Sigma 8-16mm Review (Long with many images)

Started Jun 8, 2010 | Discussions thread
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technic Veteran Member • Posts: 8,932
Re: Sigma 8-16mm Review: Centering and Focusing

Severian The Lame wrote:

Centering and Focusing

  • Two other copies of this lens featured in threads in the Canon lens forum exhibited symptoms of centering problems as did the copy reviewed by photozone.

same story with the Canon 15-85IS. Many copies around with some centering problems and one bad border/corner. I guess other WA zooms with an 'ambitious' design will show similar variation when examined carefully.

  • In the examples below I shot at 8mm at f/4.5, f/8, and f/11.

those are not full size examples? f/4.5 still looks pretty good to me, but difficult to judge at this size.

  • One thing I have found is that (on my copy at least) this problem is focus related. I have shot many landscapes where I don’t notice any issue at all but have virtually identical shots of the same subject where the problem is clear. The only difference I could discern by examining EXIF data and fiddling around with camera and lens settings has to do with the recorded subject distance, i.e., exact position of the focus ring. I would have thought at 8mm and f/8 for subjects mostly at infinity that the DOF would make precise focusing not that critical. But my copy’s defect means that’s not true. Yeah, it’s true for the center, but not for the periphery, especially the upper right corner in landscape orientation. Taking a close look at the lens internals as seen through the front element and referring to Sigma’s published diagram ( ) you can see how the inner focus groups of elements work and how they move back and forth and rotate.

interesting, I have the same problem with the Canon 15-85 at WA end of the range, and others have confirmed. Corners/ borders at infinity tend to be soft. The problem varies between shots while using the same fl, e.g. 15mm. I first thought this variation was caused by the IS unit, but later found it also occurs with IS off. The 15-85 tends to frontfocus in the WA range (not at the tele end), at least on some of the Rebel bodies, which usually increases the border sharpness problem. It is strange that the problem increases with frontfocus, because this lens shows some field curvature at 15mm where close objects seem to be sharper - one would guess that frontfocusing makes infinity objects even softer.

It might be the same issue you mention, caused by lens groups/elements that move during focusing or zooming. Just like with your Sigma this is a bit strange problem, because officially the DOF should easily cover a huge range.

My Canon 18-55IS lens had a similar problem with soft corners/borders at the WA end of the range, but ONLY if the camera was pointed strongly upwards. In normal orientation corners were always sharp even wide open. This also varied between shots and I haven't been able to nail down the cause, but I'm suspecting mechanical play of lens elements during focusing.

  • Given that the center stays sharp while one side of the image varies my guess is that the problem lies squarely with these elements and is related to their rotation and not to their travel and that in the full image circle the misfocusing is still present; it just gets rotated off of the upper right corner of the rectangular sensor.

I don't think it works like that. I'm guessing that the lens elements are not moved exactly along the optical axis, or maybe their own axis varies a little bit while they move. This is most likely due to mechanical tolerances in the parts of the lens barrel that guide the focusing/zooming actin.

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