Okay,

I am going to try intuitive. Since I am not awake, this should help.
I will ignore all the back filling issues which started this and no
FFT talk (except maybe incognito).

We all remember that the NA is a function of the sine of the half
angle formed by the focused cone of light created by the lens. As the
angle gets steeper (higher NA), light is projected to a smaller spot.
This happens because, at focus, rays that start from the lens
perimeter more closely overlap light passing through the center of
the lens. As the NA (and half angle go to zero), less and less light
overlap at focus until there is no focusing and no increase in
illumination.

Sorry if this is too intuitive and unfortunately inaccurate in some
respects. I liked Alexander's explanation, but if you are a
biologist, in most cases "intuitive" rules out talking about
diffraction patterns and interference effects, and microscopes as
FFT-IFFT devices.

Oh, and sorry Jim P. This question was addressed to you. Maybe
Barbara Foster can give us a nice explanation, too. Barbara?


>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
>Associate Professor                             lab:    (612) 624 2991
>Dept. Neuroscience                      Preferred FAX:  (612) 624 8118
>University of Minnesota                 Dept FAX:       (612) 626 5009
>Minneapolis, MN  55455                e-mail: [log in to unmask]

--

Mario M. Moronne, Ph.D.
NanoMed Technologies, Inc.
ph (510) 528-2400
Berkeley, CA
94706