Lens Sharpness - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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LENS SHARPNESS
Lens sharpness refers to the ability of a lens to render fine details and textures in a scene. Lens sharpness can vary across the frame, but it's common for lenses to exhibit at leas some sort of edge or corner softness, generally more pronounced at short focal lengths and wider apertures.

In this section, we will explore monochromatic aberrations that affect all wavelengths the same and do not produce spreading of color as with the chromatic aberrations.
Table of Contents:
1. Field Curvature:
1.1 Simple.
1.2 Wavy.
2. Spherical Aberration.
3. Astigmatism.
4. Coma Aberration.
5. Diffraction.
1. Field Curvature

It arises from the curved nature of optical elements (particularly spherical elements within a lens), which tend to focus light in a curved manner while the film/sensor is flat and, as a result, the later cannot perfectly capture simultaneously the entire image in focus across the entire frame. Therefore, parts of the image that fall outside the plane of perfect focus will be increasingly blurred. For example, in a simple field curvature scenario, if the focus is on the center of the image, the periphery will look slightly blurred and the other way around.

Although the behaviour of field curvature varies according to lens design, there are some common trends. Field curvature tends to be more prominent at wider apertures, as it allows more light to enter the lens often at steeper angles, which can exacerbate the effect. Wide-angle lenses, by their nature, often exhibit more pronounced field curvature too. This is due to the wider field of view they cover (again, with light entering at steeper angles) and the complex optical designs required to correct the distortions inherent in this type of lenses. In contrast, telephoto lenses typically have less noticeable field curvature, as the light entering the lens arrives at relatively straight angles, which minimizes effect.

1.1 Simple:

Lack of detail follows a gradual trend from the center to the periphery, or vice versa, in which either the center or periphery can be perfectly in focus.

1.2 Wavy:

Most modern lenses incorporate multiple optical elements and some are specifically set to reduce the effect of field curvature. As a result of this corrections, field curvature might not be limited to the "simple" type and can appear in different forms, often with a "wavy" type pattern. This might show sharp performance in the center and corners, while a soft mid-frame.
Field Curvature Diagram
Simple Field Curvature
Wavy Field Curvature
2. Spherical Aberration:

It is caused by the spherical shape of the lens which does not allow all the incoming rays to perfectly converge to a single point, especially those passing through the periphery. This effect is characterised by a more symmetrical blur, often resembling a disc or halo around the point source, and affects the entire image field although its impact is most noticeable towards the edges.

This phenomenon is proportional to the fourth power of the aperture diameter and therefore more pronounced when the diaphragm is wide open (low f-stop), as larger apertures allow more significant deviation of light rays. Stopping down the lens (higher f-stop, narrower aperture diameter) can significantly diminish the effect, as the diaphragm blades block the outer edges of the lens. At the same time, it is inversely proportional to the third power of the focal length and therefore more pronounced at shorter focal lengths (wide-angle lenses) because shorter focal lengths cause light rays to diverge more significantly.
Spherical Aberration Diagram
Spherical Aberration
Close up comparison
3. Astigmatism:

Light enters the lens in two axis: tangential (x-axis) and sagittal (y-axis). Astigmatism occurs when light rays entering the lens in one plane are focused at a different depth compared to those entering in the other plane. This results in one axis being in focus at the sensor while the other axis is not, causing a point of light to become spread out to some degree into a line, creating an elliptical or elongated blur pattern in one direction. Although astigmatism is often described in terms of tangential (around the circle) or sagittal (coming out of the circle) planes, in practice, it may not align perfectly with these axes and may occur at various orientations relative to them, for example, exhibiting a diagonal orientation.

Astigmatism does not usually affect the center of the image, with the effect becoming stronger further away from it, and is more pronounced at wider apertures, although it is rarely observed in modern lenses.
Astigmatism Diagram
Tangential
Sagittal
Tangential vs Sagittal
4. Coma:

Coma aberration is an optical imperfection that primarily occurs near the edges of wide-angle lenses, where light rays enter the lens at more extreme angles (especially when used at their largest aperture) and affects the way light rays are focused. Coma occurs when light rays entering the lens at an angle fail to converge at a single point. This results in a specific type of image distortion, which creates an asymmetrical blur effect that manifests itself as a tail or coma shape trailing away from the point of light in the image, rather than a sharp point. It can occur in two directions, when the tails point away from the center of the image (external coma) and the opposite (internal coma). Nonetheless, this effect is typically well-controlled and may only be noticeable in extreme shooting conditions.
Coma Diagram
Coma
Close up comparison
5. Diffraction:

Diffraction (with respect to lens sharpness) refers to the effect of images becoming progressively less sharp towards small aperture values: f/16, f/22, etc. As we have seen, diffraction is the interference or slight bending of waves (in this case, light) around the corners of an obstacle or through an opening (such as the lens aperture). This causes each pinpoint of light travelling through the lens aperture to project onto the sensor as an "airy disk."

Every opening causes diffraction,
as light always bends through an opening even if it is very large, and therefore the effect is present at every aperture. However, smaller openings (small apertures) increase interference between light waves. This is because at wide apertures (e.g. f/2.8, f/4) the wave passes through the obstacle without much interference, as the waves are not particularly disturbed and follow a relatively straight path, and the resulting "airy disk" from the diffraction is much smaller than the pixel in the image, rendering the phenomenon almost imperceptible.

In contrast, at very small aperture sizes, the vast majority of the wave is blocked and diffracted, and light waves increasingly spread out interfering more with one another, resulting in an "airy disk" that is larger than the pixel itself, making the effect considerable more perceptible. However, while larger apertures may minimise diffraction effects, they are not always the sharpest settings on a lens. Many lenses tend to be sharpest when stopped down slightly from their widest aperture, typically around f/8.
Diffraction in wide vs small aperture
Diffraction and pixel size comparison
Diffraction simulation at a narrow aperture (e.g. f/22)
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