[WinniePooh]

Hi there,

usually, I'm a big fan of primes, calling the 43 ltd, the Cosina 100 and a rare Sigma Super Wide II 24mm AF my own. However, my kids are fast, and I usually lack behind. Once I'm ready to shot, they're long gone. This is mostly because of the old primes with slow AF, but partly also of the fixed focal length (not very surprising with primes, I know)

I'm considering buying a standard zoom. Besides IQ, the AF speed is essential for me. Currently, I narrowed down to the following candidates:
- Pentax 18-135 WR
- Sigma 17-70 2.8-4.0 Contemporary
- Tamron 17-50 2.8
- Possibly Sigma 17-50 2.8, but might be a little too heavy!

My problems on deciding this:

1. The Pentax 18-135 is said to be the quickest AF lens. But from what I read I got the impression that I'd have to compromise too much in IQ (it being a super zoom).
2. Here in the forums, the review of the Sigma 17-70 Contemporary was pretty enthusiastic, but didn't say much about the AF speed. Optical quality wise, I also got the feeling from the test results that it outperforms the Pentax 18-135.
3. The Tamron is said to have superb IQ, but I'm not sure how it performs AF wise when compared to the new Pentax and Sigma 17-70 C. However, the constant 2.8 is tempting...
4. The Sigma 17-50 seems to be comparable to the Tamron, but it would probably be a little heavy. I'm used to the small primes :-).

I'd be glad to get some comments from people who can compare any of the lenses.


Cheers,
Winnie Pooh

[jimr-pdx]

If you stop using the 18-135 where the other lenses stop, the issues won't be at all apparent; it's past 100mm or so that sharpness and CA are hard to avoid. Plus WR, plus silent focus... it's an excellent 18-80mm lens that can stretch a bit with compromises. In good light the 18-55 old-kit lens focuses mighty fast when I use it (but louder).

- and oh yes: Welcome to the Forums!

[twitch]

DA 18-135 is the fastest for AF but it is an outdoor lens. If you want a standard zoom for use indoors I'd pick one of your other options.

[Adam]

The af speed of the 17-70 is almost as good as that if the 18-135, if not the same in practice. If you want a fast focusing high performance lens I'd go with the sigma.

Adam
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[crewl1]

I use a Tamron 28-75 2.8 for indoor sports due to its fast AF and low light abilities. It has excellent image quality.

Outdoors I use the 18-135 since there is more light available and the greater range comes in handy.

I had a Tamron 17-50 previously but it would BF or FF all the time so I didn't keep it long.

[Vortexian]

I can confirm that 18-135 has a great focusing speed and precision on both K20d and K3. I've tested it for a week, but it's wide angle IQ was awful (extreme lateral coma).


What's about Tamron 17-50 - it's great for action when it has an internal motor (Nikon, Canon).
But Pentax version has got very low screwdrive-to-focusing_ring ratio, so it takes over a second to throw it's focusing ring from 0,24m to infinity. Also there are lots of samples with a very complicated Back-Front-focus pattern out of the box (mine had following AF-fine-tune numbers: over -10 at 17mm, +2 at 35mm and -4 at 50mm - all corrected at the service centre). It's a good lens when the image quality and les compactness is your priority - it's very sarp and contrasty, but in terms of AF it's very slow.

Try Sigma 18-50/2.8 - I think it should be fine in both IQ and AF.

[WinniePooh]

Wow, thanks for the many replies! First, I forgot to mention that I've a K-30, if that matters.

From the replies, I think I have to decide between the Sigma 17-70 and the 18-135. Does anyone has the f-stops for the focal ranges at hand? Does it compare to the 2.8-4.0?

The Tamron seems to have not the most reliable AF, and the Sigma 17-50 is too heavy for my taste.

Btw, I've the Kit II 18-55 and don't like it very much. It is quicker in focussing than my primes, yes. But somehow the images look dull compared to the others. This might be unfair, cause the primes are known to be remarkable in IQ, but it's my feeling still. I don't know if it is due to lack of sharpness or contrast or so so bokeh, but I don't like using it...

[WinniePooh]

Ah, now I got to ask about the Sigma 17-50 / 2.8 again. How would that compare to the Pentax 18-135 and the SIgma 17-70 2.8-4.0 C? AF wise?? I might be tempted by the constant 2.8