Question

The circle of least confusion is the position where:
A. marginal rays converge
B. paraxial rays converge
C. best image with least area can be seen
D. blurred image with maximum area can be seen

Answer 1

Hint: In this question we have been asked to define the circle of least confusion. We have been given certain statements as the options. Therefore, we have to select a suitable statement that will best describe the circle of least confusion. So, we shall first discuss and define what is the circle of least confusion.

Complete step-by-step answer:
In optics, a circle of confusion is an optical spot that is formed by a cone of light rays from a lens not forming a perfect focus when an image of point source is obtained. It is also known as a disk of confusion, circle of indistinctness, blur circle, or blur spot. In other words, we can say that a circle of confusion is a spot where a blurred image is obtained in a maximum area.
If we were to define the circle of least confusion, it would be the exact opposite of the circle of confusion. Therefore, a circle of least confusion would be the spot where the best image is obtained in a least area. The image of the point source will not be seen as a spot but a distinct point.

So, the correct answer is “Option C”.

Note: Even though with the help of a perfect lens, all the rays can not be focused at a single point. Therefore, a point source will be imaged as a spot. The smallest of these types of spot imaged by a lens is known as the circle of least confusion. The circle of confusion is calculated using the ‘Zeiss’ formula.