“Distortion” might be the most widely misunderstood, or misused, word when it comes to optics. Many people will use “distortion” to refer to any aberration or error in a photographed image.
However, distortion actually has a more specific definition and refers to a certain type of photographic aberration. Understanding what distortion is, and how to measure it, can be very helpful when trying to determine the right lens for the right application.
Let’s explore the “distortion” meaning.
What distortion actually refers to is the bending of lines in an image that, in reality, are actually straight. Distortion is usually unavoidable with most lenses, though in many cases it is hardly perceptible at all.
But in some cases, distortion can have a negative impact on image quality. This is especially true for highly technical applications, such as architecture photography or medical imaging. In these fields, reducing distortion is a top priority.
There are two main types of distortion in optics to be aware of, barrel distortion and pincushion distortion. Both distortion types are named for the apparent shape of the images which are affected by distortion.
With barrel distortion, straight lines appear to bend outward, away from the center of the image. It is called barrel distortion because the effect can cause a geometric square image to take on the shape of a barrel.
Pincushion distortion has the opposite effect, causing straight lines to bend inward toward the center. The result is an image shape resembling that of a pincushion.
When accounting for levels of distortion in optics, there are a few different ways to read and measure its effects. Understanding distortion tables can help decide lens selection and lens specifications.
Distortion Tables: How to Measure Distortions
Distortion tables, or distortion charts, are tools used to measure distortion levels with certain lenses. Once again, there are two general categories for measuring distortions: Optical and TV distortion.
Optical distortion measures the space between where an object appears in a frame versus where it appears in reality, or where it should be. Optical distortion can be easily read using a distortion dot chart, or a lattice pattern chart.
Either type of distortion table superimposes a matrix of lines or dots representing the distorted image over an equivalent undistorted matrix. These charts make it very easy to gauge distortion levels at certain points in a frame. Distortion is not linear, and thus optical distortion tables are effective when accounting for field of view.
TV distortion, on the other hand, measures distortion based on a single square plane filling the vertical field of view. The resulting graphic can resemble a television screen. TV distortion tables measure the “straightness” of a line (or lack thereof) against the edge of a lens. While this method is effective for measuring within a given field of view, it does not account for distortion levels beyond that field of view.
So, why does understanding how to read distortion tables matter?
Distortion can affect the appearance of any image on any lens. Adjusting for field of vision can lessen distortion levels, but that alone does not eliminate distortion. The best way to minimize distortion is to use specialized equipment and advanced, customized lenses for exact image quality.
Contact Universe Optics to learn more about how to reduce distortion using custom lenses.