Defocus

Defocus is a term used in optics to describe a situation where an image is out of focus. This occurs when light from a point source does not converge to a single point after passing through an optical system, such as a lens or mirror. Defocus is a common issue in various optical devices, including cameras, microscopes, and telescopes.

Causes of Defocus

Defocus can be caused by several factors:

• Incorrect lens positioning: If the lens is not positioned correctly relative to the image sensor or film, the image will appear blurry.
• Lens aberrations: Imperfections in the lens can cause light to spread out rather than focus to a point.
• Depth of field: A shallow depth of field can result in parts of the image being out of focus.
• Thermal effects: Changes in temperature can cause the lens or mirror to expand or contract, leading to defocus.

Types of Defocus

There are two primary types of defocus:

• Positive defocus: Occurs when the focal plane is in front of the image sensor or film.
• Negative defocus: Occurs when the focal plane is behind the image sensor or film.

Measurement and Correction

Defocus can be measured and corrected using various techniques:

• Autofocus systems: Many modern cameras and microscopes are equipped with autofocus systems that automatically adjust the lens position to achieve sharp focus.
• Manual focus: Users can manually adjust the focus ring on the lens to achieve the desired focus.
• Focus stacking: In photography, multiple images taken at different focus distances can be combined to create a single image with a greater depth of field.

Applications

Defocus is an important consideration in several fields:

• Photography: Achieving the correct focus is crucial for capturing sharp images.
• Microscopy: Proper focus is essential for observing fine details in biological and material samples.
• Astronomy: Telescopes must be precisely focused to observe distant celestial objects clearly.

Related Pages

• Optics
• Lens (optics)
• Depth of field
• Autofocus
• Aberration (optics)
• Microscope
• Telescope