Optical Distortion of Image
Fault of optical distortion of a lens deforms a photograph. This deformation can be spherical or linear.
In any photograph, a straight line in the subject must be recorded as a straight line. However, spherical distortion of a lens records it as a curvature. The result is curved lines (and horizon), which is rarely acceptable in architectural and product photography.
Spherical distortion can be a barrel distortion or a pincushion distortion. In barrel distortion, the image bulges out and straight lines closer to edges are bent in a convex shape. In pincushion distortion, the image is pushed inside from the center and straight lines near the edges are bent in a concave shape.
Example of barrel and pincushion distortion
In both cases, lines running from center of the image is not disturbed.
A symmetrical lens design, with complementary lens elements at the front and the back of the lens will cancel out this distortion. However, retro-focus (wide angle) lens design face this problem to certain extent. The magnitude of this distortion depends on the focal length and focusing distance.
Zoom lens and low end telephoto lenses also has this distortion, which can be tested by shooting a test shot of a grid, which should be perfectly square to the camera.
This is how lines should remain straight
In food photography and model shoot, this fault can hardly be noticed unless it is at extreme level. However, in architectural and room interior shots, where vertical lines must maintain their plumb line, barrel or pincushion distortion will spoil the result. A good lens design can minimize this problem.
Linear distortion is a rare thing, (may be a manufacturing defect), but I had an experience with this lens. It was a zoom lens, which would record a perfectly squared image in perspective.
Example of linear distortion
In this optical distortion, lines are not curved. I used that lens for quite some time, especially in product photography, where a slight top angle shots were required. At that precise height, the lens was giving me a perfect vertical lines, (parallel to each other) which otherwise would show a converge in perspective. Though this is a rare fault, we must test the lens for these faults with a test shoot.
Read about these lens faults in detail:
Blur or defocus of lens: Blurring of an image due to lens fault
Chromatic aberration: How chromatic aberration or distortion affects the image quality
Diffraction: Scattering of light causing diffraction in an image
Field curvature: Field curvature aberration of a lens
Lens flare: How to take care of a lens flare
Loss of contrast: How ability of capturing contrast of a lens is lost
Spherical aberration: Optics of spherical and asherical lens
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