1. Field of the Invention
The present invention relates to a lens apparatus which has a branching optical system, disposed in a light path, and which is used for a focusing operation using light beams that have been branched.
2. Description of the Related Art
Various technologies of auto-focusing (AF) in a shooting device, such as a still camera or a video camera, have hitherto been proposed. In particular lens/image pickup devices having a branching optical system disposed in a light path and having a focus detector disposed in a branching light path are known from Japanese Patent Laid-Open Nos. 55-76312, 59-128506, 8-50227, 9-274130, 2002-365517, 2002-365518, 2002-372661, 2003-279842, 2003-279846, 2003-279847, and 2003-287673. Image pickup devices which make variable a focus detection area, which is an area where a length is measured when performing a focusing operation, are discussed in Japanese Patent Laid-Open Nos. 7-46455 and 7-191256.
As discussed in Japanese Patent Laid-Open No. 55-76312 to Japanese Patent Laid-Open No. 2003-287673, in an image pickup system that has a branching optical system disposed in a light path in a focusing optical system and that performs a focusing operation as a result of detecting a focus state by using light beams that have been branched, it is, in principle, necessary to keep the same the relationship between the optical system at the image-pickup side and the optical system at the focus detection side, disposed following the branching optical system. Therefore, when the relationship between a detection value from the focus detector and a focus state at an image-pickup plane is kept the same and focusing with respect to an object near a screen is considered, the relationship between the position of a focus detection area with respect to a predetermined position on an image-pickup screen is kept the same.
Here, it is desirable that the relationship between a focus change amount and a unit operation amount of a focus moving unit have a predetermined relationship. When the relationship between the focus change amount and the unit operation amount is a predetermined relationship, it is possible to set a position control precision and an operating speed of the focus moving unit on the basis of this relationship, so that focusing can be carried out at a high speed and with high precision.
In a focusing operation, for example, a change in focus magnification at an image-pickup side caused by, for example, mounting converter lenses, a change in an image size at an image-pickup element caused by applying the image-pickup element to a lens-replacement-type image pickup device, and a change in an image pickup range due to a difference between aspect ratios may occur. Therefore, a change may occur in the relationship between the image pickup side and the focus detection side, as a result of which a proper focusing operation cannot be carried out.
Factors, such as a change in the relationship between a focus change amount and a unit operation amount of the focus moving unit (which results from a magnification change) make it difficult to properly set a position control precision and an operating speed of the focus moving unit, thereby preventing focusing from being carried out at high speed and with high precision.
Further, for example, a user may want to give priority to a T-number at the image pickup side than to focusing precision when shooting a dark object, or to give priority to following of a focus at high speed with respect to a moving object when shooting the moving body, or to give priority to focusing precision than to focusing speed when shooting a stationary object. In such cases, depending upon shooting conditions, whether the focusing precision, the focusing speed, or the T-number at the image-pickup side is to be given priority changes frequently, thereby preventing proper focusing characteristics from being obtained for the respective shooting conditions.