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Circle of Chaos: Do you know what it means to accept circular ambiguity?
In the world of photography, depth of field (DOF) is an important concept every photographer should understand. Depth of field not only affects the beauty of a photo, but also has a profound impact on the visual story we capture. Proper depth of field setting can guide the viewer's eye, emphasize a specific subject, or keep the entire image clear.
Definition and phenomenon of depth of field
Depth of field is the distance between the nearest and farthest objects that can remain acceptably in focus in an image. In theory, "acceptably sharp focus" is defined based on a property called "circular blur".
Depth of field increases as the distance to the subject or the amount of acceptable circular blur increases, while increasing the aperture (i.e., decreasing the F-number) or increasing the focal length decreases depth of field.
Factors that affect depth of field
Among the factors that affect depth of field, the most critical are focal length, object distance, acceptable circular blur size, and aperture. In simplified terms, depth of field can be expressed as follows:
DOF ≈ 2u²Nc/f²
Here, u is the object distance, N is the f-number of the aperture, c is the size of the circular blur, and f is the focal length. This formula states that the ratio between the distance to the object and the focal length has a direct effect on depth of field.
Depth of field changes linearly with F-number and circular blur, and decreases as object distance increases or focal length increases.
How Aperture Affects Depth of Field
In photography, aperture is a key factor in controlling depth of field. When you decrease the aperture (i.e. increase the F-number), the depth of field increases because only light enters the lens at shallower angles, reducing circular blur.
The impact of circular blur and focus
Whenever we focus at a certain distance, a point-like object will form a small dot on the photosensitive plane. Conversely, if the distance is inaccurate, a larger blur circle will be produced. The size of this blur circle is called the acceptable circular blur, and its size depends on the final use of the image.
For example, traditionally for 35mm film we set the acceptable circular blur limit at 0.025mm. This means that when looking at a certain distance, any blur larger than this diameter will be seen as unsharp.
The Effects of Camera Movement
Camera motion such as pan and tilt will affect the focal plane position. Such changes will change the acceptable sharpness range as the focal plane changes.
Methods to overcome depth of field limitations
Many photographers use different methods to overcome the limitations of depth of field. This includes focus stacking, a technique where multiple photos are taken with different focus points and the resulting image is combined into a single image with greater depth of field.
Optical Diffraction and Depth of Field
When using high F-number settings, diffraction can reduce image clarity. While this isn't a huge issue in general photography, in close-up photography, small apertures can result in degraded image quality when trying to achieve maximum depth of field.
Practical depth of field calculation
Many lenses display depth of field markings to help photographers adjust focus and aperture before shooting. This allows the photographer to effectively extrapolate the necessary focal length and aperture settings from the desired depth of field.
SummaryUnderstanding the importance of depth of field is not only about improving your photographic skills, but also about improving your awareness of visual beauty. As digital photography technology continues to advance, are there other undiscovered possibilities that can expand our understanding and application of depth of field?
