The depth of field of a standard camera/lens is dependent on the lens settings. A shallow depth of field results in increased image blur on each side of the focus point. Although lens aperture can be reduced to increase depth of field, a corresponding increase in exposure time is required in order to compensate for the smaller aperture. Longer exposures can lead to undesirable motion blur in the image. Consequently, software approaches that alleviate image blur due to defocus are desirable. Extended Depth of Field (EDoF) is a combined hardware/software approach for reducing defocus blur that is recently appearing in mass market camera offerings. This approach can enable the capture of all-in-focus images. However, its significant drawback is the requirement for tailor made non-standard lenses. Extending focus by applying image processing alone (software-only approach) is possible using a deconvolution route. However, the estimation of the deconvolution kernal is problematic, the deconvolution is typically very sensitive to image noise, and a degradation of image quality due to ringing is difficult to avoid.

 

Solution

We have developed a software-only algorithm that extends image focus by reducing defocus blur. A modified lens is not required, allowing the method to be retrospectively applied to images. Our algorithm does not use deconvolution to extend focus, thus avoiding the introduction of ringing artifacts. Instead, a novel derivative shaping approach is used that enables a controlled alteration of the defocus blur. The algorithm is designed to be suitable for inclusion in the embedded image processing pipeline.

The image below shows a 2D barcode imaged under shallow depth of field conditions, where the foreground and background are defocused. On mouse-over the image after extending the focus using our software technique is shown. The depth of field has effectively been extended to bring the entire barcode into focus.

 

Key Advantages

  • Selectively extends the focus range without requiring a modified optical design
  • Mimics the effect that an increasingly stopped physical aperture produces
  • In colour images can be combined with chromatic aberration correction for improved image clarity

 

Image of barcode with significant defocus blur, and image after defocus blur reduction using our extended focus technique.
Mouse over the image to see correction.

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