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Thread: What is a decentered lens?

  1. #1
    Senior Member
    Join Date
    Dec 2007
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    What is a decentered lens?

    I have read references to lenses being "decentered" which I believe means at least one of the glass elements is not accurately aligned.

    How does this manifest itself? How does one know that one's lens has this defect? Can the lens be sent in for adjustment or is it something that cannot be fixed?

    I am not aware of any specific problems with any of my lenses, but perhaps I don't know what to look for.

  2. #2
    Ranger 9

    Re: What is a decentered lens?

    You're right about what it means. Lenses bend light because of their curved surfaces, and the center points of the curves have to be on the same axis so the bending will be consistent.

    It shows up easily via a "star test" on an optical bench, but heck, who's got one of those anymore? (For the bench test, you would have viewed a centered point of light -- generated by a collimator -- as you rotated the lens around its central axis. In a perfectly centered lens, which is rare, the point would stay still. If the lens was decentered, the point would move in a circle, or flare off in various directions.)

    Symptoms you might see in pictures include "glowing" highlights (especially if the glow tends to go in one direction more than another, although other defects can cause this too), or an uneven distribution of sharpness.

    It's rare for a lens, especially a complicated one such as a zoom, to be perfectly centered, and some designs are more sensitive to decentration than others. So if your lenses seem to be performing well, it's probably not worth worrying about.

    On the other hand, if you've (gack!) dropped a lens recently, you might want to have its centration checked as part of a repair evaluation. In traditional lens construction, the individual elements are held in place in machined rings and "cups" that are made of soft metals such as brass or aluminum. A hefty knock can bend these enough to decenter an element, even if no exterior damage is visible. Sometimes a repair technician can repair this damage by carefully straightening the internal mounts; I once had a 135/2.8 Komura lens in Leica thread mount that was almost unusable when I got it, but improved amazingly once it was straightened.

  3. #3

    Re: What is a decentered lens?

    @ Ranger 9

    Generally speaking, how much of a drop can say, a metal bodied manual focus lens withstand without there being a decentering problem?

    Like could it withstand a drop of say five feet onto a wooden floor?

    Or does any drop require that the lens be sent back for repair/checking?

    Thanks in advance.


  4. #4
    Join Date
    Nov 2007
    New Zealand
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    Re: What is a decentered lens?

    Where I am, theres mountains. I took 3 photos, mountains or horizon at top/sides, middle/sides, lower frame sides. Just to make sure left was same as right. And mostly at different aperatures to find whats better...

    This was a while ago, January 2007, Canon 400D.
    Water pic, Zeiss Flecktagon 20 2.8 @2.8, left side is out.
    A new, well taken care of, Sigma 12-24 wide open @12, right side is out.

  5. #5

    What is a decentered lens?

    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 moving and non permanent to a corner.

  6. #6
    Senior Member KETCH ROSSI's Avatar
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    Oct 2009
    Rome, Italy
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    Re: What is a decentered lens?

    This is actually something that is almost impossible to control in Zooms, but less of an issue, with Primes, in fact I have never personally had a single Prime in over twenty years show such issue, on the other hand in Zooms has happened many times.

    Most of the time I must admit it was only seen in Filming with zooms, rather then Photographing with Still Zooms, but now this has becoming more of an issue as many DSR's are now shooting video, not that I use any, since I have Pro grade Digital Cinema gear, but my friends do all the time.

    More of a clear and why this happens has appeared recently during intensive testing for S3D rigs, which use two identical Cameras with two identical Zooms which are then Genlocked and matched in calibration, zooming and focusing, yet the disparities from the two Zooms, while zooming are so evident in this set up, as truly no Zoom is identical and does show when zooming as the elements are always de centered, no matter how little of a difference this can make on a single zoom it does make a huge difference when shooting 3D.

    This off course is not an issue with stills, unless off course the de centered Zoom is not just as natural as it should be but instead greatly pronounced, then it needs to be sent in for service to arrange for the elements and internal gear to be aligned more properly.

  7. #7

    Re: What is a decentered lens?

    A lens whose optical center does not coincide with the geometrical center of the rim of the lens; has the effect of a lens combined with a weak prism.

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