Before correcting the image geometry, it is necessary to pre-correct lens distortion.
The lens distortion is all the more important as the focal length is low.
A wide angle will be more affected by distortion than a long focal lens. However, the wide angle lens precisely requires straightening the horizon and / or correct the perspective.
Hence the need to carefully execute this step for a perfect result.
Integrated PhotoLab Version
This step is transparent since performed automatically for the supported lenses.
Lense modules management is a basic function of PhotoLab, see tutorial: Efficiency with DxO PhotoLab.
Independent version plug-in for PS / PSE / LR
ViewPoint accesses the lense database characterized by DxO and has the same automatic distortion correction as PhotoLab.
One of several cases occurs at the opening of an image based on the presence or absence of EXIF allowing ViewPoint to know which lens was used.
* The module is already present and the correction is applied
* All the necessary elements have been found in the EXIF. A dialog box opens, the module is imported and the correction is applied
* There is no EXIF metadata, or they do not allow to determine which lens was used.
ViewPoint asks “Open original image”: the file (JPEG or raw) from the camera. When the module is loaded, the DC model, the lens and the original image file name are displayed, and the correction is automatically available
* The elements have been found in the EXIF, but there is no corresponding module for the purpose. In this case, only the manual setting is possible (AUTO button is gray).
Manual distortion correction
ViewPoint provides tools according to the three types of distortion.
– Barrel distortion
– Pincushion distortion
– Taken from fisheye
A monument taken from the front with a 28mm wide-angle lens (24×36 format).
Referring to the paving stones line at the edge of the image, there would be a slight barrel distortion. The examination of other photographs, taken at the same time, shows that the middle of the paved passage is slightly curved and that the 4 central studs have their vertex located on the same straight line. It is therefore on these studs that the correction must be based.
One has to be sure of the reference points.
The distortion is constant for a given focal length (except for some zooms at short distance). A simple trick is to take a photograph of a geometric shape with straight lines (door frame, …) and placing it closer to the edge of the frame and record the correction values.
This is enough for a fixed focal length. For a zoom, it will be necessary to repeat the operation for the principal focal lengths.
Photo from a 35mm vintage lens, with a complex “mustache” distortion.The complete correction of the DxO modules quality is obviously not possible in this case. Manual correction is a very acceptable alternative and far superior to no correction.
Examine a test picture by using the approach above. With such a barrel distortion, the best result is obtained with a value of 30. Some defects remain in the angles that can not be fully corrected, but the damage is reduced to a minimum.
The second illustration is a Scan from a slide taken with this lens. Barrel deformation is not immediately noticeable. The correction values thus achieved will be extended for all photos taken with this lens.
PhotoLab Elite users can create a partial preset of these corrections and apply it to each new photo of that lens.
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