Depth of Field/Depth of Focus vs. F/# Trade-Off

In a traditional incoherent optical system increased depth of field is achieved by stopping down the system aperture. Stopping down the aperture is not usually a desired method of increased depth of field since considerable optical power is lost. Through wavefront coding, an incoherent system can have an increased depth of field without any loss of optical power.

Traditional Optical System

 

The image above is from a traditional F7.3 system. Moving your mouse over the image illustrates the increased depth of field achieved by stopping the lens aperture down by a factor of 6, from F/7.3 to F/44. The first image, from the F/7.3 system, has a very small depth of focus and very few parts of the image are in-focus. The F/44 image, stopped down image has a much larger depth of focus and nearly all parts of the image are in-focus. The cost of this increased depth of field has been to decrease the aperture radius by a factor of 6. This decreased aperture captures less than 3% of the optical power captured by the F/7.3 system.

Focus-Invariant Optical/Digital System

 

The images above illustrate the benefits of wavefront coding for increased depth of field. The first image is the raw image data, or intermediate image. Moving your mouse over the image shows the final image after linear digital filtering of the intermediate image. This final image has a depth of field slightly larger than that obtained from the traditional F/44 system, yet has the light gathering power of the F/7.3 system. The achievable increase in depth of field is limited only by the particular optical mask used and the detector dynamic range.

Traditional F/44 Image vs. Focus-Invariant F/7.3 Image

traditional image
 
filtered focus-invariant image

The spatial resolution and depth of field of both images are approximately equal. The important difference is that the optical power captured by the traditional F/44 (stopped down) system is less than 3% of that captured by the focus-invariant (full aperture) system.. An image of the special-purpose optical mask used for this example of wavefront coding is given below.