[Joojoo]

Does anybody know if the Sigma 18-50mm f/2.8 EX DC Macro is compatible with the Sigma 1.4x teleconverter?

[Edgar_in_Indy]

I would take either of the Sigmas (18-50mm if you're on a budget, 17-50mm if you have more to spend and want the best) over the Tamron 17-50mm.

I bought a Tamron 17-50mm f/2.8 new a while back, but ended up returning it because in reality it performed like a f/3.3 lens. When compared to my other f/2.8 lenses, the Tamron 17-50mm underexposed by half a stop at f/2.8, and by a full stop at f10. I don't think the aperture was opening as wide as it should. I started a thread about it, with sample pics, and other people reported similar results when comparing their Tamron 17-50mm to their other lenses.

Here's the thread if you want to check it out:

tamron-17-50mm-f2-8-not-fast-bright-tamron-28-75mm-f2-8-pentax-k-x-pics

Now from everything I've heard and experienced myself, the Tamron 17-50mm is a great lens for the price. But just be aware that there may be this aperture issue.

[ChopperCharles]

Big question: Does the Sigma have any backfocus issues? I can't use my Tamron 28-75 on anything except my K-01, because the back focus is not correctable. (It's different at 28 than it is at 75, and there isn't enough correction in my K-30 to adjust it all out at 75 anyhow)

I love the lens, but it's just not useful on my K-30 unless I shoot in live view.

Charles.

[Edgar_in_Indy]

Big question: Does the Sigma have any backfocus issues?

I don't own either of the lenses, but I have not heard of any rampant back-focus problem. If you look at my signature, you will see that I have a lot of other Sigma lenses, and I have not noticed any of them to be prone to back or front focusing.

I have, however, taken advantage of Sigma's free calibration service to have all of my Sigma's professionally calibrated to my camera body. You're without your camera for a few days, but I think its worth it for the peace of mind of knowing that your lens is perfectly calibrated.

Of course most Pentax bodies have user-adjustable focus calibration, but it's not very precise since you only have several clicks in either direction for adjustment. And when you're talking about lenses where the depth of field came be extremely thin, such as the 85mm 1.4 and 100-300mm f4, even just a tiny misadjustment can result in loss of maximum sharpness. And one thing Sigma is good at is making sharp lenses, so I want to make sure I'm not leaving any pixels on the table!

The fact that Sigma offers this service for free is a big plus in my opinion since other lens makers, such as Canon, charge quite a bit for calibration.

All that being said, an APS-C f/2.8 standard zoom has more tolerance (room for error in adjustment) than something like an 85mm 1.4, so I would certainly try it out first and attempt to make adjustments yourself before sending it off. I think that most people are able to self-calibrate such lenses to their satisfaction.

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Last edited by Edgar_in_Indy; 12-13-2013 at 11:46 AM.

[Pentaxguy22]

I say it's worth it, I use to have one, but now I use my 10-24mm Tamron more than it so I sold it, but for low light levels I say yes the Sigma is a win.