Decentering - let's be clear what it is and what it isn't

Started 4 months ago | Discussions thread
ForumParentFirstPreviousNextNext unread
Flat view
Shirozina
Regular MemberPosts: 310
Like?
Decentering - let's be clear what it is and what it isn't
4 months ago

I see the term banded about here and it's obviously not being used correctly. Decentering is a specific type of lens distortion causing an asymmetry of pure radial distortion. If barrel , pincushion and complex moustache distortion are symmetrically radial  around a central point like a doughnut then decentering is the process where the radius becomes non circular and becomes a doughnut squashed into an oval. What it is not is a standard definition for off center image softness which has many causes. It's almost a certainty that all our lenses esp zooms are decentered to some degree but this may not show as uneven resolution side to side. What we may see as uneven definition is a lens having it's optical axis not striking the sensor in the middle and this being combined with a curved field of focus. Misalignment of the optical system it is but true decentering it is not. If we use shift lenses we are moving the optical axis relative to the sensor but we are not causing decentering. Also we may have a lens flange that is not perfectly parallel with the sensor and that also means the mechanical body of the lens and it's mount in relation to the optical group which may in itself be perfectly symmetrical. Here were again create this using a tilt lens to move the plane of focus to be non parallel with the sensor creating an uneven focus across the image but again decentering it is not. Resolution tests of flat subjects no matter how well aligned with a camera will not tell you if you have decentering. You need to use calibrated test targets and ideally  a 3d test object  shot from different camera positions combined with photogrammetry software  to measure and identify true decentering. Carefully shot flat subjects will reveal if you have uneven resolution across the field but it can't be correctly called decentering.

BTW my best performing lens in terms of sharpness from center to corner also has the worst decentering value as measured by a highly specialised photogrammetry lens calibration procedure.

ForumParentFirstPreviousNextNext unread
Flat view
ForumParentFirstPreviousNextNext unread
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow