Depth of Field - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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DEPTH OF FIELD
The focal point is the specific point on the optical axis where parallel rays of light coming from a specific distance (infinity for converging lenses) converge after passing through the lens elements, thus the point of focus in the image. By adjusting the focus mechanism, the arrangement of lens elements is altered, effectively shifting the focal point relative to the image sensor. Consequently, the object placed at the current focal point distance will be captured with sharp focus.

Depth of field refers to the range of distance in a scene that appear "acceptably" sharp in the image. It is often not a clear cut but a transition that goes from in focus to out of focus based on distance from the focused element in the scene (focal point). Creatively, it is often used to draw attention to the main subject and create a sense of depth and dimensionality in the scene.

Overall, depth of field is a critical aspect of photography, manipulated to achieve the desired aesthetic and storytelling goals. The amount of deph of field is influenced by several factors:
Aperture Size:

The aperture size, represented by the f-stop value (e.g. f/1.8, f/8), determines how much light enters the lens. A wider aperture (lower f-stop) results in a shallower depth of field, with only a narrow range of distance appearing sharp. Conversely, a narrower aperture (higher f-stop) increases the depth of field, with more of the scene appearing in focus.

Focal Length:

The focal length of the lens also affects depth of field. Longer focal lengths (telephoto lenses) tend to produce shallower depth of field, especially when used at wider apertures. Shorter focal lengths (wide-angle lenses) have greater inherent depth of field, resulting in more of the scene being in focus.

Subject Distance:

The distance between the camera and the subject influences depth of field. Closer subject distances result in shallower depth of field, while greater distances increase depth of field.

Sensor Size:

The size of the camera sensor also impacts depth of field. A smaller sensor, because it inherently capture less "image space", requires shorter focal lengths or a longer distance to capture the same as a full frame sensor, therefore increasing depth of field.
Bokeh:

Bokeh refers to the aesthetic quality of the out-of-focus areas in an image, particularly the way light is rendered into blurry circles or shapes. It's influenced by the shape and size of the lens aperture and can be adjusted to create various visual effects, from smooth and creamy to specular and geometric. Some lenses produce smoother and more aesthetically pleasing bokeh, while others may exhibit harsh or distracting bokeh artifacts such as onion rings or double lines. Artists often consider bokeh quality when selecting lenses for specific applications.

The bokeh can manifest differently based on multiple parameters. Here, are a few:

Number and shape of aperture blades:

The number and shape of aperture blades in a lens affect the shape and appearance of bokeh. Lenses with a higher number of aperture blades tend to produce more rounded and smoother bokeh circles, while lenses with fewer blades may result in more polygonal or angular shapes.

Circular: out-of-focus highlights that take on a circular or rounded shape in the image. This shape is a result of the lens aperture, which determines the shape of the bokeh circles. It is common when shooting with lenses with wide apertures (e.g. f/1.4) and highlights that are evenly illuminated.

Polygonal: out-of-focus highlights take on a polygonal or geometric shape rather than a smooth, circular shape. This effect is the result of fewer number of aperture blades in the lens, resulting in more distinct polygonal bokeh shapes. Is the lens or also the narrower aperture?
Lighting Conditions:

The quality, direction, and intensity of light in the scene affects the appearance of the bokeh. Strong, directional light sources can create more pronounced highlights and contrast in the bokeh, while diffused or ambient lighting can result in softer and more even background blur.

Harsh: out-of-focus highlights or areas in the image have noticeable edges or artifacts, resulting in a cluttered or distracting background. It's often caused by lenses with fewer aperture blades or certain optical imperfections, leading to less smooth and more defined bokeh shapes.

Smooth: characterized by uniformly blurred background or foreground elements, with no harsh edges or distracting artifacts.

Creamy: a particularly smooth and velvety appearance, resembling the texture of cream. It typically results from high-quality lenses with wide apertures.
Aberration:

Certain aberrations may cause bokeh shapes to appear distorted, elongated or with color fringing towards the edge of the frame.

Cat's eye: occurs when out-of-focus highlights take on a shape resembling a cat's eye or oval rather than a perfect circle. This effect is caused by optical vignetting, where light entering the lens is partially blocked by the lens barrel or lens elements, therefore increases towards the edges of the frame. If you have a fast aperture prime lens, you might have seen this effect on the lens bokeh – the bokeh shapes stay circular in the center, but gradually change in shape towards the corners. That is because: Note that such vignetting (in this case bokeh, cats eye) is mostly evident at large apertures, since it is the physical lens barrel that mostly blocks the peripheral light from the front and back of the lens barrel. Once stopped down, the smaller size of the aperture in the center is visible even from the corners, allowing the light to pass through.

This is lessened by stopping down the lens.

However, optical vignetting can also have a pronounced effect on out-of-focus parts of the image. Because the shape of an out-of focus highlight (OOFH) mimics the shape of the clear aperture, the bottom left situation of Fig. 2 leads to the so-called cat's eye effect [ 1]. Figure 3 evidences the resemblance between the appearance of OOFHs and the aperture shape. With an increasing distance from the optical axis the shape of the OOFH progressively narrows and starts to resemble a cat's eye. The larger the distance from the image center, the narrower the cat's eye becomes

https://www.cs.cmu.edu/~sensing-sensors/readings/vignetting.pdf
Lens design:

Swirly: characterized by a swirling or vortex-like pattern, which is often associated with certain vintage or specialty lenses. what causes it?

Soap bubble: out-of-focus highlights that resemble the spherical, iridescent patterns seen in soap bubbles, characterized by its multi-faceted appearance.

Double-ring: out-of-focus highlights exhibit two concentric rings or circles within each bokeh shape. This effect is caused by imperfections in the lens elements, particularly in the shape and surface quality of the lens diaphragm blades.
An aspherical element in the optical design may create a bullseye pattern of concentric circles in the bokeh — aka the aforementioned onion rings.
Focus Breathing:

Focus breathing, also called lens breathing, refers to the slight change in focal length (and hence angle of view and magnification) that occurs when the focus distance of a lens is changed (i.e. change in a lens focal length as the focal distance changes).

Technically, the focal length of a lens is defined using at focus. Hence, typically focus breathing results in a slight reduction of the lens focal length (usually no larger than -5%) when focusing at close distances, exhibiting a narrower field of view than when focusing further away. Thus, the perception of this phenomenon might be prominent when doing a pull of focus from a far element to a close one.

It's caused by the internal movement of lens elements during focusing, and a common issue that occurs to some extent on most photographic lenses, specially vintage lenses. Nonetheless, specialised high-end cinema lenses tend to include "stronger" compensation, due to the higher physical perception of this phenomenon in video.
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