Old or poorly made lenses often suffered from chromatic aberrations. Apertures One way to reduce the problems of aberrations is to limit the area of the lens being used. By using an aperture, you can select only the image being refracted in the middle of the lens. However, there are two problems with apertures. Reduces the amount of light Light diffraction limits the how small an aperture can be.
Light has wave-like properties.
This is evident when you place an aperture in the path of a light source. Instead of a single pinpoint of light, there is a bright central disk surrounded by a diffuse ring of light. This image is called an airy disc . Light diffraction is the reason that an exact image of a specimen is never achieved . In work done by Sir George Airy and Ernst Abbe in the 19th century the following equation was established to calculate the limit of resolution.