Depth of Field

DEPTH OF FIELD

Table of Contents:

1. Focal Point
2. Depth of Field:

2.1 Aperture.
2.2 Focal Length.
2.3 Subject Distance.
2.4 Sensor Size.

3. Bokeh:

3.1 Number and shape of aperture blades.
3.2 Lighting Conditions.
3.3 Physical Barriers / Aberration:

3.3.1 Cat's eye.

3.4 Lens Design.

4. Focus Breathing.

1. Focal Point:

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. Accordingly, the element located at the current focal point distance is captured with sharp focus.

2. Depth of Field:

Depth of field refers to the range of distance in a scene that appear "acceptably" sharp in the image, which 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 (i.e. focal point).

Creatively, it is a critical aspect of photography, often purposely manipulated to achieve aesthetic and storytelling goals, such as to draw attention to the main subject and to create a sense of depth and dimensionality in the scene.

The amount of depth of field is influenced by several factors:

2.1 Aperture:

The aperture size, represented by the f-stop value (e.g. f/1.8, f/8) determines the size of the lens opening and therefore how much light enters.

A wide aperture (low f-stop) allows more light rays to enter from different angles, causing more significant variation in focus (wider circle of confusion), leading to a shallow depth of field and only a narrow range of distance appearing sharp.
Conversely, a narrower aperture (higher f-stop) restricts light rays, reducing the angle variance (smaller circle of confusion) and increasing the depth of field (area in focus) with more of the scene appearing in focus.

2.2 Focal Length:

The focal length of the lens, although it does not affect the amount of depth of field, influences its appearance.

Longer focal lengths (telephoto lenses) compresses the scene, making the background appear closer to the subject. When the background is blurred due to a shallow depth of field, this compression emphasises the differences between the in-focus elements and the blurriness of the background which both appear close together, producing a perceived shallower depth of field (especially at wide apertures).

Shorter focal lengths (wide-angle lenses) capture a larger portion and distance in the scene. Even with a shallow depth of field, the background elements might be farther away in the frame and the blur might seem less pronounced.

However, at the same focusing distance and aperture, all focal lengths will capture the same depth of field. It is just that the background will appear differently due to the magnification properties of the lens. In essence, focal length doesn't change the depth of field itself, but it affects how we perceive it in the image due to background compression and magnification.

2.3 Subject Distance:

The distance between the camera and the subject influences depth of field.

Closer subject distances result in shallower depth of field. The closer the lens physically get to the subject, the shallower the depth of field becomes (a smaller portion of the scene, both in front of and behind the subject, will be in sharp focus) because the light rays coming from that subject strike the lens at a steeper angle compared to focusing on a distant subject. These steeper angles cause the light rays to diverge more after passing through the lens, resulting in a larger circle of confusion on the sensor.

On the other hand, greater distances increase depth of field (a larger portion of the scene, both near and far from your subject, will appear sharp) because focusing on a distant subject allows light rays to enter the lens at shallower angles. These rays converge more closely on the sensor, resulting in a smaller circle of confusion and a deeper depth of field.

In simpler terms, the closer the lens is to the subject, the more the light rays spread out after the lens, creating blur in areas that fall outside the acceptable focus range.

2.4 Sensor Size:

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

Depth of Field Diagram

Same focal length (20mm)

Different f-stop comparison (f3.5 vs f22)

Same f-stop (5.6)

Different focal length comparison (18mm vs 55mm)

Full frame (55mm) vs crop sensor (36mm) equivalent

3. 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 is 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 artefacts 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 some:

3.1 Number and shape of aperture blades:

The number and shape of aperture blades in a lens affect the shape and appearance of bokeh (i.e. the shape of the out-of-focus highlight mimics the shape of the clear aperture). Lenses with a higher number of aperture blades tend to produce more rounded and smoother bokeh circles due to the circular shape of the iris, while lenses with fewer blades may result in more distinct polygonal or angular shapes.

3.2 Lighting Conditions:

The quality, direction, and intensity of light in the scene also 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.

3.3 Physical Barriers / Aberrations:

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

3.3.1 Cat's eye:

Occurs when out-of-focus highlights take on a cropped shape compared to the bokeh shape encountered towards the center of the frame (e.g. circular bokeh shapes often resembling a cat's eye or oval rather than a circle). This effect is caused by optical vignetting (i.e. mechanical obstruction), where light entering the lens is partially blocked by the lens barrel or lens elements. As such, it increases towards the edges of the frame (larger the distance from the image center, the narrower the cat's eye becomes) and is mostly evident at large apertures (as it is the physical lens barrel that mostly blocks the peripheral light). Once stopped down (narrower aperture), the smaller size of the aperture is visible even from the corners, allowing the light to pass through.

3.4 Lens design:

Affects the appearance of bokeh, with some models introducing definable and memorable patterns such as swirly (or vortex-like, often associated with certain vintage or specialty lenses), soap bubble (resemble the spherical, iridescent patterns seen in bubbles, characterised by its multi-faceted appearance), double rings (two concentric rings or circles within each bokeh shape, often caused by imperfections in the lens elements, particularly in the shape and surface quality of the lens diaphragm blades) or onion-rings (an aspherical element in the optical design may create a bullseye pattern of concentric circles in the bokeh).

4. 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 focal length with the re-arrangement of lens elements due to focal distance changes).

Technically, the focal length of a lens is defined at infinity focus. Hence, focusing at close distances typically results in a slight reduction of the lens focal length (i.e. focus breathing), usually no larger than -5%, 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 is caused by the internal movement (re-arranging) of lens elements during focusing, and a common issue that occurs to some extent on most photographic lenses (especially with vintage lenses). Due to this, specialised high-end cinema lenses tend to include stronger compensation for this phenomenon due to its higher readability in video.

Focal length change (lens breathing) by focusing between a close and distant element