The “Numerical Aperture” (NA) is the most important number associated with the light gathering ability of an objective or condenser. It is directly related to the angle of the cone which is formed between a point on the specimen and the front lens of the objective or condenser, determined by the equation NA = n sin ∝.

What is the formula for F-number?

The F-number of a lens is the ratio of its focal length divided by the diameter of the aperture. Since the F-number is a ratio involving the diameter, and not the area, we lose the ability to nicely double or halve a number to calculate a stop.

How is numerical aperture related to n1 n2?

The N.A. of any glass combination may be calculated as follows: (where N1= the index of refraction of the core glass), and N2=(the index of refraction of the cladding glass): For example, taking 1.62 for N1 and 1.52 for N2 , we find the NA to be . 56. By calculating the arc sine (sin-1) of .