David Svensson Offline Upload & Sell: Off
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Bobu wrote:
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And, as I allready wrote, Canon has tested this lens a few weeks ago and I have a writen statement from Canon, that the sharpness of this lens was tested is within spec.
Best regards,
Boris
Thanks for doing these tests, Boris!
Looking at the 24-70 pictures I think there might be some element de-centered. This is easy to test.
If you do a test shoot of the same wall, but turn the camera 180 degrees between two shots (most easily done by having the camera vertical-left on a ball-head to start with, then turning the camera to vertical-right, and using good pano technique to keep the lens in the exact same lateral location), and and compare the images.
If the colour finge in the shadow of the window-frame seen at the center of the image changes at all between the two images, you can be 100% certain there is at least one lens element that is not properly centered.
I don´t know about the 24-70, but many canon lenses have internal adjustment screws to allow the service to adjust centering and tilt of the lens groups.
When checking a complex lens professionally in an optics lab, a rotating projector is used. On that device the lens under test has a reticle with a test pattern behind it (where the image sensor normally is) and the lens is being continously rotated about its optical center axis.
The projected pattern is viewd at a white screen at a distance of typically 100x the focal length of the lens, giving a 2.4 x 3.6 m enlarged image of the test pattern.
As the lens is rotating, any change in the test pattern is observed. The amount of movement of the test pattern gives an indication of how good the centering is. The lens is then adjusted and re-tested until satisfactory.
BR,
David
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