How subjective laser speckle pattern is formed

Check out speckle patterns here.

1. Different paths that light takes will cause different phases at a given point A at a screen. (That's geometry.)
2. If there's no imaging system - the light hits an object, it gets scattered/reflected off the object, and meets at the screen. The waves interfere and form a speckle pattern depending on the roughness of the object. That's an objective speckle.
3. If you take a lens and image a point A at the object. A is not a point it's an area, defined by the Airy disc. The the disc's diameter is proportional to the distance u between the object and the lens.
4. The Airy disc that can be resolved in the image is proportional to the distance v between the lens and the image.
5. The wavefronts captured within A do not necessarily interfere within the area A'.

Say the v is smaller than u, i.e. the Airy disc A is smaller than A'. The wavefronts that interfere in A' must, therefore, origin from the surrounding area of A. Different wavefronts with different phases result in a different intensity. Because the resulting pattern is depending on the imaging system, it is called subjective.

Moreover, the Airy disc depends also on the aperature (or the lens diameter) the subjective pattern also changes when changing the aperature.