Steve Cushing Impresionist Fine Art Photography

Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

Chromatic Errors

Please read the section on how lenses work first. LINK here to this section.

What I am about to write about here is rather complex, but I will try to keep it as simple as I can. Remember that light consists of various wavelengths and all of these wavelengths have a different frequency so reach differently. This leads to chromatic aberration as it is to do with colour (chroma). In CA the colours of an image are distorted because the different wavelengths that form those colours do not all converge exactly upon the focal point. The refractive index of the lens, which mathematically describes its ability to bend light, varies with different wavelengths, so each colour of light is bent at a slightly different angle, resulting in a image with rainbow effects or coloured fringes of light where there should be sharp boundaries. Think about the effects of a prism and how it separates light into its RYB colour frequencies.

The concept of RGB (Red, green, Blue) and RYB (Red Yellow, Blue) is the perspective at which light comes to you. A great analogy is the movie projector, the video projecting out of the projector is in RGB and when it hits the screen it is reflected through the scheme known as RYB.

RYB is the colour coding system that we all learned while we were in the beginning years of our schooling. We were taught that Red Yellow and Blue were the primary colours and you can make any other colours out of these three basics. Red + Yellow = Orange, Blue + Yellow = Green and so on.
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Since the concept of paint is that colour absorb every colour but the one that it is reflecting. This means that red absorbs every colour but red, blue absorbs every colour but blue and yellow absorbs every colour but yellow. When you add colours together what you are doing is adding to the amount of colours that the paints will absorb. This colour scheme is known as a subtractive colour scheme because when you add different colours together what you are actually doing is subtracting the amount of light that the paints can reflect.

RGB is the way we all see light, when light comes from a lamp or the sun, it is all travels in RGB. This scheme is what is known as an additive colour scheme because when you add lights together you are only adding the spectrum of light that can reflect back or go straight into the eye.

So if we look at lenses in terms of the RYB system of light hitting the sensor or film we can see how a lens can split these colour frequencies.

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Spherical aberration is another chromatic error but is as a result of lens curvature. Short focal length lenses demonstrate it more strongly than longer focal length lenses do. What happens in SA is that the further an entering light ray is from the centre of the lens, the less accurately it is bent toward the focal point. As a result the image will be presented with a blurry halo. Glass with a higher index of refraction is less prone to this problem than lower index glass, but it is present in all spherical lenses to some extent. Since the centre of the lens is more accurate than the outer edges, lenses of high diameter can be used and the light blocked from entering the margins of the lens.

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Aspherical lenses are available which do not have any SA. These are ground with a "perfect" shape, where the curvature is varied from the centre to the edges so there is inherent correction essentially built into the lens, but they are extremely expensive to produce.

There are five basic types of aberration which occur in photography due to the geometry of lenses and which are applicable to systems dealing with monochromatic light, they are known as the Seidel aberrations, from an 1857 paper by Ludwig von Seidel. These are the aberrations that become evident in optics, also known as Seidel optics.

The five Seidel aberrations are:

Spherical Aberration: this is the aberration affecting rays from a point on the optical axis, because rays from this point going out in different directions pass through different parts of the lens, then, if the lens is spherical, or otherwise not the exact shape needed to bring them all to a focus, then these rays will not all be focused at the same point on the other side of the lens.

Coma: this aberration affects rays from points off the optical axis. If spherical aberration is eliminated, different parts of the lens bring rays from the axis to the same focus. But the place where the image of an off-axis point is formed may still change when different parts of the lens are considered.

Astigmatism: this is aberration affecting rays from a point of the optical axis. These rays, as they head through the lens to the point in the image where they will be focused, pass through a lens that is, from their perspective, tilted. Even if neither spherical aberration nor coma prevents them from coming to a sharp focus, if we consider the rays of light that are in the plane of the tilt, and the rays of light that are in the plane perpendicular to that, these rays pass through a part of the lens with a different profile. So they may not be focused at the same distance from the lens, even if they do come to a focus in each case. See Digital Sensors HERE

Curvature of Field: even when light from every point in the object is brought to a sharp focus, the points at which they are brought into focus might lie on a curved surface instead of a flat plane.

Distortion: even when all the other aberrations have been corrected, the light from points in the object might be brought together on the image plane at the wrong distance from the optical axis, instead of being linearly proportional to the distance from the optical axis in the object. If distance increases faster than in the object, one has pincushion distortion, if more slowly, barrel distortion.

The following diagram illustrates these aberrations:

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A lens designed and corrected for the Seidel aberrations may form a spatially correct image in a focal plane of point objects at all field angles. This is possible only in monochromatic light, as the refractive indices of the optical materials are a function of wavelength. Focal length therefore is inversely proportional to wavelength, and as refractive index decreases with wavelength then focal length increases with wavelength.

The lens disperses collimated incident white light to form a primary spectrum or give primary chromatic error. This is independent of lens surface curvatures.

Along the optical axis a linear image changes colour from blue through red this is termed longitudinal chromatic aberration (LCA). In a focal plane the effect is one of concentric coloured circles of confusion.

Dispersion and Glass Type

If the spectra produced by passing white light W into two prisms C and F of crown and flint glass respectively of the same refracting angle are examined, two differ­ences are found. The crown glass disperses the light less.

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The dispersion of optical materials is non-linear, and the terms 'long' and ‘short’ applied to glasses indicate the length of the spectra formed.

Chromatic error is also found for obliquely incident light and is termed transverse chromatic aberration. Spherical aberration, which is a function of aperture radius H, combines with chromatic error to determine the image height y of a coloured ray.

But it is fair to say that chromatic aberration was likely to be less of a priority in "pre-1970's" lenses. The reason is that colour photography, though available, was not as popular as "black and white" until around the mid-1960's. The cost and availability of colour prints were prohibitive for casual photography until around this 1960's to 1970's period. Even for Pros, black and white was the staple for news, including sports, corporate photography including portraits, and architecture.

So, chromatic aberration was not going to be as serious an issue even until "later", and "later" varied from company to company.

See also "Why does the aperture not cause vignetting when it gets smaller?" in the Aperture section HERE and Digital Sensors HERE

For more information read sections on Optical Design, Glass, Chromatic Errors, Perspective, Lens Flare and Vignetting. Click on item to go to page.

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