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12-09-2011, 04:59 AM   #1
Zafar Iqbal's Avatar

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Sigma focusing performances

Continued from this thread. It's getting offtopic there and I'm not getting the attention needed, possibly because of the thread title.

I stated my impression was, that (my) Sigma lenses focus locks slower than for instance my kit lenses under low light conditions (or in the shade outside during a bright sunny day). I went though with the purchase anyway of a new Sigma (17-50 f2.8) and I doesn't look like it's any better. But all this was based on conviction - and it had been a while since I had used the kit lenses. I did some tests to refresh my memory and it doesn't look like I was in the wrong.

I miss some explanation on this one. Someone must know whats going on here/what I'm writing about.

12-09-2011, 08:01 AM   #2
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I didn't read your previous thread but there is no mystery about some lenses being faster to focus than others. You could have a dud lens but it's more likely that your expectations may be a bit high. Lens focusing speed is one of the things that are reported on in lens tests.

The body you are using will no doubt have an impact here too. Reputable camera tests specify which lens was used to measure focusing speed.

I have no experience of the Sigma 17-50 but I know the kit lens is very quick to focus on the KR. It seems to be pretty close to the same speed as the DA 16-50 on my K7.
12-09-2011, 04:20 PM   #3
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If it was just a single lens then I'd probably accept that for what it is, but it's not.

So far it seems my 18-55 and 50-200 kit lenses locks the focus in low light quicker than Sigma 17-50 f2.8, 28-70 f2.8, 70-200 f2.8 and possibly also the 70-300 f4-5.6.

And although I've only had my 35mm Macro Limited for few months, I think the locking performance of this one isn't much different from the kit lenses.

So, it looks more and more like a case of Pentax vs Sigma and not just single lens vs another and/or what body was used. The cam body was the same for all these lenses.

The HSM featured lenses definitely has a faster motor drive. But it's not very useful if I also can't get the lock quick enough. I can get it faster with the Pentax lenses so why not with the Sigmas?

The Sigma 17-50 costs exactly 10 times more than the DA L here in Denmark. I did not expect it to be 10 times better, but I did expect it to be better in all areas - and it is, except for this focus locking thing when light starts to get dim. If my DA L can get a lock, then I expect the Sigma to do the same and at least as fast as the DA L.

I recently read that wider apertures could work against AF (I have no idea how true this is), but I had already done tests with the aperture stopped down and that didn't improve the scores.

You could say the AF system in the K-x is simply not good enough to properly use the Sigmas to their fullest. It could make some sense I suppose. right now I'm thinking that the Sigmas a better on paper and so should they be in practice as well.
12-11-2011, 10:13 AM   #4
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Third party lenses have to reverse engineer their firmware for different manufacturers, so it makes sense that a camera maker's own lenses will have an advantage, especially in low light, though I have no problems with my Tamron 17-50. The only Sigma I have is the 85mm 1.4, which focus quickly on my K-5. Both have excellent optics.

12-11-2011, 12:50 PM   #5
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That makes some sense I suppose but I had hoped more would join in with their experiences to get this thing a bit more sorted out.

I do not think anything is faulty here but I'm still surprised. I own quiet a few lenses now and always read review and user comments and haven't noticed something like this being mentioned.

Last edited by Zafar Iqbal; 12-11-2011 at 02:40 PM.

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