Maximum numerical aperture and diffraction limit of a lens

Question

In this thesis, you can read page 62/63 the following:

"The lens is designed for λ = 780 nm and works diffraction limited up to NA= 0.55. However the diffraction-limited numerical aperture for λ = 1064 nm is significantly reduced to NA= 0.35"

I am confused with regards to the relation between the numerical aperture (which as I understand here would be fixed by the size of a collimated beam incident on the lens ?) and the diffraction limit.

Isn't the diffraction limit fixed by the aperture of the lens and the working wavelength? How can you work diffraction limited up to a certain numerical aperture ? Can anyone explain the sentence ?

Answer 1

It's about aberration. Generally, aberration increases with increasing numerical aperture, while diffraction decreases. Thus, there is a numerical aperture value below which diffraction limits the optical resolution. Above that value, aberration limits the resolution.

Aberration is a matter of detailed lens design. A lens optimized for minimum aberration at a particular wavelength will generally have worse aberration at other wavelengths.

So, for small apertures, diffraction will be large, and will limit your resolution. Increasing the aperture will reduce the diffraction, but if the aperture is too large, aberration will dominate and the system will no longer be diffraction limited.