoat finish they use and I just don't care for how they "handle". I know it may seem trivial but if a camera or lens doesn't feel right in my hands I'm not going to be effective with it regardless of how good it can perform.
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