Page "Speckle pattern" Paragraph 10

The light at neighbouring points in the image has been scattered from areas which have many points in common and the intensity of two such points will not differ much.

However, two points in the image which are illuminated by areas in the object which are separated by the diameter of the Airy disk, have light intensities which are unrelated.

This corresponds to a distance in the image of 2. 4λv / D where v is the distance between the lens and the image.

Thus, the ‘ size ’ of the speckles in the image is of this order.