What is "decentering" on a lens?


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dRebelXT

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May 14, 2005
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#1
http://www.photozone.de/8Reviews/lenses/sigma_150_28/
It is a little tricky to find a verdict for the Sigma AF 150mm f/2.8 APO EX DG macro HSM - as mentioned above the sample lens suffered from a rather extreme degree of decentering which pollutes the resolution potential of the lens (on the left image side) - a rather worrysome quality control issue.
Anybody managed to download the four sample images on this website? I only get a Excess traffic notice out of it. :sweat:

TIA
 

sulhan

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May 11, 2002
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#2
Hiee Rebel....

Decentering imay be a problem when lens elements or groups are not optimised in its alignement with any of its elements/group to a sweet center spot to give its best center and corner-corner resolution.

In most lens....there may be an adjustable group/element or both, which is adjusted in its final stages and "glued/locked in place" at the manufacturing line....

If any of these processes drifts then you could expect certain corner softnest even if the center resolution is super...

In lens glass making usually after grinding of lens....there will bea centering process to "final touch" the lens glass to make sure that the lens edge is on the right place with respect to its desired lens center....if this process drifts then you gonna have problems like corner resolutions...

In lens that implment internal focus that that does not have rotating elements in the lens glass line up.....versus the older type of rotating front elements lens....
The internal focus lens which does not have its elements moving may have permamant degradation may be seen at a particular corner...unlike the fornt rotating lens that may have the location of corner defect (if any) moving and non permanent to a corner...

Hope this helps....
 

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