The explanation lies in wave optics and interference.
The distance of interference fringes is d=lambda/sin(alpha), where alpha is

the angle between an interfering pair of light beams. As the angle
increases, the distance of the fringes decreases. Your microscopical object

(or the focus in reverse) can be seen after fourier transformation as a
superposition
of interference fringes of several period.

Alexander D.



Martin Wessendorf schrieb:

> James Pawley wrote:
>
> > Not filling the BFP means that no light goes through the high-NA part
> > of the lens. Hence, it operates as though it was of lower NA, the
> > spot is bigger than it should be and the peak power is less.
>
> Jim--
>
> Is there any intuitive explanation for why the illumination spot should
> be bigger with a lower NA?
>
> Thanks!
>
> Martin Wessendorf
> --
> Martin Wessendorf, Ph.D.                        office: (612) 626 0145
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