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About Lateral Chromatic Aberration (Magnification Chromatic Aberration)

This is a series of articles that conceptually explain aberration, which is an index of lens performance.

Among the aberrations, this section introduces Lateral Chromatic Aberrations (Magnification Chromatic Aberration) in a simple and easy-to-understand way using figures and simulations.

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Introduction to Lens Design

Aberration and Axial Chromatic Aberration

First, aberration is an index that shows the deviation from the ideal imaging relationship.

Camera lenses aim to accurately focus light emitted from the "subject" onto the "imaging surface" (film or CMOS).

However, it is difficult to focus light accurately, and the small amount of deviation is called "aberration".

These aberrations are classified and named according to their characteristics.

In past articles, we introduced spherical aberrations, axial chromatic aberration, distortion, etc., but this time, we will explain about "lateral chromatic aberrations".

Many people may know that chromatic aberration is "color bleeding" that can be seen when a photograph is enlarged.

Chromatic aberration is further divided into "axial chromatic aberration" and "lateral chromatic aberrations."

In the past articles, we explained about axial chromatic aberration.

Axial chromatic aberration

This article about axial chromatic aberration is an explanation of the world's gentlest axial chromatic aberration explained from "the concept of feeling colors", so I recommend you to read from here first.

In this section, I would like to explain "lateral chromatic aberrations" so that it can be understood as a simple concept as possible.

Let's imagine

On a sunny day, you have a black wall with white dots and you want to "take a picture."

There was a scene like this in the article a little while ago. it was a star at that time, but this time it is also a secret point that it is a sunny day.

Now, as a result of taking a picture of this wall, the picture is as follows.

The white dot in the center is fine, but the white dots in the periphery have color bleeding around them.

Color bleeding occurring in the center of the screen is called "axial chromatic aberration", while color bleeding occurring in the peripheral area is called "lateral chromatic aberrations".

At first glance, it looks like only the place where it occurs is different, but let's explain what kind of difference there is using simulation, etc.

Color of light and chromatic aberration

As explained in detail in axial chromatic aberration's commentary, let's briefly review the color of light first.

Axial chromatic aberration

The white light like the sun is a mixture of many colors, which are expressed in terms of wavelength. Typical colors are shown below.

Different colors of light change the "angle of bend when passing through the lens".

The phenomenon of shifting to the focus direction due to this difference in angle is called axial chromatic aberration.

The figure below shows the axial chromatic aberration caused by blue light (400 nm) and red light (750 nm).

On the other hand, the phenomenon in which the angle of light rays due to the color of light is shifted toward the image height is called lateral chromatic aberrations.

I made an image of lateral chromatic aberrations with a little exaggeration.

In contrast to the light path of the d-line (yellow), the g-line (blue) is curved at a different angle due to the difference in wavelength, and the image is focused at a position slightly shifted in the image height direction.

At this time, the deviation from the d-line (yellow) ray is the lateral chromatic aberrations.

For the sake of simplicity, only the g-line (blue) is shown in this figure. However, when shooting under sunlight, all the colors of light are included, and the amount and direction of light bending are also changed. Therefore, the color bleed due to lateral chromatic aberrations in the picture will be various colors.

Lateral Chromatic Aberrations graph

Now, let me explain the detailed lateral chromatic aberrations map using the optical path diagram.

The displacement of the g-line (blue) relative to the ray of the d-line (yellow) is illustrated. This is the conceptual diagram of the lateral chromatic aberrations graph.

Lateral chromatic aberrations is an aberration with a smaller value than spherical aberrations, so it was produced only as an image.

A normal lateral chromatic aberrations doesn't shift so much that you can see even with one lens.

In the above figure, the g-line (blue) is misaligned, but similarly, the C-line (red) and F-line (light blue) are misaligned.

Lateral Chromatic Aberrations of one lens

Optical path diagram

The lens I have prepared this time is a 28 mm F2.8 specification lens consisting of only one lens.

The aberration is already enormous visually, but what is the lateral chromatic aberrations like?

Lateral Chromatic Aberrations

Since the lateral chromatic aberrations diagram is based on the d line (yellow), the d line is a straight line. The largest deviation is the g line (blue), which falls greatly to the minus side.

This is due to the fact that the g line (436 nm) is shifted from the d line (588 nm) to the side with the largest and shortest wavelength (purple outer side) when viewed as a wavelength.

The C line (red) is shifted to the positive side in the opposite direction. This is because the wavelength of the C line is 656 nm and is shifted to the longer wavelength side (red outside) when viewed from the reference d line, so it moves in the opposite direction to the g line (blue).

Lateral Chromatic Aberrations correction

Earlier, I introduced the aberration of 28 mm F2.8 lens composed of only one lens.

In reality, Fno's bright lens with such a wide angle of view consists of many lenses.

Let's look at the characteristics of the distortion corrected lens again.

The figure below shows the lateral chromatic aberrations of a standard retro-type 6-piece wide-angle lens on the graph scale of distortion.

Optical path diagram

This optical path diagram shows the NIKON NIKKOR 28 mm F2.8D, which we have analyzed in the past. This retro-type wide-angle lens has a nostalgic look.

Lateral Chromatic Aberrations

It is still a bit difficult because it has six asymmetric wide-angle lenses, but I would say it can correct aberrations to about 1/3 compared to a single lens.

It is also possible to reduce the lateral chromatic aberrations of the same six lens system if it is a symmetrical optical system such as a double Gaussian type.

NIKON NIKKOR 50 mm F1.8D

Lateral Chromatic Aberrations and Distortion

There is another way of looking at lateral chromatic aberrations.

It can be said that "the difference in distortion aberration for each color of light is lateral chromatic aberrations."

Like other aberrations, distortion also varies with the wavelength of light.

For example, the distortion of the d-line (yellow) of the reference light is as shown in the figure below.

Each color (wavelength) of light has a different way of bending the light rays, so naturally the shape of distortion also changes with each color.

Next, let's try to exaggerate the appearance of the g line (blue).

If the image is taken with the g-line (blue), it is distorted like this.

In reality, color distortion does not occur to this extent, but the difference between the shape of the d line (yellow) and the shape of the C line (red) corresponds to lateral chromatic aberrations.

To illustrate it clearly, it is equivalent to the difference between the graphs of the d line and the g line when they are viewed in an overlapped manner.

Conclusion

To put it simply, lateral chromatic aberrations is a color shift in the direction of the image height.

The phenomena that appear in photographs do not seem to be very different from axial chromatic aberration, but in fact, they are treated as different aberrations due to the difference in the generating factor, and used as performance indicators.

I am sure that you can enjoy our blog 20 times by understanding lateral chromatic aberrations.

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Modern Optical Engineering: The Design of Optical Systems

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