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Old Jun 4, 2010, 1:15 AM   #1
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Default Sigma 17-70mm or 18-50mm

I was looking forward to buying a new lens that is suitable as a walk around lens for my sony a550.

I've researched a lot on the 17-70mm F2.8-4.5 DC Macro and 18-50mm F2.8 EX DC Macro but I can't still decide which one to get.
The cost is not a matter, they are very close.

From what i've seen on other forums and review are.
1) Both are macro (big plus)
2) The 17-70 does not have a constant aperture of 2.8
3) The 18-50 does have constant aperture of 2.8
4) The 17-70 has the extra 21mm compared to the other.
5) The 18-50 is a EX, which means its a better glass and built quality.

Now I need someone to help me choose.
The options that most interest me are:
- Best Image sharpness
- Least Chromatic and distortion.

Does the 2.8 differer that much on the f-stops? How much are we talking about?
Is the EX built quality that much greater than the non-EX?

Please help me on this decision.
Thanks.
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Old Jun 4, 2010, 1:49 AM   #2
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you will not be gaining much with the 17-70mm vs the kit lens that came with the sony. 15mm and just 1 stop wide open and 1 stop on the long end. With the 18-50 you loose a bit of reach, but have a brighter lens through out the zoom range at 2.8.

The 2.8 will make it a better indoor lens where the lighting is not as good.

Have you consider something with a bit more reach then 70mm as a walk around lens. It is a bit short, something in the 85 to 135mm range would make a better walk around lens.
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Old Jun 4, 2010, 4:14 AM   #3
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you will not be gaining much with the 17-70mm vs the kit lens that came with the sony. 15mm and just 1 stop wide open and 1 stop on the long end. With the 18-50 you loose a bit of reach, but have a brighter lens through out the zoom range at 2.8.

The 2.8 will make it a better indoor lens where the lighting is not as good.

Have you consider something with a bit more reach then 70mm as a walk around lens. It is a bit short, something in the 85 to 135mm range would make a better walk around lens.
Thanks shorturtle.

Well for me the zoom is not so important as the image quality over the kit lens.
I wanted a lens that will be on the camera most of the time, so it has to be at least a wide (18mm or less), but can go up to any reach.
I just don't want to sacrifice image quality over an all in one zoom lens (like 18-250).
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Old Jun 4, 2010, 6:25 AM   #4
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They are both very good lenses. The 17-70 has a 1:2.3 magnification ratio while the 18-50 is 1:3, so calling the 18-50 a "Macro" lens is a bit of a stretch.

Since this would be a "walk around" lens, the extra reach would be a plus. The 17-70 has a little more distortion at the wide end and a little more vignetting. The 18-50 is sharper in the center, but the sharpness falls off quickly, while the 17-70 is sharper in the edges, which would be good for cityscapes.

Also, the 18-50 has a lot more chromatic aberration than the 17-70.

Since you'll be using this as a "walk around" lens, I think the extra reach, the greater edge sharpness, and the lesser CA make the 17-70 a better choice, especially since you'll presumeably be shooting outdoors in good light, so the smaller maximum aperture at the long end won't matter much. But you'll need to stop it down a little at the wide end to avoid the vignetting.

If the "macro" capability isn't significant, you might want to consider the Tamron 17-50/2.8, which is better than either of the Sigmas.
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Old Jun 4, 2010, 6:34 AM   #5
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Note that any of the Sigma 17-70/2.8-4.5, the Sigma 18-50/2.8, or the Tamron 17-50/2.8 would be a noticeable improvement over the Sony 18-55 kit lens.
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Old Jun 4, 2010, 6:39 AM   #6
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Note that any of the Sigma 17-70/2.8-4.5, the Sigma 18-50/2.8, or the Tamron 17-50/2.8 would be a noticeable improvement over the Sony 18-55 kit lens.
Thanks TCav.
Thats what I want, "a better lens than the kit lens".
I'm leaning towards the sigma 17-70 since macro is something I need, so the tamron is out.

What about the newly realeased sigma 17-50mm F2.8 EX DC OS HSM, aparently it has this new glass which is of high quality.
I've looked everywhere for a review, but i think its still not available to purchase.
Do you know anything about it?
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Old Jun 4, 2010, 6:58 AM   #7
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The stabilized version of the 17-70 isn't as good as the unstabilized version. It's not as sharp and has more CA. Plus, it's only 1:2.7 macro compared to the 1:2.3 for the unstabilized version. See:

Sigma 17-70mm f/2.8-4.5 DC Macro (Tested)
Sigma 17-70mm f/2.8-4 DC Macro OS HSM (Tested)
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Old Jun 4, 2010, 9:00 AM   #8
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The stabilized version of the 17-70 isn't as good as the unstabilized version. It's not as sharp and has more CA. Plus, it's only 1:2.7 macro compared to the 1:2.3 for the unstabilized version.
Thanks TCav.
I don't need the stabilized version since I will be using a sony camera.
As you know sony has built in stabilization.

Event thou the stabilized has an greater aperture then the non-stablized, but its only by 0.5.

Ok one thing i don't understand is the HSM motor. Its only for nikon right?
Does that mean that it will use the built in motor from my camera to autofocus? Is it as quick as the HSM and also as quiet?

Last edited by Bruno28; Jun 4, 2010 at 9:03 AM.
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Old Jun 4, 2010, 11:18 AM   #9
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HSM lets the lens focus faster then a conventional DC motor. They make hsm for nikon, canon and pentax also.
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Old Jun 4, 2010, 1:26 PM   #10
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The stabilized 17-70 HSM is for all the mounts; the unstabilized 17-70 HSM is for only the Nikon mount, for the entry level Nikon bodies that don't have their own autofocus motor.
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