# Resolution and Rayleigh's Criteria

1. Mar 6, 2013

### prettykitty

When working problems using Rayleigh's Criteria:

θmin=1.22 [λ/D]

I'm sure it has something to do with the aperture being circular but I don't understand why.

Thanks!

2. Mar 6, 2013

### marcusl

You can change the units in the formula to degrees if you wish, in which case the constant 1.22 becomes 69.9°. Radians are the preferred unit in mathematics, however. The look direction in an optical system involves the quantity $sin(\theta)$, and $\theta$ is very small when determining resolution. We can use the approximation $$sin(\theta)≈\theta$$ in deriving the Rayleigh criterion. That approximation is valid as written only for $\theta$ in radians.

3. Mar 7, 2013

### Andy Resnick

The factor 1.22 is due to a circular aperture- for a square aperture, the numerical factor is 1.