For Sigma 18-50 f2.8 EX DC Owners

This might be of interest to you, after much testing on my copy:

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Hmm ?

I *always* use the parfocal with that lens, whats new ? I don't expect the AF system to work massively well at 18, because well, my eye doesn't see lots of small details in the viewfinder anyway !

In fact, I wouldn't be surprised this works also very well on other wide-to-normal zooms, like the 17-40 etc.
I wonder what the fuss is about tho, I mentionned that to the FM forum a couple weeks ago, and people are now discovering that that lens has a "focusing problem" when they never did before

How do I know if my lens is parfocal? Is the Sigma 24-70 EX parfocal?

Zoom to 70mm, half press, zoom out to 24mm, shoot.

If the result is sharp, you'll know

I'm sorry, what is "parfocal"?

A zoom that keeps it's focus point independentaly of it's focale position. Neat trick

Like buze, I instinctively parfocal unless I can see something with good contrast to let the lens focus on. In fact, I'd have thought that this is more body related, than lens related, since it's the body that does the focussing - would be interesting to see how different bodies perform with the same lens.

Thanks for that fstopJoJo - your samples on PBase were quite a big player in driving me towards the 18-50 EX

Thanks Buze. I googled "Parfocal technique" but i couldn't find any helpful step-by-step instruction. Do u know any website that explains this method in detail by any chance? Sorry for the annoying question.

CFC, it's really quite simple. Especially in low contrast scenes (where the AF will struggle to lock focus), you zoom all the way in to your longest FL (so on the 18-50 it's 50mm), half press your shutter to lock focus, and while keeping the shutter half-pressed, you zoom back out to 18mm, then fire. Note that exposure can change in any mode but manual (I'm almost always in either manual or Av). Just play around and you'll see it's quite easy and fast.

Goatee, you're quite welcome. I'm actually in the process of corresponding with both Sigma USA and Sigma Japan as they are assisting me with a fix for this issue, although it really isn't all that big a deal. I just got the Canon 17-85IS yesterday and have posted some preliminary findings at my site. I'd like to keep both lenses. We'll see. In short, I find that the 18-50EX is better optically but the Canon's focus and USM is much better.

Thanks fStop! Very interesting method. I'll try it with my lens.