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

Started Jun 8, 2010 | Discussions thread
Severian The Lame
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Re: Sigma 8-16mm Review: Centering and Focusing
In reply to technic, Jun 9, 2010

technic wrote:

Severian The Lame wrote:

Centering and Focusing

  • 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.

No. I uploaded to my gallery half-sized samples; i.e., 1728x2592. If you click the original image link you'll see that. I thought that was big enough to show the problem without taking up too much space in the gallery. I have no clue how DPR downsized the images when I embedded them in my post. I don't know what happened with that first shot at f/4.5 because it looks worse than is typical; something odd was definitely going on there. That's one of the reasons I abandoned the 'rotate the camera' approach.

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.

I often shoot skylines from the roof of a 30 storey building and will occasionally notice some oddities in sharpness across the frame which may be related to the downward angle I'm shooting at. It's hard to put a finger on.

  • 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.

I have no clue, really. But that was my theory. You may very well be right.

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