For a conventional lens, distortion is tied to its physical properties, but volumes can be very distorted.
…but we can see that the distortion in the image becomes very pronounced after the deformation is fixed.
In practice, the principles of optical geometry dictate that it is impossible to maintain both straight lines and volume consistency.
Let’s take the preceding two images and correct the volume deformation and distortion, respectively.
We can see that correcting one defect amplifies the other. This means it is necessary to find the best compromise, and that depends on the image content, making it particularly complicated to automate this kind of correction.
It is possible to quantify the volume deformation and to see how it evolves depending on the focal length. The graph below is plotted for an object situated at about 20% away from a corner of the image.
Elongation (%) depending on the focal length (eq. 24 x 36 mm)
The sensitivity threshold is estimated to be about 15%, or in other words, a focal length of 24mm (eq. 24 x 36 mm); deformation above 25% (which appears below 17mm, eq. 24 x 36 mm), the problem is considered too big to avoid correcting.
DxO Labs has developed volume deformation correction tools that let you control the distortion in an image. These tools apply corrections in two ways:
: the best correction, but generates distortion
: less powerful, but better-controlled distortion
The spherical correction
The spherical correction tool is powerful and can correct volume deformation throughout the image.
To activate this correction, simply click on the Volume Deformation palette in DxO ViewPoint (or choose the Diagonal option in the integrated palette in DxO Optics Pro).
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The faces and bodies have been restored to their proper proportions and the photo is uniform overall.
This correction can also be applied even when there aren’t any faces: the subject simply has to be at the edge of the image and has to have volume.
The correction restores the proper proportions of the statue.
However, the distortion generated by the correction will significantly bend the lines on the edges of the image.
Volume consistency is recovered but vertical/horizontal lines are lost.
In the above example, the distortion has generated an unsightly curving of the planks on the ground; in this case, you should use the cylindrical correction instead.
The cylindrical correction
As an alternative solution, the cylindrical correction tool does not generate a great deal of distortion, but neither does it completely correct the volume deformation.
It will, however, effectively correct the volume deformation in the horizontal direction (the “too-wide syndrome” mentioned at the beginning of this tutorial).
To activate it, click on the Volume Deformation palette in DxO ViewPoint (or choose the Horizontal / Vertical option in the integrated palette in DxO Optics Pro).
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In the preceding photo example, this turns out to be an advantageous solution.
Volume consistency is maintained while vertical/horizontal lines are recovered.
Depending on the subject, volume deformation and distortion will not affect an image equally: volume deformation will be most apparent for three-dimensional subjects, particularly when the lens focal length is short and when the subject is near the edge of the image; distortion mainly affects straight lines.
Unfortunately, it is not possible to efficiently correct the two kinds of problems at the same time: correcting the one will increase the effects of the other. This means that you will need to find a compromise, and for this, both DxO ViewPoint 2 and DxO Optics Pro offer you two correction tools:
A powerful spherical correction tool for fixing volume deformation, but which generates considerable distortion
A cylindrical correction tool that provides a less-powerful solution for handling volume deformation, but which limits the appearance of distortion
Photos credits: Patrick Gaillardin / Laetitia d'Aboville / JL Dubin / Beboy photographies / Olivier Lambolez