Focusing is slow and inaccurate

[astro]

I've been mostly using manual focus lenses up until I recently purchased my first semi-pro AF zoom lens, a Sigma 28-70mm F/2.8 EX from ebay for $180 new.
The AF speed is horrendously slow, and almost impossible to use in low light situations. I find that it hunts for the right focus a lot, even where there is a lot of light. It misses the focusing quite a bit. This doesn't seem to happen as much with the kit lens, but the focusing is still much slower than my Panasonic FZ15 superzoom P&S. I find that I can focus much faster with manual focus, not to mention having a lot more control too.
Anyone here with the same experiences as I?

[coldrain]

Focussing systems on slr's can be a bit odd, it is a collaboration between camera body and lens. With Canon and Konica Minolta bodies you can see this too, that sometimes a 3rd party lens in combination with the camera has some trouble detrmining when something is best in focus.

I can not be sure, but I bet you will see much better AF performance with a Pentax AF zoom lens (and probably with a Tamron 28-75 SP Di f2.8).

[WightWalker]

I've not experienced any undue focus issues with the kit lens or a Sigma 28-135mm that I have when using the *istDS under 'normal' conditions.

Indoors under low light it performs resonably OK but there are times when it's a tad slow or 'hunts'.

It was suggested that v2 of the SW improved the auto focus features; if you haven't done so, I would suggest try that to see if it makes a difference.

[astro]

Thanks for the replies.
I guess I'm just used to using manual focus and getting the focus I want every time and having the ability to retake shots immediately without waiting for the lens to try to refocus again.
I'm using v.2 firmware.

[Ishkabibble]

This sounds similar to my issue with the old firmware. I was getting slow AF performance after time. A manual reset of the camera corected it for a short time, and a re-flash of the firmware solved it for good. My problem was only pronounced in low-light situations.

[Rhys]

I've been mostly using manual focus lenses up until I recently purchased my first semi-pro AF zoom lens, a Sigma 28-70mm F/2.8 EX from ebay for $180 new.
The AF speed is horrendously slow, and almost impossible to use in low light situations. I find that it hunts for the right focus a lot, even where there is a lot of light. It misses the focusing quite a bit. This doesn't seem to happen as much with the kit lens, but the focusing is still much slower than my Panasonic FZ15 superzoom P&S. I find that I can focus much faster with manual focus, not to mention having a lot more control too.
Anyone here with the same experiences as I?

Lol. I have a Canon XT. I find that focussing can be a little strange too. Personally, although I have used AF for about 5 years now, I must admit that I prefer manual focus. I am somewhat upset that my XT does not have a focussing screen as I too find manual focus is a lot more accurate. As for the arguments about focus trackng - well phooey to them. Moving subjects are usually at infinity so focu tracking does nothing useful.

You're lucky in having a Pentax as you have the choice of aF or MF. Most Pentax DSLR user that I've seen have used MF out of preferance. If my XT had a focussing screen then I'd use that and wouldn't waste my time with AF, More than that - I'd get the focus in the right place every time!

[jeisner]

Welcome to the world of Sigma zoom lenses... They are well known for inaccurate focus (and not just on Pentax, not by a long shot)...

I am very happy with my Sigma DG EX primes, they focus fast and accurate, but I have sold my Sigma zooms (all except the 135-400 which will go when the new pentax high performance zoom due 2006 is released), they are just too prone to backfocus issues at wider apertures...

[speaklightly]

Well said Jeisner! Run of the mill Sigma lenses seem quite variable and sometimes are not good "focusers."
Personally, I have sold off all my Sigma "ordinary" lenses, and I have re-focused on Pentax brand lenses which seem to provide me with much better quality.

Speaklightly

[astro]

Ah too bad a Pentax *FA 28-70 2.8 will cost about 5 times more than what I paid for the sigma
I think I might go back to carrying my 35/2 and my 50/1.4

[jeisner]

Ah too bad a Pentax *FA 28-70 2.8 will cost about 5 times more than what I paid for the sigma
I think I might go back to carrying my 35/2 and my 50/1.4

I feel your pain Astro, I would love to be able to afford some * glass...

Normally I carry my primes with me like you have suggested when I need low light or shallow DOF, and for zooms I can recommend the Pentax DA 16-45 f4 and the Pentax FA 28-105 f3.2-4.5, they are not super bright but when supplimented with some good primes they are a very nice combo... The 24-90 3.5-4.5 would be an even better quality choice than the 28-105 3.2-4.5 but about double the price, not really worth it for me, but it would be almost on par with the near prime quality 16-45/4.

[astro]

I personally can't stand slow lens. I do over half my shooting indoors. I don't think I can get myself to spend over $200 on a lens yet, unless the lens works wonders(maybe like a 200mm F/2.5 or a 14mm F/2.8 prime).
Yeah I heard lots of good things about the 24-90 and the 28-105. I might get one if I can find one for cheap on ebay.
I got a 35-80 FA zoom before. I never used it once except to test it.
This sigma zoom is probably the first zoom I feel is good enough that I would use it over my primes, but the AF is just a pain in the arse to use.
You could have the best shutter lag in the world. But with a painful AF system, you'll have the worst shutter lag in the world.

[Rhys]

Well said Jeisner! Run of the mill Sigma lenses seem quite variable and sometimes are not good "focusers."
Personally, I have sold off all my Sigma "ordinary" lenses, and I have re-focused on Pentax brand lenses which seem to provide me with much better quality.

Speaklightly

Tamron lenses (which I have) are pretty good.

[speaklightly]

Rhys-

I agree with you. Tamron lenses are indeed a step up from Sigma.

Speaklightly

[Rhys]

Rhys-

I agree with you. Tamron lenses are indeed a step up from Sigma.

Speaklightly
How would you rate the Sigma f2.8 70-200?

[speaklightly]

Photozone lists the Sigma 70-200 EX (HSM) as "very good."

Speaklightly

[Rhys]

Photozone lists the Sigma 70-200 EX (HSM) as "very good."

Speaklightly

Have you tried one? I know they're alleged to be very good.

[astro]

Tamron lenses (which I have) are pretty good.
Yeah I know the Tamron version of my lens, 28-75? is better than the sigma. But the Sigma only costed me $180. Plus it's an EX lens, so I don't think it's an ordinary lens, but rather Sigma's high end lens. Shouldn't it be on the same class as the 70-200 EX?