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 Post subject: Lens decentering
PostPosted: Fri Nov 07, 2008 12:23 am 
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While looking at the Sigma AF 150mm 2.8 EX DG APO HSM IF macro review the potential issue of decentering came up.

Is the impact of decentering the loss of detail particularly in a corner? In the photozone review sample image of the industrial stairs, the bottom left is softer than the rest of the image. Is that it? While I consider myself a pixel peeper I didn't rate that as significant on a first glance, maybe as I pay more attention to center sharpness and have treated corner softness as a relatively minor concern.

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 Post subject:
PostPosted: Fri Nov 07, 2008 10:17 pm 
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The lower left corner of this lens surely shows some weakness (even at f/5.6!) and yes decentering shows esp. in the corners: some of the corners are better than the others.

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PostPosted: Fri Nov 07, 2008 10:39 pm 
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Thanks. I just wanted to be sure as I was expecting a bigger impact.

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Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
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 Post subject:
PostPosted: Fri Nov 07, 2008 10:42 pm 
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"Decentering" can go from 5% to well over 30% meaning that the resolution of the worst corner is 30% less than the resolution of the best corner. I don't know and Klaus Schroiff didn't write how much of a decentering defect his copy of this lens for the Canon mount had. So you really don't know how much of an effect you see here. I only emphasized the f/5.6 because stopped down a bit each decentering defect should show less and less. And you can see it stillquite clearly in the example image. But the next copy of this lens might be perfectly ok :idea:

It all depends of the quality control (QC) of the manufacturer - and we all know that some are more efficient at QC than others :?

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 Post subject:
PostPosted: Fri Nov 07, 2008 10:55 pm 
While it would be nice to get super high quality lenses which a uniformly sharp throughout the scene, I don't think decentering is that big a deal. You want images to be sharp at the centre. If you want sharp corners, you stop down. That's true of pretty much any lens out there.

People who diss Tamron and Sigma for poor QC, do they expect Zeiss quality at 1/3 Zeiss prices?


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PostPosted: Fri Nov 07, 2008 11:17 pm 
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To me I'm after "good enough" at the best price, not to be confused with "the best for a given price".

The photozone sample sounds like a one-off so I'm not worried about buying the Sigma. Actually I'm considering several Sigma lenses for the future. As great as many Canon L lenses are, some are getting rather old and could use a refresh. In this specific case, I don't see the value in paying for the Canon 180mm f/3.5L when the Sigma 150mm f/2.8 seems to be as good for less than half the price. Even Zeiss are not perfect. My 16-80 has a mild case of the AF issues when wide and close.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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 Post subject:
PostPosted: Fri Nov 07, 2008 11:23 pm 
popo wrote:
To me I'm after "good enough" at the best price, not to be confused with "the best for a given price".


I know that, as I noted that you're surprised that this "big" issue wasn't as big as you had originally thought.

I'm just saying that reviewers will always find something to complain about.


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 Post subject:
PostPosted: Fri Nov 07, 2008 11:24 pm 
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Decentering has nothing to do with the quality of the design and the material that goes into the lens. It's plain simply: They didn't assemble it right.
So a lens from Sigma (or Tamron btw.) can be very good if they just learn how to assemble their lenses with less junk getting off the line.
And I'm certainly not worried about corners being softer than the centre. But if you find copies of the same lens with clearly better corners than other copies you know that these guys are only sloppy in assembling their gear. And that is really infuriating :evil:

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