Chromatic aberration:
Precise corrections

Kah-Wai Lin

Svalbard, Norway

Fujifilm X-T3

140-560mm f/6.4-8

ISO 160 - 1/500 - f/8 - 560mm

DxO PureRAW - DxO PhotoLab

DxO’s scientific approach removes
fringing without impacting image quality.

Chromatic aberration causes unsightly fringing around the edge of objects caused by the limitations of a lens.

While clumsy corrections can be too heavy-handed, DxO’s meticulous corrections ensure images are refined without any unwanted consequences.

What is chromatic aberration?

Chromatic aberration occurs when a lens is unable to converge all the colors it is transmitting into a single point on the camera’s sensor. The red, green, or purple fringing typically occurs in areas of high contrast, such as bare branches against a white sky.

The amount of chromatic aberration depends on the lens type, construction, quality, and coatings, but also factors such as aperture, focal length, and focal distance.

Understanding
the limits of physics

First, it’s useful to understand the intended performance of a lens.

All of the different wavelengths (i.e., colors) of light pass through the lens and are focused in a single point on the sensor. Because of the precision of the lens, no chromatic aberration is produced — as seen in expensive apochromatic lenses that are designed specifically to avoid chromatic aberrations.

In a lens, this ideal can fail in two ways, producing two different types of chromatic aberration: longitudinal and lateral.

With longitudinal chromatic aberration — also known as LoCA or axial chromatic aberration — the lens fails to bring all of the wavelengths of light to a single point, a phenomenon that becomes more severe when using the lens at larger apertures.

As a result, objects across the frame might appear with a color fringe — often pink, purple, or green, but always a mixture of red, green, and blue.

Subjects that are out-of-focus can acquire different color fringes: subjects behind the focal plane can take on one color, while subjects in front can take on another.

Lateral chromatic aberration

Lateral (or transverse) chromatic aberration occurs when the wavelengths of light arrive at different angles and therefore come into focus on different parts of the sensor.

Because of the way the light falls on the sensor, this type of aberration is only seen in the corners of the frame.

How DxO assesses
chromatic aberration

DxO’s technicians capture images of a dot chart at different focal lengths and distances. It’s then possible to calculate the distance between different color components (red, blue, and green) of the same dot.

And because chromatic aberration varies according to the focal length and distance, DxO algorithms factor in these two key parameters to correct chromatic aberration in RAW and JPEG images more effectively.

The resulting values can be placed on a scale. Under 0.03 mm, it is not perceptible. Between 0.03 mm and 0.06 mm, it becomes visible. Values exceeding 0.07 mm are problematic.

Precise corrections

With these measurements, DxO technicians compile data inside a DxO Module that describes the tiny inaccuracies of each color channel, pixel by pixel. DxO software then uses algorithms that apply this data to realign the geometric position of each pixel, producing an image that is free from fringing and other color shifts.

As a result, DxO software removes colored outlines with absolute precision, and without degrading a photograph’s color and detail.