Method to Speed up Autofocus using Temporary Aperture

Summary

For autofocus, procedure speed is essential.  In a low f-number lens system, the focus depth is highly limited. Samples of image focus criteria must be taken often relative to the lens movement in order to not miss the position where best focus exists.  By temporarily increasing f-number, and thus getting a better focus depth, the lens movement between samples can be increased. The time for a total autofocus cycle could be reduced.

Background

Performance IR cameras want to use low f-number in the optical system to get a good signal-noise ratio on sampled detector signal.  Lower f-number gives less noisy IR images.  Less noisy IR images makes it possible to detect smaller temperature differences.

 

Problem

A problem is that focus depth is narrowed when f-number gets lower.  Correct focus position is more important when f-number is low making robust and fast auto focus procedures important.

For an imaging system that uses the image sensor and the lens system itself to focus.

Finding best focus involves moving the focusing part of the lens system and sampling images or parts thereof from the video system and calculating focus quality values.  Sampling is done in different focus sweep positions.

At the best focus position there will be a peak in the focus quality value.

During a focus position sweep, samples must be frequent enough to find where quality values changes due to the passed focus peak.

A low f-number lens has a very narrow focus depth.  When video image frequency is given, the time for a focus search will be limited by the number of necessary samples.  Low f-number lenses will then need a longer focus search procedure than a system with higher f-number to find focus.

Solution

During focus search, an aperture is temporarily inserted into the optical system.  This increases the focus depth, and thus fewer samples are necessary to find the focus peak.

Benefits

Less time is necessary to perform an auto focus procedure.

Claims

  1. A method in an imaging system having an autofocusing lens system, comprising:
    1. Sampling image focus criteria relative to lens movement in order to detect a lens position with best focus; and
    2. Temporarily increasing the f-number to improve focus depth, thereby increasing the lens movement between samples of image focus criteria and reducing the time for a total autofocus cycle.
  2. The method of claim 1, wherein the imaging system is or is comprised in an IR camera.
  3. The method of claim 2, wherein the optical system of the imaging system has a low f-number, for the purpose of getting a good signal-noise ratio on sampled detector signal making it possible to detect smaller temperature differences.
  4. The method of claim 1, wherein the imaging system is configured to use the image sensor and the lens system itself to focus.
  5. The method of claim 4, wherein the imaging system is a video imaging system.
  6. The method of claim 5, wherein finding best focus comprises moving the focusing part of the lens system and sampling images or parts of images from the video imaging system and calculating focus quality values.
  7. The method of claim 6, wherein sampling is done in different focus sweep positions.
  8. The method of claim 6, wherein the best focus position is found at a peak in the focus quality value.
  9. The method of claim 6, comprising sampling images during a focus position sweep at a frequency sufficient to detect where quality values changes due to a passed focus peak.
  10. The method of claim 6, comprising at a certain video image frequency, decreasing the duration of a focus search by providing a higher f-number thus limiting the number of necessary samples.
  11. The method of claim 1 or 3, comprising temporarily inserting an aperture into the optical system, thereby increasing the focus depth and decreasing the number of samples necessary to find the focus peak.
  12. An imaging system having an autofocusing lens system, being configured to
    1. During a focus search, sampling image focus criteria relative to lens movement in order to detect a lens position with best focus;
    2. During said focus search, temporarily inserting an aperture into the lens system (=optical system) to increase the f-number and thus improve focus depth, thereby increasing the lens movement between samples of image focus criteria and reducing the time for a total autofocus cycle.