Dispersion affects the focus of a lens by causing different colors of light to come to focus at different points. This happens because the refractive index of the lens material varies with the wavelength of light.
When white light passes through a lens, shorter wavelengths like blue or violet bend more than longer wavelengths like red. As a result, blue light focuses closer to the lens, while red light focuses farther away. This creates a problem called chromatic aberration, where the lens fails to bring all colors to the same focal point.
Due to this effect, images formed by the lens may appear blurred or fringed with color, especially around the edges of objects. The focus varies slightly for each color, reducing the sharpness and clarity of the image.
To reduce this problem, optical designers use special combinations of lenses made from materials with different dispersion properties. These combinations, called achromatic or apochromatic lenses, help bring multiple wavelengths to the same focus and minimize the impact of dispersion on image quality.