In a photograph

Photograph

A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...

, the area of critical focus is the portion of the picture that is optically in focus

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

. This does not relate to depth of field

Depth of field

In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

 which describes apparent sharpness.
Reducing the size of the aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 will increase the depth of field but the plane of critical focus will not change. Depth of field extends away from the plane of critical sharpness.

The image is only critically in focus within a plane.
The formula that describes the relationship between plane of sharpness, lens

Photographic lens

A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

 and film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 is,
where is the film to lens distance, is the distance from the lens to the plane of critical focus, and is the focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 of the lens.

'Critical Focus' is also the title of a regular column by Brian J. Ford

Brian J. Ford

Brian J. Ford is an independent research biologist, author, and lecturer, who publishes on scientific issues for the general public...

in the American magazine The Microscope.