Question

obtain expression for the equivalent focal length of two thin convex lenses kept in contact ​

Answer 1

Explanation:

Quoted 35 mm equivalent focal lengths typically ignore depth of field (DOF), which depends on both focal length and aperture. The perceived DOF of smaller sensors is deeper due to the shorter focal length lenses.

Equivalent depth of field can be calculated the same way using the crop factor.[3] For example, a 50mm f/2 lens on a 2× crop factor Micro Four Thirds camera would be equivalent to a 100 mm (= 2×50 mm) f/4 (= f/(2×2)) lens on a Full-frame digital SLR in terms of field of view, depth of field, total light gathered,[4] and diffraction effects.