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11-11-2009, 04:36 AM   #1
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Sigma 18-200 or Pentax 18-250 good for K-x?

I'm considering getting a new K-x with an all-in-one lens (Sigma 18-200 f/3.5-6.3 or Pentax 18-250 f/3.5-6.3). These lenses are not really "fast", but they are convenient for use by the family. I need to shoot in low light (e.g. school concert, indoor sports) from time to time, so I'm hoping that the K-x excellent high ISO performance can offset the disadvantages of the small aperture.

I've read some reviews of both lenses but I'm interested to hear your particular experience/view regarding the use of these lenses on the K-x body:

1. Autofocus performance/speed and noise
2. Shake reduction effectiveness
3. Handling and ease-of-use

Also, the Pentax 18-250 is A$280 (~US$260) dearer that the Sigma 18-200. Is the 50mm extra zoom worth the extra cost, or is Pentax 18-250 image quality much better than Sigma's?

11-11-2009, 05:40 AM   #2
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Hi Lenchan,

Welcome to the forum.

I've a KX, and used to have Tamron 18-250 (which is the same as the Pentax one).
I've not use the 18-250 on Kx, as I gave my lens away to my bro, but I've used heavier lens on Kx. So from that point of view, I should be able to share my experience.

As Kx is so small and light, it feels a bit more balance if you use a hand grip to shoot with bigger lens.

1. AF is fast for my Tamron. Noise is subjective, but I'm ok with it.
2. SR is very effective with KX.
3. You can use one hand operation for Kx.

If you can get the Tamron version, I think it is cheaper. The image quality of Tamron/Pentax is better than the Sigma. Lots of reviews have proof it.

I've shot my trip last yr to Tassie mainly using the Tamron 18-250, and I was very happy with the pic.
11-11-2009, 06:00 AM   #3
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I am also considering a vacation zoom but my consideration is between the tamron 18-250 and the Sigma 18-250 OS. I read that the OS feature on the Sigma + the SR on the Pentax body is a pretty good combo. Also it would appear the Sigma has almost the same IQ as the Tammy. Perhaps some other ppl with this Sigma lens can share better.
11-11-2009, 06:12 AM   #4
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QuoteOriginally posted by raider Quote
I am also considering a vacation zoom but my consideration is between the tamron 18-250 and the Sigma 18-250 OS. I read that the OS feature on the Sigma + the SR on the Pentax body is a pretty good combo. Also it would appear the Sigma has almost the same IQ as the Tammy. Perhaps some other ppl with this Sigma lens can share better.
As far as I know Sigma doesn't include OS in Pentax mount lenses. If you use shake reduction in the lens and the camera, they cancel each other out. It's equivalent to using no shake reduction. I saw this graphically demonstrated in a video with an Olympus system.

To the OP, you're going to get improved high ISO performance with a K-x compared to most cameras, but I still don't see a superzoom turning into a low light lens for the kinds of use you intend. Shake reduction helps for still photos, but it won't freeze motion. A K-x has ISO 12800 but have you seen examples? Pretty fugly.

I agree with the post above, the Pentax/Tamron 18-250 beats the Sigma and Tamron 18-200's as shown in multiple tests. If you can find a Pentax 18-250, you will benefit from the lens correction features in the K-x. Off brands are not corrected.


Last edited by audiobomber; 11-11-2009 at 06:25 AM.
11-11-2009, 06:16 AM   #5
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Hi mcjm78,

Glad to meet you. Thanks for your experience/comments on the Tamron 18-250.

Yes, the Tamron is cheaper than the Pentax if I buy the lens separately, but the Pentax is cheap when bundled with the K-x.

I checked out your Tassie 08 photos, and they are quite nice. The weather was quite good at Cradle Mountain :-)
11-11-2009, 06:29 AM   #6
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Hi audiobomber,

Yes, my understanding is also that lens image stabilisation and body shake reduction cannot be mixed. The Sigma 18-250 OS does come with a Pentax mount version (with OS disabled), but it is quite heavy and bulky compared with the Tamron/Pentax 18-250.

I agree that the Sigma 18-200 or Pentax/Tamron 18-250 would not be a good low light lens, but I'm hoping that it might be ok for some of these events. Based on my calculations using ISO 3200/6400, for venues with better lighting, it might be ok.

I have seen a few ISO 12800 photos, and have run them through NeatImage. I think they are still usable if properly resized. It is perhaps better to get a shot than to miss out under difficult lighting conditions.
11-11-2009, 06:35 AM   #7
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This is what I find on BHPhoto.

Sigma | 18 -250 f/3.5 - 6.3 DC OS HSM Zoom Lens For | 880-109

One of the ppl who bought this lens say in the review section:
"The one of the best things about the lens is the OS. The Pentax K7 has inbuilt image stabilisation, however i find at telephoto lengths (over 150mm) the lens stabilisaion is better by approx 2 stops and really allows you to hand hold at 250mm.


Perhaps the OS is not disabled by default but may be disabled by a switch? I am not sure.
11-11-2009, 07:05 AM   #8
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Hi raider,

I checked out the user review on B&H you referred to, and it seems that the Sigma 18-250 OS for Pentax does have OS! I'm pretty sure that you have to turn off SR in the body though, so you can choose either lens or body stabilisation, but not both.

I found a small discussion thread on dpreview related to this topic: Sigma OS lenses, anyone tried them yet?

11-11-2009, 09:05 AM   #9
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I've had trouble locking focus at the long end using my Tamron 18-250. This is true for both my K10 and K-7.

ISO isn't a help, here. The max 6.3 aperture simply doesn't seem to let in enough light for the focus sensor. I've had to manually focus when shooting large jet black lettering and a black/gold clock face on the shady side of a light grey building on a clear, sunny day. (Royal Courts of Justice, if that info helps. Shooting in Fleet St. about two hours before sunset.)

No problem with a Pentax 55-300 5.8. With quick shift manual focus and a crisper image at the long end, it's a great choice where you don't need 18-55.

If others here report that their 18-250 focuses just as well as their 55-300, perhaps mine should go back to Tamron.
11-11-2009, 10:27 AM   #10
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QuoteOriginally posted by glanglois Quote
If others here report that their 18-250 focuses just as well as their 55-300, perhaps mine should go back to Tamron.
It's a good thing my 55-300 has quick-shift because it does get lost once in a while. In difficult conditions the DA 18-250 seems to get close, then fine-tune a couple of times before it finds focus. In good light, focus on both is acceptable, neither is great.

As a telephoto, the 55-300 is a better lens; half a stop faster, sharper at the long end, and much longer because of the 18-250 design. I'd recommend the 55-300 for what the OP wants, over the 18-250.
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