Depth of Field, Lens Diffraction, and Circles of Confusion
Focus Issues and Autofocus Considerations
When dealing with camera focus issues, the first thing to check is the (auto-)focus setting in the camera.
- Ensure the camera is set to the preferred focus mode.
- Verify that the subject is positioned in the center of the image, as most cameras use this area as the focal reference point.
Autofocus relies on an algorithm that detects pixel contrast, which may not always yield the best results. While manual focus is preferable in controlled environments, autofocus remains the only option in situations where manual adjustments are not feasible.
If the subject cannot be positioned in the center of the frame, consider using manual focus.
Manual Focus and Depth of Field
Manual focus is ideal when the distance between the camera and the subject remains relatively constant. A common reason for an image appearing blurry is an iris that is opened too wide.
By default, the camera adjusts to provide the brightest image possible. However, a fully opened iris results in more diffused light entering the sensor, creating a shallow depth of field.
Understanding Circles of Confusion
The absolute focal point, known as the circle of least confusion, remains unchanged. However, areas in front of and behind this point experience 'acceptable sharpness' until they reach the Circles of Confusion (CoC), beyond which objects appear out of focus.
In the diagram below, the upper section illustrates a nearly fully open iris, where the CoC threshold is close to the absolute sharpness point.
In contrast, the lower section shows a nearly closed iris, resulting in a more focused beam of light hitting the sensor. This extends the depth of field significantly.
Practical Applications
Understanding depth of field is particularly useful in scenarios such as filming conferences, where speakers may lean forward or recline. Adjusting the iris helps keep the entire subject in focus.
Lens Diffraction and the Trade-off
While a smaller iris opening increases depth of field, it also reduces the amount of light reaching the sensor. If set too small, the sensor may struggle to compensate, introducing noise into the image. This phenomenon is known as lens diffraction.
Examples of Depth of Field Adjustments
The following examples illustrate how iris settings affect focus. The images were taken with a CM71-IP-W using identical settings, except for the f-stop value.
Wide Open Iris (F1.8)
At F1.8, the iris is fully open. The focus is on the Lego minifig in the background, while the black minifig in the foreground is out of focus, just past the CoC threshold.
Smaller Iris Opening (F5.6)
At F5.6, the iris is reduced, increasing depth of field. Both minifigs are now in focus, improving the overall image quality.
Excessively Small Iris (F9.6)
Reducing the iris further may seem beneficial, but it introduces new issues:
- Longer shutter times make filming fast-moving objects challenging.
- Electronic gain compensation can introduce noise if there is insufficient ambient light.
- Lens diffraction reduces overall sharpness as less natural light reaches the sensor.
The image below demonstrates the effect of lens diffraction at F9.6, where sharpness is compromised.
Conclusion
To achieve optimal focus, balance must be maintained between iris size, shutter speed, and gain settings. While a smaller iris improves depth of field, excessive reduction leads to lens diffraction, reducing image sharpness. Adjust settings according to the filming conditions to ensure the best possible image quality.