An automatic focus detection device for microscopes, comprising:
- an optical system for forming an optical image of a sample to be observed, said optical system including:
- a stage for holding the sample placed thereon,
- an objective lens, and
- an imaging lens;
- and
- an automatic focus detection section including:
- an image sensor for photoelectrically converting the optical image derived from said optical system into an electric signal;
- means for storing image data that is obtained by analog-to-digital conversion of the electric signal from said image sensor and for processing the image data to compute a contrast value,
- means for comparing contrast values,
- means for moving one of said objective lens and said stage, and
- means for storing a position of the one of said objective lens and said stage;
- said automatic focus detecting section performing a focus detecting operation in such a manner that a first contrast value measured at a present position of the one of said objective lens and said stage is compared with a second contrast value previously measured where the one of said objective lens and said stage was located at a position shifted from the present position along an optical axis by a predetermined distance, a distance by which the one of said objective lens and said stage is to be moved next is determined from a contrast change obtained by comparison between the first and second contrast values, and if the distance required for next movement and a resultant contrast change predicted are below predetermined constant values, the present position of one of said objective lens and said stage is judged as an in-focus position,
- wherein said optical system further includes an illumination system with an annular aperture, and
- wherein said automatic focus detecting section proceeds to perform the focus detecting operation in such a manner that the position judged as the in-focus position is redefined as a temporary in-focus position, and the one of said objective lens and said stage is allowed to be moved, for contrast measurement from the temporary in-focus position by a space equal to a distance between a true in-focus position and a false in-focus position to change a distance between said objective lens and the sample,
- the distance between the true in-focus position and the false in-focus position being determined by a ratio of a numerical aperture corresponding to an inside diameter of a conjugate image of said annular aperture at a pupil position of said objective lens to a numerical aperture of said objective lens, a ratio of a numerical aperture corresponding to an outside diameter of the conjugate image of said annular aperture at the pupil position of said objective lens to the numerical aperture of said objective lens, and a first frequency which is determined by one value selected among a pixel size of said image sensor and integral multiples of the pixel size, so that a contrast measurement is made at least where the one of said objective lens and said stage is located at one place between the true in-focus position and the false in-focus position and then the true in-focus position is detected by comparison of contrast values.