Method of performing autofocus, autofocus system, and camera comprising an autofocus module
Autofocusing a camera while zooming may include zooming the lens to a first zoom position, measuring a first object distance from the camera to an object on which to focus, determining a first focus start position using the first object distance, performing a first autofocus using the first focus start position as a starting point, thereby determining a first focus position. A first lookup object distance may be determined based on the first determined focus position. A first correction factor may be calculated as a ratio between the first lookup object distance and the first object distance. The lens may be zoomed to a second zoom position, and a second focus position may be determined using a second object distance based on the first object distance and a second correction factor based on depths of field at the second and first zoom positions.
1. A method of autofocusing a camera during a zooming operation, the camera comprising a zoom lens, the method comprising
zooming the lens to a first zoom position,
measuring a first object distance from the camera to an object on which to focus,
determining a first focus start position using said first object distance, and using a relationship between object distance, zoom position, and focus position,
performing a first autofocusing operation using a passive autofocus algorithm and using the first focus start position as a starting point for the algorithm, thereby determining a first determined focus position,
determining a first lookup object distance based on the first determined focus position,
calculating a first correction factor as a ratio between the first lookup object distance and the first object distance,
determining a first depth of field for said first zoom position,
zooming the lens to the second zoom position,
determining a second depth of field for said second zoom position,
calculating a second correction factor as a product of the first correction factor and a ratio between the second depth of field and the first depth of field,
calculating a second object distance as a product of the first object distance and the second correction factor, and
determining a second focus position using the second object distance, and using the relationship between object distance, zoom position, and focus position.
2. The method of claim 1 , further comprising
performing a second autofocusing operation using the passive autofocus algorithm and using the second focus position as a starting point for the algorithm, thereby determining a second determined focus position,
determining a second lookup object distance based on the second determined focus position, and
calculating a new correction factor as a ratio between the second lookup object distance and the first object distance.
3. The method of claim 1 , further comprising zooming the lens to an intermediate zoom position between the first zoom position and a second zoom position,
determining an intermediate depth of field for said intermediate zoom position,
calculating an intermediate correction factor as a product of the first correction factor and a ratio between the intermediate depth of field and the first depth of field,
calculating an intermediate object distance as a product of the first object distance and the intermediate correction factor,
determining an intermediate focus position using the intermediate object distance.
4. The method of claim 3 , further comprising performing an intermediate autofocusing operation using the passive autofocus algorithm and using the intermediate focus position as a starting point for the algorithm, thereby determining an intermediate determined focus position.
5. The method of claim 4 , further comprising determining an intermediate lookup object distance based on the intermediate determined focus position, and calculating a new intermediate correction factor as a ratio between the intermediate lookup object distance and the first object distance.
6. The method of claim 1 , wherein depth of field is determined by requesting it from a lens controller controlling the lens.
7. The method of claim 1 , wherein at a predetermined transitional zoom position a previously stored correction factor is used, which is based on a depth of field for a zoom position between said transitional zoom position and a third zoom position, which is beyond said second zoom position, instead of based on a depth of field for a zoom position between said first zoom position and said transitional zoom position.
8. The method of claim 1 , wherein measuring the first object distance from the camera to the object on which to focus is performed using a distance measurement system comprised in the camera.
9. An autofocus system for autofocusing a camera during a zooming operation comprising
a camera,
a distance measurement system arranged to measure a first object distance from the camera to an object on which to focus, and
an autofocus module arranged to perform the method of claim 1 .
10. A camera comprising a zoom lens and an autofocus module arranged to perform the method of claim 1 .
11. The camera of claim 10 , further comprising an integrated object distance measurement system arranged to measure the first object distance from the camera to the object on which to focus.
12. A computer program product comprising a non-transitory computer-readable storage medium with instructions adapted to carry out the method according to claim 1 when executed by a processor.